// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:math' as math;
import 'dart:ui' as ui show lerpDouble;

import 'package:flutter/foundation.dart';
import 'package:flutter/gestures.dart';

import 'package:vector_math/vector_math_64.dart';

import 'debug.dart';
import 'object.dart';

// This class should only be used in debug builds.
class _DebugSize extends Size {
  _DebugSize(Size source, this._owner, this._canBeUsedByParent) : super.copy(source);
  final RenderBox _owner;
  final bool _canBeUsedByParent;
}

/// Immutable layout constraints for [RenderBox] layout.
///
/// A [Size] respects a [BoxConstraints] if, and only if, all of the following
/// relations hold:
///
/// * [minWidth] <= [Size.width] <= [maxWidth]
/// * [minHeight] <= [Size.height] <= [maxHeight]
///
/// The constraints themselves must satisfy these relations:
///
/// * 0.0 <= [minWidth] <= [maxWidth] <= [double.infinity]
/// * 0.0 <= [minHeight] <= [maxHeight] <= [double.infinity]
///
/// [double.infinity] is a legal value for each constraint.
///
/// ## The box layout model
///
/// Render objects in the Flutter framework are laid out by a one-pass layout
/// model which walks down the render tree passing constraints, then walks back
/// up the render tree passing concrete geometry.
///
/// For boxes, the constraints are [BoxConstraints], which, as described herein,
/// consist of four numbers: a minimum width [minWidth], a maximum width
/// [maxWidth], a minimum height [minHeight], and a maximum height [maxHeight].
///
/// The geometry for boxes consists of a [Size], which must satisfy the
/// constraints described above.
///
/// Each [RenderBox] (the objects that provide the layout models for box
/// widgets) receives [BoxConstraints] from its parent, then lays out each of
/// its children, then picks a [Size] that satisfies the [BoxConstraints].
///
/// Render objects position their children independently of laying them out.
/// Frequently, the parent will use the children's sizes to determine their
/// position. A child does not know its position and will not necessarily be
/// laid out again, or repainted, if its position changes.
///
/// ## Terminology
///
/// When the minimum constraints and the maximum constraint in an axis are the
/// same, that axis is _tightly_ constrained. See: [new
/// BoxConstraints.tightFor], [new BoxConstraints.tightForFinite], [tighten],
/// [hasTightWidth], [hasTightHeight], [isTight].
///
/// An axis with a minimum constraint of 0.0 is _loose_ (regardless of the
/// maximum constraint; if it is also 0.0, then the axis is simultaneously tight
/// and loose!). See: [new BoxConstraints.loose], [loosen].
///
/// An axis whose maximum constraint is not infinite is _bounded_. See:
/// [hasBoundedWidth], [hasBoundedHeight].
///
/// An axis whose maximum constraint is infinite is _unbounded_. An axis is
/// _expanding_ if it is tightly infinite (its minimum and maximum constraints
/// are both infinite). See: [new BoxConstraints.expand].
///
/// A size is _constrained_ when it satisfies a [BoxConstraints] description.
/// See: [constrain], [constrainWidth], [constrainHeight],
/// [constrainDimensions], [constrainSizeAndAttemptToPreserveAspectRatio],
/// [isSatisfiedBy].
class BoxConstraints extends Constraints {
  /// Creates box constraints with the given constraints.
  const BoxConstraints({
    this.minWidth = 0.0,
    this.maxWidth = double.infinity,
    this.minHeight = 0.0,
    this.maxHeight = double.infinity
  });

  /// The minimum width that satisfies the constraints.
  final double minWidth;

  /// The maximum width that satisfies the constraints.
  ///
  /// Might be [double.infinity].
  final double maxWidth;

  /// The minimum height that satisfies the constraints.
  final double minHeight;

  /// The maximum height that satisfies the constraints.
  ///
  /// Might be [double.infinity].
  final double maxHeight;

  /// Creates box constraints that is respected only by the given size.
  BoxConstraints.tight(Size size)
    : minWidth = size.width,
      maxWidth = size.width,
      minHeight = size.height,
      maxHeight = size.height;

  /// Creates box constraints that require the given width or height.
  ///
  /// See also:
  ///
  ///  * [new BoxConstraints.tightForFinite], which is similar but instead of
  ///    being tight if the value is non-null, is tight if the value is not
  ///    infinite.
  const BoxConstraints.tightFor({
    double width,
    double height
  }): minWidth = width != null ? width : 0.0,
      maxWidth = width != null ? width : double.infinity,
      minHeight = height != null ? height : 0.0,
      maxHeight = height != null ? height : double.infinity;

  /// Creates box constraints that require the given width or height, except if
  /// they are infinite.
  ///
  /// See also:
  ///
  ///  * [new BoxConstraints.tightFor], which is similar but instead of being
  ///    tight if the value is not infinite, is tight if the value is non-null.
  const BoxConstraints.tightForFinite({
    double width = double.infinity,
    double height = double.infinity
  }): minWidth = width != double.infinity ? width : 0.0,
      maxWidth = width != double.infinity ? width : double.infinity,
      minHeight = height != double.infinity ? height : 0.0,
      maxHeight = height != double.infinity ? height : double.infinity;

  /// Creates box constraints that forbid sizes larger than the given size.
  BoxConstraints.loose(Size size)
    : minWidth = 0.0,
      maxWidth = size.width,
      minHeight = 0.0,
      maxHeight = size.height;

  /// Creates box constraints that expand to fill another box constraints.
  ///
  /// If width or height is given, the constraints will require exactly the
  /// given value in the given dimension.
  const BoxConstraints.expand({
    double width,
    double height
  }): minWidth = width != null ? width : double.infinity,
      maxWidth = width != null ? width : double.infinity,
      minHeight = height != null ? height : double.infinity,
      maxHeight = height != null ? height : double.infinity;

  /// Creates a copy of this box constraints but with the given fields replaced with the new values.
  BoxConstraints copyWith({
    double minWidth,
    double maxWidth,
    double minHeight,
    double maxHeight
  }) {
    return new BoxConstraints(
      minWidth: minWidth ?? this.minWidth,
      maxWidth: maxWidth ?? this.maxWidth,
      minHeight: minHeight ?? this.minHeight,
      maxHeight: maxHeight ?? this.maxHeight
    );
  }

  /// Returns new box constraints that are smaller by the given edge dimensions.
  BoxConstraints deflate(EdgeInsets edges) {
    assert(edges != null);
    assert(debugAssertIsValid());
    final double horizontal = edges.horizontal;
    final double vertical = edges.vertical;
    final double deflatedMinWidth = math.max(0.0, minWidth - horizontal);
    final double deflatedMinHeight = math.max(0.0, minHeight - vertical);
    return new BoxConstraints(
      minWidth: deflatedMinWidth,
      maxWidth: math.max(deflatedMinWidth, maxWidth - horizontal),
      minHeight: deflatedMinHeight,
      maxHeight: math.max(deflatedMinHeight, maxHeight - vertical)
    );
  }

  /// Returns new box constraints that remove the minimum width and height requirements.
  BoxConstraints loosen() {
    assert(debugAssertIsValid());
    return new BoxConstraints(
      minWidth: 0.0,
      maxWidth: maxWidth,
      minHeight: 0.0,
      maxHeight: maxHeight
    );
  }

  /// Returns new box constraints that respect the given constraints while being
  /// as close as possible to the original constraints.
  BoxConstraints enforce(BoxConstraints constraints) {
    return new BoxConstraints(
      minWidth: minWidth.clamp(constraints.minWidth, constraints.maxWidth),
      maxWidth: maxWidth.clamp(constraints.minWidth, constraints.maxWidth),
      minHeight: minHeight.clamp(constraints.minHeight, constraints.maxHeight),
      maxHeight: maxHeight.clamp(constraints.minHeight, constraints.maxHeight)
    );
  }

  /// Returns new box constraints with a tight width and/or height as close to
  /// the given width and height as possible while still respecting the original
  /// box constraints.
  BoxConstraints tighten({ double width, double height }) {
    return new BoxConstraints(minWidth: width == null ? minWidth : width.clamp(minWidth, maxWidth),
                              maxWidth: width == null ? maxWidth : width.clamp(minWidth, maxWidth),
                              minHeight: height == null ? minHeight : height.clamp(minHeight, maxHeight),
                              maxHeight: height == null ? maxHeight : height.clamp(minHeight, maxHeight));
  }

  /// A box constraints with the width and height constraints flipped.
  BoxConstraints get flipped {
    return new BoxConstraints(
      minWidth: minHeight,
      maxWidth: maxHeight,
      minHeight: minWidth,
      maxHeight: maxWidth
    );
  }

  /// Returns box constraints with the same width constraints but with
  /// unconstrained height.
  BoxConstraints widthConstraints() => new BoxConstraints(minWidth: minWidth, maxWidth: maxWidth);

  /// Returns box constraints with the same height constraints but with
  /// unconstrained width
  BoxConstraints heightConstraints() => new BoxConstraints(minHeight: minHeight, maxHeight: maxHeight);

  /// Returns the width that both satisfies the constraints and is as close as
  /// possible to the given width.
  double constrainWidth([double width = double.infinity]) {
    assert(debugAssertIsValid());
    return width.clamp(minWidth, maxWidth);
  }

  /// Returns the height that both satisfies the constraints and is as close as
  /// possible to the given height.
  double constrainHeight([double height = double.infinity]) {
    assert(debugAssertIsValid());
    return height.clamp(minHeight, maxHeight);
  }

  Size _debugPropagateDebugSize(Size size, Size result) {
    assert(() {
      if (size is _DebugSize)
        result = new _DebugSize(result, size._owner, size._canBeUsedByParent);
      return true;
    }());
    return result;
  }

  /// Returns the size that both satisfies the constraints and is as close as
  /// possible to the given size.
  ///
  /// See also [constrainDimensions], which applies the same algorithm to
  /// separately provided widths and heights.
  Size constrain(Size size) {
    Size result = new Size(constrainWidth(size.width), constrainHeight(size.height));
    assert(() { result = _debugPropagateDebugSize(size, result); return true; }());
    return result;
  }

  /// Returns the size that both satisfies the constraints and is as close as
  /// possible to the given width and height.
  ///
  /// When you already have a [Size], prefer [constrain], which applies the same
  /// algorithm to a [Size] directly.
  Size constrainDimensions(double width, double height) {
    return new Size(constrainWidth(width), constrainHeight(height));
  }

  /// Returns a size that attempts to meet the following conditions, in order:
  ///
  ///  * The size must satisfy these constraints.
  ///  * The aspect ratio of the returned size matches the aspect ratio of the
  ///    given size.
  ///  * The returned size as big as possible while still being equal to or
  ///    smaller than the given size.
  Size constrainSizeAndAttemptToPreserveAspectRatio(Size size) {
    if (isTight) {
      Size result = smallest;
      assert(() { result = _debugPropagateDebugSize(size, result); return true; }());
      return result;
    }

    double width = size.width;
    double height = size.height;
    assert(width > 0.0);
    assert(height > 0.0);
    final double aspectRatio = width / height;

    if (width > maxWidth) {
      width = maxWidth;
      height = width / aspectRatio;
    }

    if (height > maxHeight) {
      height = maxHeight;
      width = height * aspectRatio;
    }

    if (width < minWidth) {
      width = minWidth;
      height = width / aspectRatio;
    }

    if (height < minHeight) {
      height = minHeight;
      width = height * aspectRatio;
    }

    Size result = new Size(constrainWidth(width), constrainHeight(height));
    assert(() { result = _debugPropagateDebugSize(size, result); return true; }());
    return result;
  }

  /// The biggest size that satisfies the constraints.
  Size get biggest => new Size(constrainWidth(), constrainHeight());

  /// The smallest size that satisfies the constraints.
  Size get smallest => new Size(constrainWidth(0.0), constrainHeight(0.0));

  /// Whether there is exactly one width value that satisfies the constraints.
  bool get hasTightWidth => minWidth >= maxWidth;

  /// Whether there is exactly one height value that satisfies the constraints.
  bool get hasTightHeight => minHeight >= maxHeight;

  /// Whether there is exactly one size that satisfies the constraints.
  @override
  bool get isTight => hasTightWidth && hasTightHeight;

  /// Whether there is an upper bound on the maximum width.
  bool get hasBoundedWidth => maxWidth < double.infinity;

  /// Whether there is an upper bound on the maximum height.
  bool get hasBoundedHeight => maxHeight < double.infinity;

  /// Whether the given size satisfies the constraints.
  bool isSatisfiedBy(Size size) {
    assert(debugAssertIsValid());
    return (minWidth <= size.width) && (size.width <= maxWidth) &&
           (minHeight <= size.height) && (size.height <= maxHeight);
  }

  /// Scales each constraint parameter by the given factor.
  BoxConstraints operator*(double factor) {
    return new BoxConstraints(
      minWidth: minWidth * factor,
      maxWidth: maxWidth * factor,
      minHeight: minHeight * factor,
      maxHeight: maxHeight * factor
    );
  }

  /// Scales each constraint parameter by the inverse of the given factor.
  BoxConstraints operator/(double factor) {
    return new BoxConstraints(
      minWidth: minWidth / factor,
      maxWidth: maxWidth / factor,
      minHeight: minHeight / factor,
      maxHeight: maxHeight / factor
    );
  }

  /// Scales each constraint parameter by the inverse of the given factor, rounded to the nearest integer.
  BoxConstraints operator~/(double factor) {
    return new BoxConstraints(
      minWidth: (minWidth ~/ factor).toDouble(),
      maxWidth: (maxWidth ~/ factor).toDouble(),
      minHeight: (minHeight ~/ factor).toDouble(),
      maxHeight: (maxHeight ~/ factor).toDouble()
    );
  }

  /// Computes the remainder of each constraint parameter by the given value.
  BoxConstraints operator%(double value) {
    return new BoxConstraints(
      minWidth: minWidth % value,
      maxWidth: maxWidth % value,
      minHeight: minHeight % value,
      maxHeight: maxHeight % value
    );
  }

  /// Linearly interpolate between two BoxConstraints.
  ///
  /// If either is null, this function interpolates from a [BoxConstraints]
  /// object whose fields are all set to 0.0.
  ///
  /// The `t` argument represents position on the timeline, with 0.0 meaning
  /// that the interpolation has not started, returning `a` (or something
  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
  /// returning `b` (or something equivalent to `b`), and values in between
  /// meaning that the interpolation is at the relevant point on the timeline
  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
  /// 1.0, so negative values and values greater than 1.0 are valid (and can
  /// easily be generated by curves such as [Curves.elasticInOut]).
  ///
  /// Values for `t` are usually obtained from an [Animation<double>], such as
  /// an [AnimationController].
  static BoxConstraints lerp(BoxConstraints a, BoxConstraints b, double t) {
    assert(t != null);
    if (a == null && b == null)
      return null;
    if (a == null)
      return b * t;
    if (b == null)
      return a * (1.0 - t);
    assert(a.debugAssertIsValid());
    assert(b.debugAssertIsValid());
    assert((a.minWidth.isFinite && b.minWidth.isFinite) || (a.minWidth == double.infinity && b.minWidth == double.infinity), 'Cannot interpolate between finite constraints and unbounded constraints.');
    assert((a.maxWidth.isFinite && b.maxWidth.isFinite) || (a.maxWidth == double.infinity && b.maxWidth == double.infinity), 'Cannot interpolate between finite constraints and unbounded constraints.');
    assert((a.minHeight.isFinite && b.minHeight.isFinite) || (a.minHeight == double.infinity && b.minHeight == double.infinity), 'Cannot interpolate between finite constraints and unbounded constraints.');
    assert((a.maxHeight.isFinite && b.maxHeight.isFinite) || (a.maxHeight == double.infinity && b.maxHeight == double.infinity), 'Cannot interpolate between finite constraints and unbounded constraints.');
    return new BoxConstraints(
      minWidth: a.minWidth.isFinite ? ui.lerpDouble(a.minWidth, b.minWidth, t) : double.infinity,
      maxWidth: a.maxWidth.isFinite ? ui.lerpDouble(a.maxWidth, b.maxWidth, t) : double.infinity,
      minHeight: a.minHeight.isFinite ? ui.lerpDouble(a.minHeight, b.minHeight, t) : double.infinity,
      maxHeight: a.maxHeight.isFinite ? ui.lerpDouble(a.maxHeight, b.maxHeight, t) : double.infinity,
    );
  }

  /// Returns whether the object's constraints are normalized.
  /// Constraints are normalized if the minimums are less than or
  /// equal to the corresponding maximums.
  ///
  /// For example, a BoxConstraints object with a minWidth of 100.0
  /// and a maxWidth of 90.0 is not normalized.
  ///
  /// Most of the APIs on BoxConstraints expect the constraints to be
  /// normalized and have undefined behavior when they are not. In
  /// checked mode, many of these APIs will assert if the constraints
  /// are not normalized.
  @override
  bool get isNormalized {
    return minWidth >= 0.0 &&
           minWidth <= maxWidth &&
           minHeight >= 0.0 &&
           minHeight <= maxHeight;
  }

  @override
  bool debugAssertIsValid({
    bool isAppliedConstraint = false,
    InformationCollector informationCollector,
  }) {
    assert(() {
      void throwError(String message) {
        final StringBuffer information = new StringBuffer();
        if (informationCollector != null)
          informationCollector(information);
        throw new FlutterError('$message\n${information}The offending constraints were:\n  $this');
      }
      if (minWidth.isNaN || maxWidth.isNaN || minHeight.isNaN || maxHeight.isNaN) {
        final List<String> affectedFieldsList = <String>[];
        if (minWidth.isNaN)
          affectedFieldsList.add('minWidth');
        if (maxWidth.isNaN)
          affectedFieldsList.add('maxWidth');
        if (minHeight.isNaN)
          affectedFieldsList.add('minHeight');
        if (maxHeight.isNaN)
          affectedFieldsList.add('maxHeight');
        assert(affectedFieldsList.isNotEmpty);
        if (affectedFieldsList.length > 1)
          affectedFieldsList.add('and ${affectedFieldsList.removeLast()}');
        String whichFields = '';
        if (affectedFieldsList.length > 2) {
          whichFields = affectedFieldsList.join(', ');
        } else if (affectedFieldsList.length == 2) {
          whichFields = affectedFieldsList.join(' ');
        } else {
          whichFields = affectedFieldsList.single;
        }
        throwError('BoxConstraints has ${affectedFieldsList.length == 1 ? 'a NaN value' : 'NaN values' } in $whichFields.');
      }
      if (minWidth < 0.0 && minHeight < 0.0)
        throwError('BoxConstraints has both a negative minimum width and a negative minimum height.');
      if (minWidth < 0.0)
        throwError('BoxConstraints has a negative minimum width.');
      if (minHeight < 0.0)
        throwError('BoxConstraints has a negative minimum height.');
      if (maxWidth < minWidth && maxHeight < minHeight)
        throwError('BoxConstraints has both width and height constraints non-normalized.');
      if (maxWidth < minWidth)
        throwError('BoxConstraints has non-normalized width constraints.');
      if (maxHeight < minHeight)
        throwError('BoxConstraints has non-normalized height constraints.');
      if (isAppliedConstraint) {
        if (minWidth.isInfinite && minHeight.isInfinite)
          throwError('BoxConstraints forces an infinite width and infinite height.');
        if (minWidth.isInfinite)
          throwError('BoxConstraints forces an infinite width.');
        if (minHeight.isInfinite)
          throwError('BoxConstraints forces an infinite height.');
      }
      assert(isNormalized);
      return true;
    }());
    return isNormalized;
  }

  /// Returns a box constraints that [isNormalized].
  ///
  /// The returned [maxWidth] is at least as large as the [minWidth]. Similarly,
  /// the returned [maxHeight] is at least as large as the [minHeight].
  BoxConstraints normalize() {
    if (isNormalized)
      return this;
    final double minWidth = this.minWidth >= 0.0 ? this.minWidth : 0.0;
    final double minHeight = this.minHeight >= 0.0 ? this.minHeight : 0.0;
    return new BoxConstraints(
      minWidth: minWidth,
      maxWidth: minWidth > maxWidth ? minWidth : maxWidth,
      minHeight: minHeight,
      maxHeight: minHeight > maxHeight ? minHeight : maxHeight
    );
  }

  @override
  bool operator ==(dynamic other) {
    assert(debugAssertIsValid());
    if (identical(this, other))
      return true;
    if (other is! BoxConstraints)
      return false;
    final BoxConstraints typedOther = other;
    assert(typedOther.debugAssertIsValid());
    return minWidth == typedOther.minWidth &&
           maxWidth == typedOther.maxWidth &&
           minHeight == typedOther.minHeight &&
           maxHeight == typedOther.maxHeight;
  }

  @override
  int get hashCode {
    assert(debugAssertIsValid());
    return hashValues(minWidth, maxWidth, minHeight, maxHeight);
  }

  @override
  String toString() {
    final String annotation = isNormalized ? '' : '; NOT NORMALIZED';
    if (minWidth == double.infinity && minHeight == double.infinity)
      return 'BoxConstraints(biggest$annotation)';
    if (minWidth == 0 && maxWidth == double.infinity &&
        minHeight == 0 && maxHeight == double.infinity)
      return 'BoxConstraints(unconstrained$annotation)';
    String describe(double min, double max, String dim) {
      if (min == max)
        return '$dim=${min.toStringAsFixed(1)}';
      return '${min.toStringAsFixed(1)}<=$dim<=${max.toStringAsFixed(1)}';
    }
    final String width = describe(minWidth, maxWidth, 'w');
    final String height = describe(minHeight, maxHeight, 'h');
    return 'BoxConstraints($width, $height$annotation)';
  }
}

/// A hit test entry used by [RenderBox].
class BoxHitTestEntry extends HitTestEntry {
  /// Creates a box hit test entry.
  ///
  /// The [localPosition] argument must not be null.
  const BoxHitTestEntry(RenderBox target, this.localPosition)
      : assert(localPosition != null),
        super(target);

  @override
  RenderBox get target => super.target;

  /// The position of the hit test in the local coordinates of [target].
  final Offset localPosition;

  @override
  String toString() => '${describeIdentity(target)}@$localPosition';
}

/// Parent data used by [RenderBox] and its subclasses.
class BoxParentData extends ParentData {
  /// The offset at which to paint the child in the parent's coordinate system.
  Offset offset = Offset.zero;

  @override
  String toString() => 'offset=$offset';
}

/// Abstract ParentData subclass for RenderBox subclasses that want the
/// ContainerRenderObjectMixin.
///
/// This is a convenience class that mixes in the relevant classes with
/// the relevant type arguments.
abstract class ContainerBoxParentData<ChildType extends RenderObject> extends BoxParentData with ContainerParentDataMixin<ChildType> { }

enum _IntrinsicDimension { minWidth, maxWidth, minHeight, maxHeight }

@immutable
class _IntrinsicDimensionsCacheEntry {
  const _IntrinsicDimensionsCacheEntry(this.dimension, this.argument);

  final _IntrinsicDimension dimension;
  final double argument;

  @override
  bool operator ==(dynamic other) {
    if (other is! _IntrinsicDimensionsCacheEntry)
      return false;
    final _IntrinsicDimensionsCacheEntry typedOther = other;
    return dimension == typedOther.dimension &&
           argument == typedOther.argument;
  }

  @override
  int get hashCode => hashValues(dimension, argument);
}

/// A render object in a 2D Cartesian coordinate system.
///
/// The [size] of each box is expressed as a width and a height. Each box has
/// its own coordinate system in which its upper left corner is placed at (0,
/// 0). The lower right corner of the box is therefore at (width, height). The
/// box contains all the points including the upper left corner and extending
/// to, but not including, the lower right corner.
///
/// Box layout is performed by passing a [BoxConstraints] object down the tree.
/// The box constraints establish a min and max value for the child's width and
/// height. In determining its size, the child must respect the constraints
/// given to it by its parent.
///
/// This protocol is sufficient for expressing a number of common box layout
/// data flows. For example, to implement a width-in-height-out data flow, call
/// your child's [layout] function with a set of box constraints with a tight
/// width value (and pass true for parentUsesSize). After the child determines
/// its height, use the child's height to determine your size.
///
/// ## Writing a RenderBox subclass
///
/// One would implement a new [RenderBox] subclass to describe a new layout
/// model, new paint model, new hit-testing model, or new semantics model, while
/// remaining in the Cartesian space defined by the [RenderBox] protocol.
///
/// To create a new protocol, consider subclassing [RenderObject] instead.
///
/// ### Constructors and properties of a new RenderBox subclass
///
/// The constructor will typically take a named argument for each property of
/// the class. The value is then passed to a private field of the class and the
/// constructor asserts its correctness (e.g. if it should not be null, it
/// asserts it's not null).
///
/// Properties have the form of a getter/setter/field group like the following:
///
/// ```dart
/// AxisDirection get axis => _axis;
/// AxisDirection _axis;
/// set axis(AxisDirection value) {
///   assert(value != null); // same check as in the constructor
///   if (value == _axis)
///     return;
///   _axis = value;
///   markNeedsLayout();
/// }
/// ```
///
/// The setter will typically finish with either a call to [markNeedsLayout], if
/// the layout uses this property, or [markNeedsPaint], if only the painter
/// function does. (No need to call both, [markNeedsLayout] implies
/// [markNeedsPaint].)
///
/// Consider layout and paint to be expensive; be conservative about calling
/// [markNeedsLayout] or [markNeedsPaint]. They should only be called if the
/// layout (or paint, respectively) has actually changed.
///
/// ### Children
///
/// If a render object is a leaf, that is, it cannot have any children, then
/// ignore this section. (Examples of leaf render objects are [RenderImage] and
/// [RenderParagraph].)
///
/// For render objects with children, there are four possible scenarios:
///
/// * A single [RenderBox] child. In this scenario, consider inheriting from
///   [RenderProxyBox] (if the render object sizes itself to match the child) or
///   [RenderShiftedBox] (if the child will be smaller than the box and the box
///   will align the child inside itself).
///
/// * A single child, but it isn't a [RenderBox]. Use the
///   [RenderObjectWithChildMixin] mixin.
///
/// * A single list of children. Use the [ContainerRenderObjectMixin] mixin.
///
/// * A more complicated child model.
///
/// #### Using RenderProxyBox
///
/// By default, a [RenderProxyBox] render object sizes itself to fit its child, or
/// to be as small as possible if there is no child; it passes all hit testing
/// and painting on to the child, and intrinsic dimensions and baseline
/// measurements similarly are proxied to the child.
///
/// A subclass of [RenderProxyBox] just needs to override the parts of the
/// [RenderBox] protocol that matter. For example, [RenderOpacity] just
/// overrides the paint method (and [alwaysNeedsCompositing] to reflect what the
/// paint method does, and the [visitChildrenForSemantics] method so that the
/// child is hidden from accessibility tools when it's invisible), and adds an
/// [RenderOpacity.opacity] field.
///
/// [RenderProxyBox] assumes that the child is the size of the parent and
/// positioned at 0,0. If this is not true, then use [RenderShiftedBox] instead.
///
/// See
/// [proxy_box.dart](https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/rendering/proxy_box.dart)
/// for examples of inheriting from [RenderProxyBox].
///
/// #### Using RenderShiftedBox
///
/// By default, a [RenderShiftedBox] acts much like a [RenderProxyBox] but
/// without assuming that the child is positioned at 0,0 (the actual position
/// recorded in the child's [parentData] field is used), and without providing a
/// default layout algorithm.
///
/// See
/// [shifted_box.dart](https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/rendering/shifted_box.dart)
/// for examples of inheriting from [RenderShiftedBox].
///
/// #### Kinds of children and child-specific data
///
/// A [RenderBox] doesn't have to have [RenderBox] children. One can use another
/// subclass of [RenderObject] for a [RenderBox]'s children. See the discussion
/// at [RenderObject].
///
/// Children can have additional data owned by the parent but stored on the
/// child using the [parentData] field. The class used for that data must
/// inherit from [ParentData]. The [setupParentData] method is used to
/// initialize the [parentData] field of a child when the child is attached.
///
/// By convention, [RenderBox] objects that have [RenderBox] children use the
/// [BoxParentData] class, which has a [BoxParentData.offset] field to store the
/// position of the child relative to the parent. ([RenderProxyBox] does not
/// need this offset and therefore is an exception to this rule.)
///
/// #### Using RenderObjectWithChildMixin
///
/// If a render object has a single child but it isn't a [RenderBox], then the
/// [RenderObjectWithChildMixin] class, which is a mixin that will handle the
/// boilerplate of managing a child, will be useful.
///
/// It's a generic class with one type argument, the type of the child. For
/// example, if you are building a `RenderFoo` class which takes a single
/// `RenderBar` child, you would use the mixin as follows:
///
/// ```dart
/// class RenderFoo extends RenderBox
///   with RenderObjectWithChildMixin<RenderBar> {
///   // ...
/// }
/// ```
///
/// Since the `RenderFoo` class itself is still a [RenderBox] in this case, you
/// still have to implement the [RenderBox] layout algorithm, as well as
/// features like intrinsics and baselines, painting, and hit testing.
///
/// #### Using ContainerRenderObjectMixin
///
/// If a render box can have multiple children, then the
/// [ContainerRenderObjectMixin] mixin can be used to handle the boilerplate. It
/// uses a linked list to model the children in a manner that is easy to mutate
/// dynamically and that can be walked efficiently. Random access is not
/// efficient in this model; if you need random access to the children consider
/// the next section on more complicated child models.
///
/// The [ContainerRenderObjectMixin] class has two type arguments. The first is
/// the type of the child objects. The second is the type for their
/// [parentData]. The class used for [parentData] must itself have the
/// [ContainerParentDataMixin] class mixed into it; this is where
/// [ContainerRenderObjectMixin] stores the linked list. A [ParentData] class
/// can extend [ContainerBoxParentData]; this is essentially
/// [BoxParentData] mixed with [ContainerParentDataMixin]. For example, if a
/// `RenderFoo` class wanted to have a linked list of [RenderBox] children, one
/// might create a `FooParentData` class as follows:
///
/// ```dart
/// class FooParentData extends ContainerBoxParentData<RenderBox> {
///   // (any fields you might need for these children)
/// }
/// ```
///
/// When using [ContainerRenderObjectMixin] in a [RenderBox], consider mixing in
/// [RenderBoxContainerDefaultsMixin], which provides a collection of utility
/// methods that implement common parts of the [RenderBox] protocol (such as
/// painting the children).
///
/// The declaration of the `RenderFoo` class itself would thus look like this:
///
/// ```dart
/// class RenderFlex extends RenderBox with
///   ContainerRenderObjectMixin<RenderBox, FooParentData>,
///   RenderBoxContainerDefaultsMixin<RenderBox, FooParentData> {
///   // ...
/// }
/// ```
///
/// When walking the children (e.g. during layout), the following pattern is
/// commonly used (in this case assuming that the children are all [RenderBox]
/// objects and that this render object uses `FooParentData` objects for its
/// children's [parentData] fields):
///
/// ```dart
/// RenderBox child = firstChild;
/// while (child != null) {
///   final FooParentData childParentData = child.parentData;
///   // ...operate on child and childParentData...
///   assert(child.parentData == childParentData);
///   child = childParentData.nextSibling;
/// }
/// ```
///
/// #### More complicated child models
///
/// Render objects can have more complicated models, for example a map of
/// children keyed on an enum, or a 2D grid of efficiently randomly-accessible
/// children, or multiple lists of children, etc. If a render object has a model
/// that can't be handled by the mixins above, it must implement the
/// [RenderObject] child protocol, as follows:
///
/// * Any time a child is removed, call [dropChild] with the child.
///
/// * Any time a child is added, call [adoptChild] with the child.
///
/// * Implement the [attach] method such that it calls [attach] on each child.
///
/// * Implement the [detach] method such that it calls [detach] on each child.
///
/// * Implement the [redepthChildren] method such that it calls [redepthChild]
///   on each child.
///
/// * Implement the [visitChildren] method such that it calls its argument for
///   each child, typically in paint order (back-most to front-most).
///
/// * Implement [debugDescribeChildren] such that it outputs a [DiagnosticsNode]
///   for each child.
///
/// Implementing these seven bullet points is essentially all that the two
/// aforementioned mixins do.
///
/// ### Layout
///
/// [RenderBox] classes implement a layout algorithm. They have a set of
/// constraints provided to them, and they size themselves based on those
/// constraints and whatever other inputs they may have (for example, their
/// children or properties).
///
/// When implementing a [RenderBox] subclass, one must make a choice. Does it
/// size itself exclusively based on the constraints, or does it use any other
/// information in sizing itself? An example of sizing purely based on the
/// constraints would be growing to fit the parent.
///
/// Sizing purely based on the constraints allows the system to make some
/// significant optimizations. Classes that use this approach should override
/// [sizedByParent] to return true, and then override [performResize] to set the
/// [size] using nothing but the constraints, e.g.:
///
/// ```dart
/// @override
/// bool get sizedByParent => true;
///
/// @override
/// void performResize() {
///   size = constraints.smallest;
/// }
/// ```
///
/// Otherwise, the size is set in the [performLayout] function.
///
/// The [performLayout] function is where render boxes decide, if they are not
/// [sizedByParent], what [size] they should be, and also where they decide
/// where their children should be.
///
/// #### Layout of RenderBox children
///
/// The [performLayout] function should call the [layout] function of each (box)
/// child, passing it a [BoxConstraints] object describing the constraints
/// within which the child can render. Passing tight constraints (see
/// [BoxConstraints.isTight]) to the child will allow the rendering library to
/// apply some optimizations, as it knows that if the constraints are tight, the
/// child's dimensions cannot change even if the layout of the child itself
/// changes.
///
/// If the [performLayout] function will use the child's size to affect other
/// aspects of the layout, for example if the render box sizes itself around the
/// child, or positions several children based on the size of those children,
/// then it must specify the `parentUsesSize` argument to the child's [layout]
/// function, setting it to true.
///
/// This flag turns off some optimizations; algorithms that do not rely on the
/// children's sizes will be more efficient. (In particular, relying on the
/// child's [size] means that if the child is marked dirty for layout, the
/// parent will probably also be marked dirty for layout, unless the
/// [constraints] given by the parent to the child were tight constraints.)
///
/// For [RenderBox] classes that do not inherit from [RenderProxyBox], once they
/// have laid out their children, should also position them, by setting the
/// [BoxParentData.offset] field of each child's [parentData] object.
///
/// #### Layout of non-RenderBox children
///
/// The children of a [RenderBox] do not have to be [RenderBox]es themselves. If
/// they use another protocol (as discussed at [RenderObject]), then instead of
/// [BoxConstraints], the parent would pass in the appropriate [Constraints]
/// subclass, and instead of reading the child's size, the parent would read
/// whatever the output of [layout] is for that layout protocol. The
/// `parentUsesSize` flag is still used to indicate whether the parent is going
/// to read that output, and optimizations still kick in if the child has tight
/// constraints (as defined by [Constraints.isTight]).
///
/// ### Painting
///
/// To describe how a render box paints, implement the [paint] method. It is
/// given a [PaintingContext] object and an [Offset]. The painting context
/// provides methods to affect the layer tree as well as a
/// [PaintingContext.canvas] which can be used to add drawing commands. The
/// canvas object should not be cached across calls to the [PaintingContext]'s
/// methods; every time a method on [PaintingContext] is called, there is a
/// chance that the canvas will change identity. The offset specifies the
/// position of the top left corner of the box in the coordinate system of the
/// [PaintingContext.canvas].
///
/// To draw text on a canvas, use a [TextPainter].
///
/// To draw an image to a canvas, use the [paintImage] method.
///
/// A [RenderBox] that uses methods on [PaintingContext] that introduce new
/// layers should override the [alwaysNeedsCompositing] getter and set it to
/// true. If the object sometimes does and sometimes does not, it can have that
/// getter return true in some cases and false in others. In that case, whenever
/// the return value would change, call [markNeedsCompositingBitsUpdate]. (This
/// is done automatically when a child is added or removed, so you don't have to
/// call it explicitly if the [alwaysNeedsCompositing] getter only changes value
/// based on the presence or absence of children.)
///
/// Anytime anything changes on the object that would cause the [paint] method
/// to paint something different (but would not cause the layout to change),
/// the object should call [markNeedsPaint].
///
/// #### Painting children
///
/// The [paint] method's `context` argument has a [PaintingContext.paintChild]
/// method, which should be called for each child that is to be painted. It
/// should be given a reference to the child, and an [Offset] giving the
/// position of the child relative to the parent.
///
/// If the [paint] method applies a transform to the painting context before
/// painting children (or generally applies an additional offset beyond the
/// offset it was itself given as an argument), then the [applyPaintTransform]
/// method should also be overridden. That method must adjust the matrix that it
/// is given in the same manner as it transformed the painting context and
/// offset before painting the given child. This is used by the [globalToLocal]
/// and [localToGlobal] methods.
///
/// #### Hit Tests
///
/// Hit testing for render boxes is implemented by the [hitTest] method. The
/// default implementation of this method defers to [hitTestSelf] and
/// [hitTestChildren]. When implementing hit testing, you can either override
/// these latter two methods, or ignore them and just override [hitTest].
///
/// The [hitTest] method itself is given an [Offset], and must return true if the
/// object or one of its children has absorbed the hit (preventing objects below
/// this one from being hit), or false if the hit can continue to other objects
/// below this one.
///
/// For each child [RenderBox], the [hitTest] method on the child should be
/// called with the same [HitTestResult] argument and with the point transformed
/// into the child's coordinate space (in the same manner that the
/// [applyPaintTransform] method would). The default implementation defers to
/// [hitTestChildren] to call the children. [RenderBoxContainerDefaultsMixin]
/// provides a [RenderBoxContainerDefaultsMixin.defaultHitTestChildren] method
/// that does this assuming that the children are axis-aligned, not transformed,
/// and positioned according to the [BoxParentData.offset] field of the
/// [parentData]; more elaborate boxes can override [hitTestChildren]
/// accordingly.
///
/// If the object is hit, then it should also add itself to the [HitTestResult]
/// object that is given as an argument to the [hitTest] method, using
/// [HitTestResult.add]. The default implementation defers to [hitTestSelf] to
/// determine if the box is hit. If the object adds itself before the children
/// can add themselves, then it will be as if the object was above the children.
/// If it adds itself after the children, then it will be as if it was below the
/// children. Entries added to the [HitTestResult] object should use the
/// [BoxHitTestEntry] class. The entries are subsequently walked by the system
/// in the order they were added, and for each entry, the target's [handleEvent]
/// method is called, passing in the [HitTestEntry] object.
///
/// Hit testing cannot rely on painting having happened.
///
/// ### Semantics
///
/// For a render box to be accessible, implement the
/// [describeApproximatePaintClip] and [visitChildrenForSemantics] methods, and
/// the [semanticsAnnotator] getter. The default implementations are sufficient
/// for objects that only affect layout, but nodes that represent interactive
/// components or information (diagrams, text, images, etc) should provide more
/// complete implementations. For more information, see the documentation for
/// these members.
///
/// ### Intrinsics and Baselines
///
/// The layout, painting, hit testing, and semantics protocols are common to all
/// render objects. [RenderBox] objects must implement two additional protocols:
/// intrinsic sizing and baseline measurements.
///
/// There are four methods to implement for intrinsic sizing, to compute the
/// minimum and maximum intrinsic width and height of the box. The documentation
/// for these methods discusses the protocol in detail:
/// [computeMinIntrinsicWidth], [computeMaxIntrinsicWidth],
/// [computeMinIntrinsicHeight], [computeMaxIntrinsicHeight].
///
/// In addition, if the box has any children, it must implement
/// [computeDistanceToActualBaseline]. [RenderProxyBox] provides a simple
/// implementation that forwards to the child; [RenderShiftedBox] provides an
/// implementation that offsets the child's baseline information by the position
/// of the child relative to the parent. If you do not inherited from either of
/// these classes, however, you must implement the algorithm yourself.
abstract class RenderBox extends RenderObject {
  @override
  void setupParentData(covariant RenderObject child) {
    if (child.parentData is! BoxParentData)
      child.parentData = new BoxParentData();
  }

  Map<_IntrinsicDimensionsCacheEntry, double> _cachedIntrinsicDimensions;

  double _computeIntrinsicDimension(_IntrinsicDimension dimension, double argument, double computer(double argument)) {
    assert(RenderObject.debugCheckingIntrinsics || !debugDoingThisResize); // performResize should not depend on anything except the incoming constraints
    bool shouldCache = true;
    assert(() {
      // we don't want the checked-mode intrinsic tests to affect
      // who gets marked dirty, etc.
      if (RenderObject.debugCheckingIntrinsics)
        shouldCache = false;
      return true;
    }());
    if (shouldCache) {
      _cachedIntrinsicDimensions ??= <_IntrinsicDimensionsCacheEntry, double>{};
      return _cachedIntrinsicDimensions.putIfAbsent(
        new _IntrinsicDimensionsCacheEntry(dimension, argument),
        () => computer(argument)
      );
    }
    return computer(argument);
  }

  /// Returns the minimum width that this box could be without failing to
  /// correctly paint its contents within itself, without clipping.
  ///
  /// The height argument may give a specific height to assume. The given height
  /// can be infinite, meaning that the intrinsic width in an unconstrained
  /// environment is being requested. The given height should never be negative
  /// or null.
  ///
  /// This function should only be called on one's children. Calling this
  /// function couples the child with the parent so that when the child's layout
  /// changes, the parent is notified (via [markNeedsLayout]).
  ///
  /// Calling this function is expensive and as it can result in O(N^2)
  /// behavior.
  ///
  /// Do not override this method. Instead, implement [computeMinIntrinsicWidth].
  @mustCallSuper
  double getMinIntrinsicWidth(double height) {
    assert(() {
      if (height == null) {
        throw new FlutterError(
          'The height argument to getMinIntrinsicWidth was null.\n'
          'The argument to getMinIntrinsicWidth must not be negative or null. '
          'If you do not have a specific height in mind, then pass double.infinity instead.'
        );
      }
      if (height < 0.0) {
        throw new FlutterError(
          'The height argument to getMinIntrinsicWidth was negative.\n'
          'The argument to getMinIntrinsicWidth must not be negative or null. '
          'If you perform computations on another height before passing it to '
          'getMinIntrinsicWidth, consider using math.max() or double.clamp() '
          'to force the value into the valid range.'
        );
      }
      return true;
    }());
    return _computeIntrinsicDimension(_IntrinsicDimension.minWidth, height, computeMinIntrinsicWidth);
  }

  /// Computes the value returned by [getMinIntrinsicWidth]. Do not call this
  /// function directly, instead, call [getMinIntrinsicWidth].
  ///
  /// Override in subclasses that implement [performLayout]. This method should
  /// return the minimum width that this box could be without failing to
  /// correctly paint its contents within itself, without clipping.
  ///
  /// If the layout algorithm is independent of the context (e.g. it always
  /// tries to be a particular size), or if the layout algorithm is
  /// width-in-height-out, or if the layout algorithm uses both the incoming
  /// width and height constraints (e.g. it always sizes itself to
  /// [BoxConstraints.biggest]), then the `height` argument should be ignored.
  ///
  /// If the layout algorithm is strictly height-in-width-out, or is
  /// height-in-width-out when the width is unconstrained, then the height
  /// argument is the height to use.
  ///
  /// The `height` argument will never be negative or null. It may be infinite.
  ///
  /// If this algorithm depends on the intrinsic dimensions of a child, the
  /// intrinsic dimensions of that child should be obtained using the functions
  /// whose names start with `get`, not `compute`.
  ///
  /// This function should never return a negative or infinite value.
  ///
  /// ## Examples
  ///
  /// ### Text
  ///
  /// Text is the canonical example of a width-in-height-out algorithm. The
  /// `height` argument is therefore ignored.
  ///
  /// Consider the string "Hello World" The _maximum_ intrinsic width (as
  /// returned from [computeMaxIntrinsicWidth]) would be the width of the string
  /// with no line breaks.
  ///
  /// The minimum intrinsic width would be the width of the widest word, "Hello"
  /// or "World". If the text is rendered in an even narrower width, however, it
  /// might still not overflow. For example, maybe the rendering would put a
  /// line-break half-way through the words, as in "Hel⁞lo⁞Wor⁞ld". However,
  /// this wouldn't be a _correct_ rendering, and [computeMinIntrinsicWidth] is
  /// supposed to render the minimum width that the box could be without failing
  /// to _correctly_ paint the contents within itself.
  ///
  /// The minimum intrinsic _height_ for a given width smaller than the minimum
  /// intrinsic width could therefore be greater than the minimum intrinsic
  /// height for the minimum intrinsic width.
  ///
  /// ### Viewports (e.g. scrolling lists)
  ///
  /// Some render boxes are intended to clip their children. For example, the
  /// render box for a scrolling list might always size itself to its parents'
  /// size (or rather, to the maximum incoming constraints), regardless of the
  /// children's sizes, and then clip the children and position them based on
  /// the current scroll offset.
  ///
  /// The intrinsic dimensions in these cases still depend on the children, even
  /// though the layout algorithm sizes the box in a way independent of the
  /// children. It is the size that is needed to paint the box's contents (in
  /// this case, the children) _without clipping_ that matters.
  ///
  /// ### When the intrinsic dimensions cannot be known
  ///
  /// There are cases where render objects do not have an efficient way to
  /// compute their intrinsic dimensions. For example, it may be prohibitively
  /// expensive to reify and measure every child of a lazy viewport (viewports
  /// generally only instantiate the actually visible children), or the
  /// dimensions may be computed by a callback about which the render object
  /// cannot reason.
  ///
  /// In such cases, it may be impossible (or at least impractical) to actually
  /// return a valid answer. In such cases, the intrinsic functions should throw
  /// when [RenderObject.debugCheckingIntrinsics] is false and asserts are
  /// enabled, and return 0.0 otherwise.
  ///
  /// See the implementations of [LayoutBuilder] or [RenderViewportBase] for
  /// examples (in particular,
  /// [RenderViewportBase.debugThrowIfNotCheckingIntrinsics]).
  ///
  /// ### Aspect-ratio-driven boxes
  ///
  /// Some boxes always return a fixed size based on the constraints. For these
  /// boxes, the intrinsic functions should return the appropriate size when the
  /// incoming `height` or `width` argument is finite, treating that as a tight
  /// constraint in the respective direction and treating the other direction's
  /// constraints as unbounded. This is because the definitions of
  /// [computeMinIntrinsicWidth] and [computeMinIntrinsicHeight] are in terms of
  /// what the dimensions _could be_, and such boxes can only be one size in
  /// such cases.
  ///
  /// When the incoming argument is not finite, then they should return the
  /// actual intrinsic dimensions based on the contents, as any other box would.
  @protected
  double computeMinIntrinsicWidth(double height) {
    return 0.0;
  }

  /// Returns the smallest width beyond which increasing the width never
  /// decreases the preferred height. The preferred height is the value that
  /// would be returned by [getMinIntrinsicHeight] for that width.
  ///
  /// The height argument may give a specific height to assume. The given height
  /// can be infinite, meaning that the intrinsic width in an unconstrained
  /// environment is being requested. The given height should never be negative
  /// or null.
  ///
  /// This function should only be called on one's children. Calling this
  /// function couples the child with the parent so that when the child's layout
  /// changes, the parent is notified (via [markNeedsLayout]).
  ///
  /// Calling this function is expensive and as it can result in O(N^2)
  /// behavior.
  ///
  /// Do not override this method. Instead, implement
  /// [computeMaxIntrinsicWidth].
  @mustCallSuper
  double getMaxIntrinsicWidth(double height) {
    assert(() {
      if (height == null) {
        throw new FlutterError(
          'The height argument to getMaxIntrinsicWidth was null.\n'
          'The argument to getMaxIntrinsicWidth must not be negative or null. '
          'If you do not have a specific height in mind, then pass double.infinity instead.'
        );
      }
      if (height < 0.0) {
        throw new FlutterError(
          'The height argument to getMaxIntrinsicWidth was negative.\n'
          'The argument to getMaxIntrinsicWidth must not be negative or null. '
          'If you perform computations on another height before passing it to '
          'getMaxIntrinsicWidth, consider using math.max() or double.clamp() '
          'to force the value into the valid range.'
        );
      }
      return true;
    }());
    return _computeIntrinsicDimension(_IntrinsicDimension.maxWidth, height, computeMaxIntrinsicWidth);
  }

  /// Computes the value returned by [getMaxIntrinsicWidth]. Do not call this
  /// function directly, instead, call [getMaxIntrinsicWidth].
  ///
  /// Override in subclasses that implement [performLayout]. This should return
  /// the smallest width beyond which increasing the width never decreases the
  /// preferred height. The preferred height is the value that would be returned
  /// by [computeMinIntrinsicHeight] for that width.
  ///
  /// If the layout algorithm is strictly height-in-width-out, or is
  /// height-in-width-out when the width is unconstrained, then this should
  /// return the same value as [computeMinIntrinsicWidth] for the same height.
  ///
  /// Otherwise, the height argument should be ignored, and the returned value
  /// should be equal to or bigger than the value returned by
  /// [computeMinIntrinsicWidth].
  ///
  /// The `height` argument will never be negative or null. It may be infinite.
  ///
  /// The value returned by this method might not match the size that the object
  /// would actually take. For example, a [RenderBox] subclass that always
  /// exactly sizes itself using [BoxConstraints.biggest] might well size itself
  /// bigger than its max intrinsic size.
  ///
  /// If this algorithm depends on the intrinsic dimensions of a child, the
  /// intrinsic dimensions of that child should be obtained using the functions
  /// whose names start with `get`, not `compute`.
  ///
  /// This function should never return a negative or infinite value.
  ///
  /// See also examples in the definition of [computeMinIntrinsicWidth].
  @protected
  double computeMaxIntrinsicWidth(double height) {
    return 0.0;
  }

  /// Returns the minimum height that this box could be without failing to
  /// correctly paint its contents within itself, without clipping.
  ///
  /// The width argument may give a specific width to assume. The given width
  /// can be infinite, meaning that the intrinsic height in an unconstrained
  /// environment is being requested. The given width should never be negative
  /// or null.
  ///
  /// This function should only be called on one's children. Calling this
  /// function couples the child with the parent so that when the child's layout
  /// changes, the parent is notified (via [markNeedsLayout]).
  ///
  /// Calling this function is expensive and as it can result in O(N^2)
  /// behavior.
  ///
  /// Do not override this method. Instead, implement
  /// [computeMinIntrinsicHeight].
  @mustCallSuper
  double getMinIntrinsicHeight(double width) {
    assert(() {
      if (width == null) {
        throw new FlutterError(
          'The width argument to getMinIntrinsicHeight was null.\n'
          'The argument to getMinIntrinsicHeight must not be negative or null. '
          'If you do not have a specific width in mind, then pass double.infinity instead.'
        );
      }
      if (width < 0.0) {
        throw new FlutterError(
          'The width argument to getMinIntrinsicHeight was negative.\n'
          'The argument to getMinIntrinsicHeight must not be negative or null. '
          'If you perform computations on another width before passing it to '
          'getMinIntrinsicHeight, consider using math.max() or double.clamp() '
          'to force the value into the valid range.'
        );
      }
      return true;
    }());
    return _computeIntrinsicDimension(_IntrinsicDimension.minHeight, width, computeMinIntrinsicHeight);
  }

  /// Computes the value returned by [getMinIntrinsicHeight]. Do not call this
  /// function directly, instead, call [getMinIntrinsicHeight].
  ///
  /// Override in subclasses that implement [performLayout]. Should return the
  /// minimum height that this box could be without failing to correctly paint
  /// its contents within itself, without clipping.
  ///
  /// If the layout algorithm is independent of the context (e.g. it always
  /// tries to be a particular size), or if the layout algorithm is
  /// height-in-width-out, or if the layout algorithm uses both the incoming
  /// height and width constraints (e.g. it always sizes itself to
  /// [BoxConstraints.biggest]), then the `width` argument should be ignored.
  ///
  /// If the layout algorithm is strictly width-in-height-out, or is
  /// width-in-height-out when the height is unconstrained, then the width
  /// argument is the width to use.
  ///
  /// The `width` argument will never be negative or null. It may be infinite.
  ///
  /// If this algorithm depends on the intrinsic dimensions of a child, the
  /// intrinsic dimensions of that child should be obtained using the functions
  /// whose names start with `get`, not `compute`.
  ///
  /// This function should never return a negative or infinite value.
  ///
  /// See also examples in the definition of [computeMinIntrinsicWidth].
  @protected
  double computeMinIntrinsicHeight(double width) {
    return 0.0;
  }

  /// Returns the smallest height beyond which increasing the height never
  /// decreases the preferred width. The preferred width is the value that
  /// would be returned by [getMinIntrinsicWidth] for that height.
  ///
  /// The width argument may give a specific width to assume. The given width
  /// can be infinite, meaning that the intrinsic height in an unconstrained
  /// environment is being requested. The given width should never be negative
  /// or null.
  ///
  /// This function should only be called on one's children. Calling this
  /// function couples the child with the parent so that when the child's layout
  /// changes, the parent is notified (via [markNeedsLayout]).
  ///
  /// Calling this function is expensive and as it can result in O(N^2)
  /// behavior.
  ///
  /// Do not override this method. Instead, implement
  /// [computeMaxIntrinsicHeight].
  @mustCallSuper
  double getMaxIntrinsicHeight(double width) {
    assert(() {
      if (width == null) {
        throw new FlutterError(
          'The width argument to getMaxIntrinsicHeight was null.\n'
          'The argument to getMaxIntrinsicHeight must not be negative or null. '
          'If you do not have a specific width in mind, then pass double.infinity instead.'
        );
      }
      if (width < 0.0) {
        throw new FlutterError(
          'The width argument to getMaxIntrinsicHeight was negative.\n'
          'The argument to getMaxIntrinsicHeight must not be negative or null. '
          'If you perform computations on another width before passing it to '
          'getMaxIntrinsicHeight, consider using math.max() or double.clamp() '
          'to force the value into the valid range.'
        );
      }
      return true;
    }());
    return _computeIntrinsicDimension(_IntrinsicDimension.maxHeight, width, computeMaxIntrinsicHeight);
  }

  /// Computes the value returned by [getMaxIntrinsicHeight]. Do not call this
  /// function directly, instead, call [getMaxIntrinsicHeight].
  ///
  /// Override in subclasses that implement [performLayout]. Should return the
  /// smallest height beyond which increasing the height never decreases the
  /// preferred width. The preferred width is the value that would be returned
  /// by [computeMinIntrinsicWidth] for that height.
  ///
  /// If the layout algorithm is strictly width-in-height-out, or is
  /// width-in-height-out when the height is unconstrained, then this should
  /// return the same value as [computeMinIntrinsicHeight] for the same width.
  ///
  /// Otherwise, the width argument should be ignored, and the returned value
  /// should be equal to or bigger than the value returned by
  /// [computeMinIntrinsicHeight].
  ///
  /// The `width` argument will never be negative or null. It may be infinite.
  ///
  /// The value returned by this method might not match the size that the object
  /// would actually take. For example, a [RenderBox] subclass that always
  /// exactly sizes itself using [BoxConstraints.biggest] might well size itself
  /// bigger than its max intrinsic size.
  ///
  /// If this algorithm depends on the intrinsic dimensions of a child, the
  /// intrinsic dimensions of that child should be obtained using the functions
  /// whose names start with `get`, not `compute`.
  ///
  /// This function should never return a negative or infinite value.
  ///
  /// See also examples in the definition of [computeMinIntrinsicWidth].
  @protected
  double computeMaxIntrinsicHeight(double width) {
    return 0.0;
  }

  /// Whether this render object has undergone layout and has a [size].
  bool get hasSize => _size != null;

  /// The size of this render box computed during layout.
  ///
  /// This value is stale whenever this object is marked as needing layout.
  /// During [performLayout], do not read the size of a child unless you pass
  /// true for parentUsesSize when calling the child's [layout] function.
  ///
  /// The size of a box should be set only during the box's [performLayout] or
  /// [performResize] functions. If you wish to change the size of a box outside
  /// of those functions, call [markNeedsLayout] instead to schedule a layout of
  /// the box.
  Size get size {
    assert(hasSize, 'RenderBox was not laid out: ${toString()}');
    assert(() {
      if (_size is _DebugSize) {
        final _DebugSize _size = this._size;
        assert(_size._owner == this);
        if (RenderObject.debugActiveLayout != null) {
          // We are always allowed to access our own size (for print debugging
          // and asserts if nothing else). Other than us, the only object that's
          // allowed to read our size is our parent, if they've said they will.
          // If you hit this assert trying to access a child's size, pass
          // "parentUsesSize: true" to that child's layout().
          assert(debugDoingThisResize || debugDoingThisLayout ||
                 (RenderObject.debugActiveLayout == parent && _size._canBeUsedByParent));
        }
        assert(_size == this._size);
      }
      return true;
    }());
    return _size;
  }
  Size _size;
  @protected
  /// Setting the size, in checked mode, triggers some analysis of the render box,
  /// as implemented by [debugAssertDoesMeetConstraints], including calling the intrinsic
  /// sizing methods and checking that they meet certain invariants.
  set size(Size value) {
    assert(!(debugDoingThisResize && debugDoingThisLayout));
    assert(sizedByParent || !debugDoingThisResize);
    assert(() {
      if ((sizedByParent && debugDoingThisResize) ||
          (!sizedByParent && debugDoingThisLayout))
        return true;
      assert(!debugDoingThisResize);
      String contract, violation, hint;
      if (debugDoingThisLayout) {
        assert(sizedByParent);
        violation = 'It appears that the size setter was called from performLayout().';
        hint = '';
      } else {
        violation = 'The size setter was called from outside layout (neither performResize() nor performLayout() were being run for this object).';
        if (owner != null && owner.debugDoingLayout)
          hint = 'Only the object itself can set its size. It is a contract violation for other objects to set it.';
      }
      if (sizedByParent)
        contract = 'Because this RenderBox has sizedByParent set to true, it must set its size in performResize().';
      else
        contract = 'Because this RenderBox has sizedByParent set to false, it must set its size in performLayout().';
      throw new FlutterError(
        'RenderBox size setter called incorrectly.\n'
        '$violation\n'
        '$hint\n'
        '$contract\n'
        'The RenderBox in question is:\n'
        '  $this'
      );
    }());
    assert(() {
      value = debugAdoptSize(value);
      return true;
    }());
    _size = value;
    assert(() { debugAssertDoesMeetConstraints(); return true; }());
  }

  /// Claims ownership of the given [Size].
  ///
  /// In debug mode, the [RenderBox] class verifies that [Size] objects obtained
  /// from other [RenderBox] objects are only used according to the semantics of
  /// the [RenderBox] protocol, namely that a [Size] from a [RenderBox] can only
  /// be used by its parent, and then only if `parentUsesSize` was set.
  ///
  /// Sometimes, a [Size] that can validly be used ends up no longer being valid
  /// over time. The common example is a [Size] taken from a child that is later
  /// removed from the parent. In such cases, this method can be called to first
  /// check whether the size can legitimately be used, and if so, to then create
  /// a new [Size] that can be used going forward, regardless of what happens to
  /// the original owner.
  Size debugAdoptSize(Size value) {
    Size result = value;
    assert(() {
      if (value is _DebugSize) {
        if (value._owner != this) {
          if (value._owner.parent != this) {
            throw new FlutterError(
              'The size property was assigned a size inappropriately.\n'
              'The following render object:\n'
              '  $this\n'
              '...was assigned a size obtained from:\n'
              '  ${value._owner}\n'
              'However, this second render object is not, or is no longer, a '
              'child of the first, and it is therefore a violation of the '
              'RenderBox layout protocol to use that size in the layout of the '
              'first render object.\n'
              'If the size was obtained at a time where it was valid to read '
              'the size (because the second render object above was a child '
              'of the first at the time), then it should be adopted using '
              'debugAdoptSize at that time.\n'
              'If the size comes from a grandchild or a render object from an '
              'entirely different part of the render tree, then there is no '
              'way to be notified when the size changes and therefore attempts '
              'to read that size are almost certainly a source of bugs. A different '
              'approach should be used.'
            );
          }
          if (!value._canBeUsedByParent) {
            throw new FlutterError(
              'A child\'s size was used without setting parentUsesSize.\n'
              'The following render object:\n'
              '  $this\n'
              '...was assigned a size obtained from its child:\n'
              '  ${value._owner}\n'
              'However, when the child was laid out, the parentUsesSize argument '
              'was not set or set to false. Subsequently this transpired to be '
              'inaccurate: the size was nonetheless used by the parent.\n'
              'It is important to tell the framework if the size will be used or not '
              'as several important performance optimizations can be made if the '
              'size will not be used by the parent.'
            );
          }
        }
      }
      result = new _DebugSize(value, this, debugCanParentUseSize);
      return true;
    }());
    return result;
  }

  @override
  Rect get semanticBounds => Offset.zero & size;

  @override
  void debugResetSize() {
    // updates the value of size._canBeUsedByParent if necessary
    size = size;
  }

  Map<TextBaseline, double> _cachedBaselines;
  static bool _debugDoingBaseline = false;
  static bool _debugSetDoingBaseline(bool value) {
    _debugDoingBaseline = value;
    return true;
  }

  /// Returns the distance from the y-coordinate of the position of the box to
  /// the y-coordinate of the first given baseline in the box's contents.
  ///
  /// Used by certain layout models to align adjacent boxes on a common
  /// baseline, regardless of padding, font size differences, etc. If there is
  /// no baseline, this function returns the distance from the y-coordinate of
  /// the position of the box to the y-coordinate of the bottom of the box
  /// (i.e., the height of the box) unless the caller passes true
  /// for `onlyReal`, in which case the function returns null.
  ///
  /// Only call this function after calling [layout] on this box. You
  /// are only allowed to call this from the parent of this box during
  /// that parent's [performLayout] or [paint] functions.
  ///
  /// When implementing a [RenderBox] subclass, to override the baseline
  /// computation, override [computeDistanceToActualBaseline].
  double getDistanceToBaseline(TextBaseline baseline, { bool onlyReal = false }) {
    assert(!_debugDoingBaseline, 'Please see the documentation for computeDistanceToActualBaseline for the required calling conventions of this method.');
    assert(!debugNeedsLayout);
    assert(() {
      final RenderObject parent = this.parent;
      if (owner.debugDoingLayout)
        return (RenderObject.debugActiveLayout == parent) && parent.debugDoingThisLayout;
      if (owner.debugDoingPaint)
        return ((RenderObject.debugActivePaint == parent) && parent.debugDoingThisPaint) ||
               ((RenderObject.debugActivePaint == this) && debugDoingThisPaint);
      assert(parent == this.parent);
      return false;
    }());
    assert(_debugSetDoingBaseline(true));
    final double result = getDistanceToActualBaseline(baseline);
    assert(_debugSetDoingBaseline(false));
    if (result == null && !onlyReal)
      return size.height;
    return result;
  }

  /// Calls [computeDistanceToActualBaseline] and caches the result.
  ///
  /// This function must only be called from [getDistanceToBaseline] and
  /// [computeDistanceToActualBaseline]. Do not call this function directly from
  /// outside those two methods.
  @protected
  @mustCallSuper
  double getDistanceToActualBaseline(TextBaseline baseline) {
    assert(_debugDoingBaseline, 'Please see the documentation for computeDistanceToActualBaseline for the required calling conventions of this method.');
    _cachedBaselines ??= <TextBaseline, double>{};
    _cachedBaselines.putIfAbsent(baseline, () => computeDistanceToActualBaseline(baseline));
    return _cachedBaselines[baseline];
  }

  /// Returns the distance from the y-coordinate of the position of the box to
  /// the y-coordinate of the first given baseline in the box's contents, if
  /// any, or null otherwise.
  ///
  /// Do not call this function directly. If you need to know the baseline of a
  /// child from an invocation of [performLayout] or [paint], call
  /// [getDistanceToBaseline].
  ///
  /// Subclasses should override this method to supply the distances to their
  /// baselines. When implementing this method, there are generally three
  /// strategies:
  ///
  ///  * For classes that use the [ContainerRenderObjectMixin] child model,
  ///    consider mixing in the [RenderBoxContainerDefaultsMixin] class and
  ///    using
  ///    [RenderBoxContainerDefaultsMixin.defaultComputeDistanceToFirstActualBaseline].
  ///
  ///  * For classes that define a particular baseline themselves, return that
  ///    value directly.
  ///
  ///  * For classes that have a child to which they wish to defer the
  ///    computation, call [getDistanceToActualBaseline] on the child (not
  ///    [computeDistanceToActualBaseline], the internal implementation, and not
  ///    [getDistanceToBaseline], the public entry point for this API).
  @protected
  double computeDistanceToActualBaseline(TextBaseline baseline) {
    assert(_debugDoingBaseline, 'Please see the documentation for computeDistanceToActualBaseline for the required calling conventions of this method.');
    return null;
  }

  /// The box constraints most recently received from the parent.
  @override
  BoxConstraints get constraints => super.constraints;

  @override
  void debugAssertDoesMeetConstraints() {
    assert(constraints != null);
    assert(() {
      if (!hasSize) {
        assert(!debugNeedsLayout); // this is called in the size= setter during layout, but in that case we have a size
        String contract;
        if (sizedByParent)
          contract = 'Because this RenderBox has sizedByParent set to true, it must set its size in performResize().\n';
        else
          contract = 'Because this RenderBox has sizedByParent set to false, it must set its size in performLayout().\n';
        throw new FlutterError(
          'RenderBox did not set its size during layout.\n'
          '$contract'
          'It appears that this did not happen; layout completed, but the size property is still null.\n'
          'The RenderBox in question is:\n'
          '  $this'
        );
      }
      // verify that the size is not infinite
      if (!_size.isFinite) {
        final StringBuffer information = new StringBuffer();
        if (!constraints.hasBoundedWidth) {
          RenderBox node = this;
          while (!node.constraints.hasBoundedWidth && node.parent is RenderBox)
            node = node.parent;
          information.writeln('The nearest ancestor providing an unbounded width constraint is:');
          information.write('  ');
          information.writeln(node.toStringShallow(joiner: '\n  '));
         }
        if (!constraints.hasBoundedHeight) {
          RenderBox node = this;
          while (!node.constraints.hasBoundedHeight && node.parent is RenderBox)
            node = node.parent;
          information.writeln('The nearest ancestor providing an unbounded height constraint is:');
          information.write('  ');
          information.writeln(node.toStringShallow(joiner: '\n  '));

        }
        throw new FlutterError(
          '$runtimeType object was given an infinite size during layout.\n'
          'This probably means that it is a render object that tries to be '
          'as big as possible, but it was put inside another render object '
          'that allows its children to pick their own size.\n'
          '$information'
          'The constraints that applied to the $runtimeType were:\n'
          '  $constraints\n'
          'The exact size it was given was:\n'
          '  $_size\n'
          'See https://flutter.io/layout/ for more information.'
        );
      }
      // verify that the size is within the constraints
      if (!constraints.isSatisfiedBy(_size)) {
        throw new FlutterError(
          '$runtimeType does not meet its constraints.\n'
          'Constraints: $constraints\n'
          'Size: $_size\n'
          'If you are not writing your own RenderBox subclass, then this is not '
          'your fault. Contact support: https://github.com/flutter/flutter/issues/new'
        );
      }
      if (debugCheckIntrinsicSizes) {
        // verify that the intrinsics are sane
        assert(!RenderObject.debugCheckingIntrinsics);
        RenderObject.debugCheckingIntrinsics = true;
        final StringBuffer failures = new StringBuffer();
        int failureCount = 0;

        double testIntrinsic(double function(double extent), String name, double constraint) {
          final double result = function(constraint);
          if (result < 0) {
            failures.writeln(' * $name($constraint) returned a negative value: $result');
            failureCount += 1;
          }
          if (!result.isFinite) {
            failures.writeln(' * $name($constraint) returned a non-finite value: $result');
            failureCount += 1;
          }
          return result;
        }

        void testIntrinsicsForValues(double getMin(double extent), double getMax(double extent), String name, double constraint) {
          final double min = testIntrinsic(getMin, 'getMinIntrinsic$name', constraint);
          final double max = testIntrinsic(getMax, 'getMaxIntrinsic$name', constraint);
          if (min > max) {
            failures.writeln(' * getMinIntrinsic$name($constraint) returned a larger value ($min) than getMaxIntrinsic$name($constraint) ($max)');
            failureCount += 1;
          }
        }

        testIntrinsicsForValues(getMinIntrinsicWidth, getMaxIntrinsicWidth, 'Width', double.infinity);
        testIntrinsicsForValues(getMinIntrinsicHeight, getMaxIntrinsicHeight, 'Height', double.infinity);
        if (constraints.hasBoundedWidth)
          testIntrinsicsForValues(getMinIntrinsicWidth, getMaxIntrinsicWidth, 'Width', constraints.maxWidth);
        if (constraints.hasBoundedHeight)
          testIntrinsicsForValues(getMinIntrinsicHeight, getMaxIntrinsicHeight, 'Height', constraints.maxHeight);

        // TODO(ianh): Test that values are internally consistent in more ways than the above.

        RenderObject.debugCheckingIntrinsics = false;
        if (failures.isNotEmpty) {
          assert(failureCount > 0);
          throw new FlutterError(
            'The intrinsic dimension methods of the $runtimeType class returned values that violate the intrinsic protocol contract.\n'
            'The following ${failureCount > 1 ? "failures" : "failure"} was detected:\n'
            '$failures'
            'If you are not writing your own RenderBox subclass, then this is not\n'
            'your fault. Contact support: https://github.com/flutter/flutter/issues/new'
          );
        }
      }
      return true;
    }());
  }

  @override
  void markNeedsLayout() {
    if ((_cachedBaselines != null && _cachedBaselines.isNotEmpty) ||
        (_cachedIntrinsicDimensions != null && _cachedIntrinsicDimensions.isNotEmpty)) {
      // If we have cached data, then someone must have used our data.
      // Since the parent will shortly be marked dirty, we can forget that they
      // used the baseline and/or intrinsic dimensions. If they use them again,
      // then we'll fill the cache again, and if we get dirty again, we'll
      // notify them again.
      _cachedBaselines?.clear();
      _cachedIntrinsicDimensions?.clear();
       if (parent is RenderObject) {
        markParentNeedsLayout();
        return;
      }
    }
    super.markNeedsLayout();
  }

  @override
  void performResize() {
    // default behavior for subclasses that have sizedByParent = true
    size = constraints.smallest;
    assert(size.isFinite);
  }

  @override
  void performLayout() {
    assert(() {
      if (!sizedByParent) {
        throw new FlutterError(
          '$runtimeType did not implement performLayout().\n'
          'RenderBox subclasses need to either override performLayout() to '
          'set a size and lay out any children, or, set sizedByParent to true '
          'so that performResize() sizes the render object.'
        );
      }
      return true;
    }());
  }

  /// Determines the set of render objects located at the given position.
  ///
  /// Returns true, and adds any render objects that contain the point to the
  /// given hit test result, if this render object or one of its descendants
  /// absorbs the hit (preventing objects below this one from being hit).
  /// Returns false if the hit can continue to other objects below this one.
  ///
  /// The caller is responsible for transforming [position] from global
  /// coordinates to its location relative to the origin of this [RenderBox].
  /// This [RenderBox] is responsible for checking whether the given position is
  /// within its bounds.
  ///
  /// Hit testing requires layout to be up-to-date but does not require painting
  /// to be up-to-date. That means a render object can rely upon [performLayout]
  /// having been called in [hitTest] but cannot rely upon [paint] having been
  /// called. For example, a render object might be a child of a [RenderOpacity]
  /// object, which calls [hitTest] on its children when its opacity is zero
  /// even through it does not [paint] its children.
  bool hitTest(HitTestResult result, { @required Offset position }) {
    assert(() {
      if (!hasSize) {
        if (debugNeedsLayout) {
          throw new FlutterError(
            'Cannot hit test a render box that has never been laid out.\n'
            'The hitTest() method was called on this RenderBox:\n'
            '  $this\n'
            'Unfortunately, this object\'s geometry is not known at this time, '
            'probably because it has never been laid out. '
            'This means it cannot be accurately hit-tested. If you are trying '
            'to perform a hit test during the layout phase itself, make sure '
            'you only hit test nodes that have completed layout (e.g. the node\'s '
            'children, after their layout() method has been called).'
          );
        }
        throw new FlutterError(
          'Cannot hit test a render box with no size.\n'
          'The hitTest() method was called on this RenderBox:\n'
          '  $this\n'
          'Although this node is not marked as needing layout, '
          'its size is not set. A RenderBox object must have an '
          'explicit size before it can be hit-tested. Make sure '
          'that the RenderBox in question sets its size during layout.'
        );
      }
      return true;
    }());
    if (_size.contains(position)) {
      if (hitTestChildren(result, position: position) || hitTestSelf(position)) {
        result.add(new BoxHitTestEntry(this, position));
        return true;
      }
    }
    return false;
  }

  /// Override this method if this render object can be hit even if its
  /// children were not hit.
  ///
  /// The caller is responsible for transforming [position] from global
  /// coordinates to its location relative to the origin of this [RenderBox].
  /// This [RenderBox] is responsible for checking whether the given position is
  /// within its bounds.
  ///
  /// Used by [hitTest]. If you override [hitTest] and do not call this
  /// function, then you don't need to implement this function.
  @protected
  bool hitTestSelf(Offset position) => false;

  /// Override this method to check whether any children are located at the
  /// given position.
  ///
  /// Typically children should be hit-tested in reverse paint order so that
  /// hit tests at locations where children overlap hit the child that is
  /// visually "on top" (i.e., paints later).
  ///
  /// The caller is responsible for transforming [position] from global
  /// coordinates to its location relative to the origin of this [RenderBox].
  /// This [RenderBox] is responsible for checking whether the given position is
  /// within its bounds.
  ///
  /// Used by [hitTest]. If you override [hitTest] and do not call this
  /// function, then you don't need to implement this function.
  @protected
  bool hitTestChildren(HitTestResult result, { Offset position }) => false;

  /// Multiply the transform from the parent's coordinate system to this box's
  /// coordinate system into the given transform.
  ///
  /// This function is used to convert coordinate systems between boxes.
  /// Subclasses that apply transforms during painting should override this
  /// function to factor those transforms into the calculation.
  ///
  /// The [RenderBox] implementation takes care of adjusting the matrix for the
  /// position of the given child as determined during layout and stored on the
  /// child's [parentData] in the [BoxParentData.offset] field.
  @override
  void applyPaintTransform(RenderObject child, Matrix4 transform) {
    assert(child != null);
    assert(child.parent == this);
    assert(() {
      if (child.parentData is! BoxParentData) {
        throw new FlutterError(
          '$runtimeType does not implement applyPaintTransform.\n'
          'The following $runtimeType object:\n'
          '  ${toStringShallow()}\n'
          '...did not use a BoxParentData class for the parentData field of the following child:\n'
          '  ${child.toStringShallow()}\n'
          'The $runtimeType class inherits from RenderBox. '
          'The default applyPaintTransform implementation provided by RenderBox assumes that the '
          'children all use BoxParentData objects for their parentData field. '
          'Since $runtimeType does not in fact use that ParentData class for its children, it must '
          'provide an implementation of applyPaintTransform that supports the specific ParentData '
          'subclass used by its children (which apparently is ${child.parentData.runtimeType}).'
        );
      }
      return true;
    }());
    final BoxParentData childParentData = child.parentData;
    final Offset offset = childParentData.offset;
    transform.translate(offset.dx, offset.dy);
  }

  /// Convert the given point from the global coordinate system in logical pixels
  /// to the local coordinate system for this box.
  ///
  /// If the transform from global coordinates to local coordinates is
  /// degenerate, this function returns [Offset.zero].
  ///
  /// If `ancestor` is non-null, this function converts the given point from the
  /// coordinate system of `ancestor` (which must be an ancestor of this render
  /// object) instead of from the global coordinate system.
  ///
  /// This method is implemented in terms of [getTransformTo].
  Offset globalToLocal(Offset point, { RenderObject ancestor }) {
    final Matrix4 transform = getTransformTo(ancestor);
    final double det = transform.invert();
    if (det == 0.0)
      return Offset.zero;
    return MatrixUtils.transformPoint(transform, point);
  }

  /// Convert the given point from the local coordinate system for this box to
  /// the global coordinate system in logical pixels.
  ///
  /// If `ancestor` is non-null, this function converts the given point to the
  /// coordinate system of `ancestor` (which must be an ancestor of this render
  /// object) instead of to the global coordinate system.
  ///
  /// This method is implemented in terms of [getTransformTo].
  Offset localToGlobal(Offset point, { RenderObject ancestor }) {
    return MatrixUtils.transformPoint(getTransformTo(ancestor), point);
  }

  /// Returns a rectangle that contains all the pixels painted by this box.
  ///
  /// The paint bounds can be larger or smaller than [size], which is the amount
  /// of space this box takes up during layout. For example, if this box casts a
  /// shadow, that shadow might extend beyond the space allocated to this box
  /// during layout.
  ///
  /// The paint bounds are used to size the buffers into which this box paints.
  /// If the box attempts to paints outside its paint bounds, there might not be
  /// enough memory allocated to represent the box's visual appearance, which
  /// can lead to undefined behavior.
  ///
  /// The returned paint bounds are in the local coordinate system of this box.
  @override
  Rect get paintBounds => Offset.zero & size;

  /// Override this method to handle pointer events that hit this render object.
  ///
  /// For [RenderBox] objects, the `entry` argument is a [BoxHitTestEntry]. From this
  /// object you can determine the [PointerDownEvent]'s position in local coordinates.
  /// (This is useful because [PointerEvent.position] is in global coordinates.)
  ///
  /// If you override this, consider calling [debugHandleEvent] as follows, so
  /// that you can support [debugPaintPointersEnabled]:
  ///
  /// ```dart
  /// @override
  /// void handleEvent(PointerEvent event, HitTestEntry entry) {
  ///   assert(debugHandleEvent(event, entry));
  ///   // ... handle the event ...
  /// }
  /// ```
  // TODO(ianh): Fix the type of the argument here once https://github.com/dart-lang/sdk/issues/25232 is fixed
  @override
  void handleEvent(PointerEvent event, covariant HitTestEntry entry) {
    super.handleEvent(event, entry);
  }

  int _debugActivePointers = 0;

  /// Implements the [debugPaintPointersEnabled] debugging feature.
  ///
  /// [RenderBox] subclasses that implement [handleEvent] should call
  /// [debugHandleEvent] from their [handleEvent] method, as follows:
  ///
  /// ```dart
  /// @override
  /// void handleEvent(PointerEvent event, HitTestEntry entry) {
  ///   assert(debugHandleEvent(event, entry));
  ///   // ... handle the event ...
  /// }
  /// ```
  ///
  /// If you call this for a [PointerDownEvent], make sure you also call it for
  /// the corresponding [PointerUpEvent] or [PointerCancelEvent].
  bool debugHandleEvent(PointerEvent event, HitTestEntry entry) {
    assert(() {
      if (debugPaintPointersEnabled) {
        if (event is PointerDownEvent) {
          _debugActivePointers += 1;
        } else if (event is PointerUpEvent || event is PointerCancelEvent) {
          _debugActivePointers -= 1;
        }
        markNeedsPaint();
      }
      return true;
    }());
    return true;
  }

  @override
  void debugPaint(PaintingContext context, Offset offset) {
    assert(() {
      if (debugPaintSizeEnabled)
        debugPaintSize(context, offset);
      if (debugPaintBaselinesEnabled)
        debugPaintBaselines(context, offset);
      if (debugPaintPointersEnabled)
        debugPaintPointers(context, offset);
      return true;
    }());
  }

  /// In debug mode, paints a border around this render box.
  ///
  /// Called for every [RenderBox] when [debugPaintSizeEnabled] is true.
  @protected
  void debugPaintSize(PaintingContext context, Offset offset) {
    assert(() {
      final Paint paint = new Paint()
       ..style = PaintingStyle.stroke
       ..strokeWidth = 1.0
       ..color = const Color(0xFF00FFFF);
      context.canvas.drawRect((offset & size).deflate(0.5), paint);
      return true;
    }());
  }

  /// In debug mode, paints a line for each baseline.
  ///
  /// Called for every [RenderBox] when [debugPaintBaselinesEnabled] is true.
  @protected
  void debugPaintBaselines(PaintingContext context, Offset offset) {
    assert(() {
      final Paint paint = new Paint()
       ..style = PaintingStyle.stroke
       ..strokeWidth = 0.25;
      Path path;
      // ideographic baseline
      final double baselineI = getDistanceToBaseline(TextBaseline.ideographic, onlyReal: true);
      if (baselineI != null) {
        paint.color = const Color(0xFFFFD000);
        path = new Path();
        path.moveTo(offset.dx, offset.dy + baselineI);
        path.lineTo(offset.dx + size.width, offset.dy + baselineI);
        context.canvas.drawPath(path, paint);
      }
      // alphabetic baseline
      final double baselineA = getDistanceToBaseline(TextBaseline.alphabetic, onlyReal: true);
      if (baselineA != null) {
        paint.color = const Color(0xFF00FF00);
        path = new Path();
        path.moveTo(offset.dx, offset.dy + baselineA);
        path.lineTo(offset.dx + size.width, offset.dy + baselineA);
        context.canvas.drawPath(path, paint);
      }
      return true;
    }());
  }

  /// In debug mode, paints a rectangle if this render box has counted more
  /// pointer downs than pointer up events.
  ///
  /// Called for every [RenderBox] when [debugPaintPointersEnabled] is true.
  ///
  /// By default, events are not counted. For details on how to ensure that
  /// events are counted for your class, see [debugHandleEvent].
  @protected
  void debugPaintPointers(PaintingContext context, Offset offset) {
    assert(() {
      if (_debugActivePointers > 0) {
        final Paint paint = new Paint()
         ..color = new Color(0x00BBBB | ((0x04000000 * depth) & 0xFF000000));
        context.canvas.drawRect(offset & size, paint);
      }
      return true;
    }());
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DiagnosticsProperty<Size>('size', _size, missingIfNull: true));
  }
}

/// A mixin that provides useful default behaviors for boxes with children
/// managed by the [ContainerRenderObjectMixin] mixin.
///
/// By convention, this class doesn't override any members of the superclass.
/// Instead, it provides helpful functions that subclasses can call as
/// appropriate.
abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, ParentDataType extends ContainerBoxParentData<ChildType>> implements ContainerRenderObjectMixin<ChildType, ParentDataType> {
  // This class is intended to be used as a mixin, and should not be
  // extended directly.
  factory RenderBoxContainerDefaultsMixin._() => null;

  /// Returns the baseline of the first child with a baseline.
  ///
  /// Useful when the children are displayed vertically in the same order they
  /// appear in the child list.
  double defaultComputeDistanceToFirstActualBaseline(TextBaseline baseline) {
    assert(!debugNeedsLayout);
    ChildType child = firstChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      final double result = child.getDistanceToActualBaseline(baseline);
      if (result != null)
        return result + childParentData.offset.dy;
      child = childParentData.nextSibling;
    }
    return null;
  }

  /// Returns the minimum baseline value among every child.
  ///
  /// Useful when the vertical position of the children isn't determined by the
  /// order in the child list.
  double defaultComputeDistanceToHighestActualBaseline(TextBaseline baseline) {
    assert(!debugNeedsLayout);
    double result;
    ChildType child = firstChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      double candidate = child.getDistanceToActualBaseline(baseline);
      if (candidate != null) {
        candidate += childParentData.offset.dy;
        if (result != null)
          result = math.min(result, candidate);
        else
          result = candidate;
      }
      child = childParentData.nextSibling;
    }
    return result;
  }

  /// Performs a hit test on each child by walking the child list backwards.
  ///
  /// Stops walking once after the first child reports that it contains the
  /// given point. Returns whether any children contain the given point.
  ///
  /// See also:
  ///
  ///  * [defaultPaint], which paints the children appropriate for this
  ///    hit-testing strategy.
  bool defaultHitTestChildren(HitTestResult result, { Offset position }) {
    // the x, y parameters have the top left of the node's box as the origin
    ChildType child = lastChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      if (child.hitTest(result, position: position - childParentData.offset))
        return true;
      child = childParentData.previousSibling;
    }
    return false;
  }

  /// Paints each child by walking the child list forwards.
  ///
  /// See also:
  ///
  ///  * [defaultHitTestChildren], which implements hit-testing of the children
  ///    in a manner appropriate for this painting strategy.
  void defaultPaint(PaintingContext context, Offset offset) {
    ChildType child = firstChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      context.paintChild(child, childParentData.offset + offset);
      child = childParentData.nextSibling;
    }
  }

  /// Returns a list containing the children of this render object.
  ///
  /// This function is useful when you need random-access to the children of
  /// this render object. If you're accessing the children in order, consider
  /// walking the child list directly.
  List<ChildType> getChildrenAsList() {
    final List<ChildType> result = <ChildType>[];
    RenderBox child = firstChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      result.add(child);
      child = childParentData.nextSibling;
    }
    return result;
  }
}