packages/flutter/lib/src/material/floating_action_button_location.dart - mirrors/flutter - Git at Google

 // Copyright 2014 The Flutter 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 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';

 import 'scaffold.dart';

 // TODO(hmuller): should be device dependent.
 /// The margin that a [FloatingActionButton] should leave between it and the
 /// edge of the screen.
 ///
 /// [FloatingActionButtonLocation.endFloat] uses this to set the appropriate margin
 /// between the [FloatingActionButton] and the end of the screen.
 const double kFloatingActionButtonMargin = 16.0;

 /// The amount of time the [FloatingActionButton] takes to transition in or out.
 ///
 /// The [Scaffold] uses this to set the duration of [FloatingActionButton]
 /// motion, entrance, and exit animations.
 const Duration kFloatingActionButtonSegue = Duration(milliseconds: 200);

 /// The fraction of a circle the [FloatingActionButton] should turn when it enters.
 ///
 /// Its value corresponds to 0.125 of a full circle, equivalent to 45 degrees or pi/4 radians.
 const double kFloatingActionButtonTurnInterval = 0.125;

 /// An object that defines a position for the [FloatingActionButton]
 /// based on the [Scaffold]'s [ScaffoldPrelayoutGeometry].
 ///
 /// Flutter provides [FloatingActionButtonLocation]s for the common
 /// [FloatingActionButton] placements in Material Design applications. These
 /// locations are available as static members of this class.
 ///
 /// See also:
 ///
 ///  * [FloatingActionButton], which is a circular button typically shown in the
 ///    bottom right corner of the app.
 ///  * [FloatingActionButtonAnimator], which is used to animate the
 ///    [Scaffold.floatingActionButton] from one [FloatingActionButtonLocation] to
 ///    another.
 ///  * [ScaffoldPrelayoutGeometry], the geometry that
 ///    [FloatingActionButtonLocation]s use to position the [FloatingActionButton].
 abstract class FloatingActionButtonLocation {
   /// Abstract const constructor. This constructor enables subclasses to provide
   /// const constructors so that they can be used in const expressions.
   const FloatingActionButtonLocation();

   /// End-aligned [FloatingActionButton], floating at the bottom of the screen.
   ///
   /// This is the default alignment of [FloatingActionButton]s in Material applications.
   static const FloatingActionButtonLocation endFloat = _EndFloatFloatingActionButtonLocation();

   /// Centered [FloatingActionButton], floating at the bottom of the screen.
   static const FloatingActionButtonLocation centerFloat = _CenterFloatFloatingActionButtonLocation();

   /// End-aligned [FloatingActionButton], floating over the
   /// [Scaffold.bottomNavigationBar] so that the center of the floating
   /// action button lines up with the top of the bottom navigation bar.
   ///
   /// If the value of [Scaffold.bottomNavigationBar] is a [BottomAppBar],
   /// the bottom app bar can include a "notch" in its shape that accommodates
   /// the overlapping floating action button.
   ///
   /// This is unlikely to be a useful location for apps that lack a bottom
   /// navigation bar.
   static const FloatingActionButtonLocation endDocked = _EndDockedFloatingActionButtonLocation();

   /// Center-aligned [FloatingActionButton], floating over the
   /// [Scaffold.bottomNavigationBar] so that the center of the floating
   /// action button lines up with the top of the bottom navigation bar.
   ///
   /// If the value of [Scaffold.bottomNavigationBar] is a [BottomAppBar],
   /// the bottom app bar can include a "notch" in its shape that accommodates
   /// the overlapping floating action button.
   ///
   /// This is unlikely to be a useful location for apps that lack a bottom
   /// navigation bar.
   static const FloatingActionButtonLocation centerDocked = _CenterDockedFloatingActionButtonLocation();

   /// Start-aligned [FloatingActionButton], floating over the transition between
   /// the [Scaffold.appBar] and the [Scaffold.body].
   ///
   /// To align a floating action button with [FloatingActionButton.mini] set to
   /// true with [CircleAvatar]s in the [ListTile.leading] slots of [ListTile]s
   /// in a [ListView] in the [Scaffold.body], consider using [miniStartTop].
   ///
   /// This is unlikely to be a useful location for apps that lack a top [AppBar]
   /// or that use a [SliverAppBar] in the scaffold body itself.
   static const FloatingActionButtonLocation startTop = _StartTopFloatingActionButtonLocation();

   /// Start-aligned [FloatingActionButton], floating over the transition between
   /// the [Scaffold.appBar] and the [Scaffold.body], optimized for mini floating
   /// action buttons.
   ///
   /// This is intended to be used with [FloatingActionButton.mini] set to true,
   /// so that the floating action button appears to align with [CircleAvatar]s
   /// in the [ListTile.leading] slot of a [ListTile] in a [ListView] in the
   /// [Scaffold.body].
   ///
   /// This is unlikely to be a useful location for apps that lack a top [AppBar]
   /// or that use a [SliverAppBar] in the scaffold body itself.
   static const FloatingActionButtonLocation miniStartTop = _MiniStartTopFloatingActionButtonLocation();

   /// End-aligned [FloatingActionButton], floating over the transition between
   /// the [Scaffold.appBar] and the [Scaffold.body].
   ///
   /// This is unlikely to be a useful location for apps that lack a top [AppBar]
   /// or that use a [SliverAppBar] in the scaffold body itself.
   static const FloatingActionButtonLocation endTop = _EndTopFloatingActionButtonLocation();

   /// Places the [FloatingActionButton] based on the [Scaffold]'s layout.
   ///
   /// This uses a [ScaffoldPrelayoutGeometry], which the [Scaffold] constructs
   /// during its layout phase after it has laid out every widget it can lay out
   /// except the [FloatingActionButton]. The [Scaffold] uses the [Offset]
   /// returned from this method to position the [FloatingActionButton] and
   /// complete its layout.
   Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry);

   @override
   String toString() => '${objectRuntimeType(this, 'FloatingActionButtonLocation')}';
 }

 double _leftOffset(ScaffoldPrelayoutGeometry scaffoldGeometry, { double offset = 0.0 }) {
   return kFloatingActionButtonMargin
        + scaffoldGeometry.minInsets.left
        - offset;
 }

 double _rightOffset(ScaffoldPrelayoutGeometry scaffoldGeometry, { double offset = 0.0 }) {
   return scaffoldGeometry.scaffoldSize.width
        - kFloatingActionButtonMargin
        - scaffoldGeometry.minInsets.right
        - scaffoldGeometry.floatingActionButtonSize.width
        + offset;
 }

 double _endOffset(ScaffoldPrelayoutGeometry scaffoldGeometry, { double offset = 0.0 }) {
   assert(scaffoldGeometry.textDirection != null);
   switch (scaffoldGeometry.textDirection) {
     case TextDirection.rtl:
       return _leftOffset(scaffoldGeometry, offset: offset);
     case TextDirection.ltr:
       return _rightOffset(scaffoldGeometry, offset: offset);
   }
   return null;
 }

 double _startOffset(ScaffoldPrelayoutGeometry scaffoldGeometry, { double offset = 0.0 }) {
   assert(scaffoldGeometry.textDirection != null);
   switch (scaffoldGeometry.textDirection) {
     case TextDirection.rtl:
       return _rightOffset(scaffoldGeometry, offset: offset);
     case TextDirection.ltr:
       return _leftOffset(scaffoldGeometry, offset: offset);
   }
   return null;
 }

 class _CenterFloatFloatingActionButtonLocation extends FloatingActionButtonLocation {
   const _CenterFloatFloatingActionButtonLocation();

   @override
   Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
     // Compute the x-axis offset.
     final double fabX = (scaffoldGeometry.scaffoldSize.width - scaffoldGeometry.floatingActionButtonSize.width) / 2.0;

     // Compute the y-axis offset.
     final double contentBottom = scaffoldGeometry.contentBottom;
     final double bottomSheetHeight = scaffoldGeometry.bottomSheetSize.height;
     final double fabHeight = scaffoldGeometry.floatingActionButtonSize.height;
     final double snackBarHeight = scaffoldGeometry.snackBarSize.height;
     double fabY = contentBottom - fabHeight - kFloatingActionButtonMargin;
     if (snackBarHeight > 0.0)
       fabY = math.min(fabY, contentBottom - snackBarHeight - fabHeight - kFloatingActionButtonMargin);
     if (bottomSheetHeight > 0.0)
       fabY = math.min(fabY, contentBottom - bottomSheetHeight - fabHeight / 2.0);

     return Offset(fabX, fabY);
   }

   @override
   String toString() => 'FloatingActionButtonLocation.centerFloat';
 }

 class _EndFloatFloatingActionButtonLocation extends FloatingActionButtonLocation {
   const _EndFloatFloatingActionButtonLocation();

   @override
   Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
     // Compute the x-axis offset.
     final double fabX = _endOffset(scaffoldGeometry);

     // Compute the y-axis offset.
     final double contentBottom = scaffoldGeometry.contentBottom;
     final double bottomSheetHeight = scaffoldGeometry.bottomSheetSize.height;
     final double fabHeight = scaffoldGeometry.floatingActionButtonSize.height;
     final double snackBarHeight = scaffoldGeometry.snackBarSize.height;

     double fabY = contentBottom - fabHeight - kFloatingActionButtonMargin;
     if (snackBarHeight > 0.0)
       fabY = math.min(fabY, contentBottom - snackBarHeight - fabHeight - kFloatingActionButtonMargin);
     if (bottomSheetHeight > 0.0)
       fabY = math.min(fabY, contentBottom - bottomSheetHeight - fabHeight / 2.0);

     return Offset(fabX, fabY);
   }

   @override
   String toString() => 'FloatingActionButtonLocation.endFloat';
 }

 // Provider of common logic for [FloatingActionButtonLocation]s that
 // dock to the [BottomAppBar].
 abstract class _DockedFloatingActionButtonLocation extends FloatingActionButtonLocation {
   const _DockedFloatingActionButtonLocation();

   // Positions the Y coordinate of the [FloatingActionButton] at a height
   // where it docks to the [BottomAppBar].
   @protected
   double getDockedY(ScaffoldPrelayoutGeometry scaffoldGeometry) {
     final double contentBottom = scaffoldGeometry.contentBottom;
     final double bottomSheetHeight = scaffoldGeometry.bottomSheetSize.height;
     final double fabHeight = scaffoldGeometry.floatingActionButtonSize.height;
     final double snackBarHeight = scaffoldGeometry.snackBarSize.height;

     double fabY = contentBottom - fabHeight / 2.0;
     // The FAB should sit with a margin between it and the snack bar.
     if (snackBarHeight > 0.0)
       fabY = math.min(fabY, contentBottom - snackBarHeight - fabHeight - kFloatingActionButtonMargin);
     // The FAB should sit with its center in front of the top of the bottom sheet.
     if (bottomSheetHeight > 0.0)
       fabY = math.min(fabY, contentBottom - bottomSheetHeight - fabHeight / 2.0);

     final double maxFabY = scaffoldGeometry.scaffoldSize.height - fabHeight;
     return math.min(maxFabY, fabY);
   }
 }

 class _EndDockedFloatingActionButtonLocation extends _DockedFloatingActionButtonLocation {
   const _EndDockedFloatingActionButtonLocation();

   @override
   Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
     final double fabX = _endOffset(scaffoldGeometry);
     return Offset(fabX, getDockedY(scaffoldGeometry));
   }

   @override
   String toString() => 'FloatingActionButtonLocation.endDocked';
 }

 class _CenterDockedFloatingActionButtonLocation extends _DockedFloatingActionButtonLocation {
   const _CenterDockedFloatingActionButtonLocation();

   @override
   Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
     final double fabX = (scaffoldGeometry.scaffoldSize.width - scaffoldGeometry.floatingActionButtonSize.width) / 2.0;
     return Offset(fabX, getDockedY(scaffoldGeometry));
   }

   @override
   String toString() => 'FloatingActionButtonLocation.centerDocked';
 }

 double _straddleAppBar(ScaffoldPrelayoutGeometry scaffoldGeometry) {
   final double fabHalfHeight = scaffoldGeometry.floatingActionButtonSize.height / 2.0;
   return scaffoldGeometry.contentTop - fabHalfHeight;
 }

 class _StartTopFloatingActionButtonLocation extends FloatingActionButtonLocation {
   const _StartTopFloatingActionButtonLocation();

   @override
   Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
     return Offset(_startOffset(scaffoldGeometry), _straddleAppBar(scaffoldGeometry));
   }

   @override
   String toString() => 'FloatingActionButtonLocation.startTop';
 }

 class _MiniStartTopFloatingActionButtonLocation extends FloatingActionButtonLocation {
   const _MiniStartTopFloatingActionButtonLocation();

   @override
   Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
     // We have to offset the FAB by four pixels because the FAB itself _adds_
     // four pixels in every direction in order to have a hit target area of 48
     // pixels in each dimension, despite being a circle of radius 40.
     return Offset(_startOffset(scaffoldGeometry, offset: 4.0), _straddleAppBar(scaffoldGeometry));
   }

   @override
   String toString() => 'FloatingActionButtonLocation.miniStartTop';
 }

 class _EndTopFloatingActionButtonLocation extends FloatingActionButtonLocation {
   const _EndTopFloatingActionButtonLocation();

   @override
   Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
     return Offset(_endOffset(scaffoldGeometry), _straddleAppBar(scaffoldGeometry));
   }

   @override
   String toString() => 'FloatingActionButtonLocation.endTop';
 }

 /// Provider of animations to move the [FloatingActionButton] between [FloatingActionButtonLocation]s.
 ///
 /// The [Scaffold] uses [Scaffold.floatingActionButtonAnimator] to define:
 ///
 ///  * The [Offset] of the [FloatingActionButton] between the old and new
 ///    [FloatingActionButtonLocation]s as part of the transition animation.
 ///  * An [Animation] to scale the [FloatingActionButton] during the transition.
 ///  * An [Animation] to rotate the [FloatingActionButton] during the transition.
 ///  * Where to start a new animation from if an animation is interrupted.
 ///
 /// See also:
 ///
 ///  * [FloatingActionButton], which is a circular button typically shown in the
 ///    bottom right corner of the app.
 ///  * [FloatingActionButtonLocation], which the [Scaffold] uses to place the
 ///    [Scaffold.floatingActionButton] within the [Scaffold]'s layout.
 abstract class FloatingActionButtonAnimator {
   /// Abstract const constructor. This constructor enables subclasses to provide
   /// const constructors so that they can be used in const expressions.
   const FloatingActionButtonAnimator();

   /// Moves the [FloatingActionButton] by scaling out and then in at a new
   /// [FloatingActionButtonLocation].
   ///
   /// This animator shrinks the [FloatingActionButton] down until it disappears, then
   /// grows it back to full size at its new [FloatingActionButtonLocation].
   ///
   /// This is the default [FloatingActionButton] motion animation.
   static const FloatingActionButtonAnimator scaling = _ScalingFabMotionAnimator();

   /// Gets the [FloatingActionButton]'s position relative to the origin of the
   /// [Scaffold] based on [progress].
   ///
   /// [begin] is the [Offset] provided by the previous
   /// [FloatingActionButtonLocation].
   ///
   /// [end] is the [Offset] provided by the new
   /// [FloatingActionButtonLocation].
   ///
   /// [progress] is the current progress of the transition animation.
   /// When [progress] is 0.0, the returned [Offset] should be equal to [begin].
   /// when [progress] is 1.0, the returned [Offset] should be equal to [end].
   Offset getOffset({ @required Offset begin, @required Offset end, @required double progress });

   /// Animates the scale of the [FloatingActionButton].
   ///
   /// The animation should both start and end with a value of 1.0.
   ///
   /// For example, to create an animation that linearly scales out and then back in,
   /// you could join animations that pass each other:
   ///
   /// ```dart
   ///   @override
   ///   Animation<double> getScaleAnimation({@required Animation<double> parent}) {
   ///     // The animations will cross at value 0, and the train will return to 1.0.
   ///     return TrainHoppingAnimation(
   ///       Tween<double>(begin: 1.0, end: -1.0).animate(parent),
   ///       Tween<double>(begin: -1.0, end: 1.0).animate(parent),
   ///     );
   ///   }
   /// ```
   Animation<double> getScaleAnimation({ @required Animation<double> parent });

   /// Animates the rotation of [Scaffold.floatingActionButton].
   ///
   /// The animation should both start and end with a value of 0.0 or 1.0.
   ///
   /// The animation values are a fraction of a full circle, with 0.0 and 1.0
   /// corresponding to 0 and 360 degrees, while 0.5 corresponds to 180 degrees.
   ///
   /// For example, to create a rotation animation that rotates the
   /// [FloatingActionButton] through a full circle:
   ///
   /// ```dart
   /// @override
   /// Animation<double> getRotationAnimation({@required Animation<double> parent}) {
   ///   return Tween<double>(begin: 0.0, end: 1.0).animate(parent);
   /// }
   /// ```
   Animation<double> getRotationAnimation({ @required Animation<double> parent });

   /// Gets the progress value to restart a motion animation from when the animation is interrupted.
   ///
   /// [previousValue] is the value of the animation before it was interrupted.
   ///
   /// The restart of the animation will affect all three parts of the motion animation:
   /// offset animation, scale animation, and rotation animation.
   ///
   /// An interruption triggers if the [Scaffold] is given a new [FloatingActionButtonLocation]
   /// while it is still animating a transition between two previous [FloatingActionButtonLocation]s.
   ///
   /// A sensible default is usually 0.0, which is the same as restarting
   /// the animation from the beginning, regardless of the original state of the animation.
   double getAnimationRestart(double previousValue) => 0.0;

   @override
   String toString() => '${objectRuntimeType(this, 'FloatingActionButtonAnimator')}';
 }

 class _ScalingFabMotionAnimator extends FloatingActionButtonAnimator {
   const _ScalingFabMotionAnimator();

   @override
   Offset getOffset({ Offset begin, Offset end, double progress }) {
     if (progress < 0.5) {
       return begin;
     } else {
       return end;
     }
   }

   @override
   Animation<double> getScaleAnimation({ Animation<double> parent }) {
     // Animate the scale down from 1 to 0 in the first half of the animation
     // then from 0 back to 1 in the second half.
     const Curve curve = Interval(0.5, 1.0, curve: Curves.ease);
     return _AnimationSwap<double>(
       ReverseAnimation(parent.drive(CurveTween(curve: curve.flipped))),
       parent.drive(CurveTween(curve: curve)),
       parent,
       0.5,
     );
   }

   // Because we only see the last half of the rotation tween,
   // it needs to go twice as far.
   static final Animatable<double> _rotationTween = Tween<double>(
     begin: 1.0 - kFloatingActionButtonTurnInterval * 2.0,
     end: 1.0,
   );

   static final Animatable<double> _thresholdCenterTween = CurveTween(curve: const Threshold(0.5));

   @override
   Animation<double> getRotationAnimation({ Animation<double> parent }) {
     // This rotation will turn on the way in, but not on the way out.
     return _AnimationSwap<double>(
       parent.drive(_rotationTween),
       ReverseAnimation(parent.drive(_thresholdCenterTween)),
       parent,
       0.5,
     );
   }

   // If the animation was just starting, we'll continue from where we left off.
   // If the animation was finishing, we'll treat it as if we were starting at that point in reverse.
   // This avoids a size jump during the animation.
   @override
   double getAnimationRestart(double previousValue) => math.min(1.0 - previousValue, previousValue);
 }

 /// An animation that swaps from one animation to the next when the [parent] passes [swapThreshold].
 ///
 /// The [value] of this animation is the value of [first] when [parent.value] < [swapThreshold]
 /// and the value of [next] otherwise.
 class _AnimationSwap<T> extends CompoundAnimation<T> {
   /// Creates an [_AnimationSwap].
   ///
   /// Both arguments must be non-null. Either can be an [_AnimationSwap] itself
   /// to combine multiple animations.
   _AnimationSwap(Animation<T> first, Animation<T> next, this.parent, this.swapThreshold) : super(first: first, next: next);

   final Animation<double> parent;
   final double swapThreshold;

   @override
   T get value => parent.value < swapThreshold ? first.value : next.value;
 }
	// Copyright 2014 The Flutter 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 'package:flutter/foundation.dart';
	import 'package:flutter/widgets.dart';

	import 'scaffold.dart';

	// TODO(hmuller): should be device dependent.
	/// The margin that a [FloatingActionButton] should leave between it and the
	/// edge of the screen.
	///
	/// [FloatingActionButtonLocation.endFloat] uses this to set the appropriate margin
	/// between the [FloatingActionButton] and the end of the screen.
	const double kFloatingActionButtonMargin = 16.0;

	/// The amount of time the [FloatingActionButton] takes to transition in or out.
	///
	/// The [Scaffold] uses this to set the duration of [FloatingActionButton]
	/// motion, entrance, and exit animations.
	const Duration kFloatingActionButtonSegue = Duration(milliseconds: 200);

	/// The fraction of a circle the [FloatingActionButton] should turn when it enters.
	///
	/// Its value corresponds to 0.125 of a full circle, equivalent to 45 degrees or pi/4 radians.
	const double kFloatingActionButtonTurnInterval = 0.125;

	/// An object that defines a position for the [FloatingActionButton]
	/// based on the [Scaffold]'s [ScaffoldPrelayoutGeometry].
	///
	/// Flutter provides [FloatingActionButtonLocation]s for the common
	/// [FloatingActionButton] placements in Material Design applications. These
	/// locations are available as static members of this class.
	///
	/// See also:
	///
	/// * [FloatingActionButton], which is a circular button typically shown in the
	/// bottom right corner of the app.
	/// * [FloatingActionButtonAnimator], which is used to animate the
	/// [Scaffold.floatingActionButton] from one [FloatingActionButtonLocation] to
	/// another.
	/// * [ScaffoldPrelayoutGeometry], the geometry that
	/// [FloatingActionButtonLocation]s use to position the [FloatingActionButton].
	abstract class FloatingActionButtonLocation {
	/// Abstract const constructor. This constructor enables subclasses to provide
	/// const constructors so that they can be used in const expressions.
	const FloatingActionButtonLocation();

	/// End-aligned [FloatingActionButton], floating at the bottom of the screen.
	///
	/// This is the default alignment of [FloatingActionButton]s in Material applications.
	static const FloatingActionButtonLocation endFloat = _EndFloatFloatingActionButtonLocation();

	/// Centered [FloatingActionButton], floating at the bottom of the screen.
	static const FloatingActionButtonLocation centerFloat = _CenterFloatFloatingActionButtonLocation();

	/// End-aligned [FloatingActionButton], floating over the
	/// [Scaffold.bottomNavigationBar] so that the center of the floating
	/// action button lines up with the top of the bottom navigation bar.
	///
	/// If the value of [Scaffold.bottomNavigationBar] is a [BottomAppBar],
	/// the bottom app bar can include a "notch" in its shape that accommodates
	/// the overlapping floating action button.
	///
	/// This is unlikely to be a useful location for apps that lack a bottom
	/// navigation bar.
	static const FloatingActionButtonLocation endDocked = _EndDockedFloatingActionButtonLocation();

	/// Center-aligned [FloatingActionButton], floating over the
	/// [Scaffold.bottomNavigationBar] so that the center of the floating
	/// action button lines up with the top of the bottom navigation bar.
	///
	/// If the value of [Scaffold.bottomNavigationBar] is a [BottomAppBar],
	/// the bottom app bar can include a "notch" in its shape that accommodates
	/// the overlapping floating action button.
	///
	/// This is unlikely to be a useful location for apps that lack a bottom
	/// navigation bar.
	static const FloatingActionButtonLocation centerDocked = _CenterDockedFloatingActionButtonLocation();

	/// Start-aligned [FloatingActionButton], floating over the transition between
	/// the [Scaffold.appBar] and the [Scaffold.body].
	///
	/// To align a floating action button with [FloatingActionButton.mini] set to
	/// true with [CircleAvatar]s in the [ListTile.leading] slots of [ListTile]s
	/// in a [ListView] in the [Scaffold.body], consider using [miniStartTop].
	///
	/// This is unlikely to be a useful location for apps that lack a top [AppBar]
	/// or that use a [SliverAppBar] in the scaffold body itself.
	static const FloatingActionButtonLocation startTop = _StartTopFloatingActionButtonLocation();

	/// Start-aligned [FloatingActionButton], floating over the transition between
	/// the [Scaffold.appBar] and the [Scaffold.body], optimized for mini floating
	/// action buttons.
	///
	/// This is intended to be used with [FloatingActionButton.mini] set to true,
	/// so that the floating action button appears to align with [CircleAvatar]s
	/// in the [ListTile.leading] slot of a [ListTile] in a [ListView] in the
	/// [Scaffold.body].
	///
	/// This is unlikely to be a useful location for apps that lack a top [AppBar]
	/// or that use a [SliverAppBar] in the scaffold body itself.
	static const FloatingActionButtonLocation miniStartTop = _MiniStartTopFloatingActionButtonLocation();

	/// End-aligned [FloatingActionButton], floating over the transition between
	/// the [Scaffold.appBar] and the [Scaffold.body].
	///
	/// This is unlikely to be a useful location for apps that lack a top [AppBar]
	/// or that use a [SliverAppBar] in the scaffold body itself.
	static const FloatingActionButtonLocation endTop = _EndTopFloatingActionButtonLocation();

	/// Places the [FloatingActionButton] based on the [Scaffold]'s layout.
	///
	/// This uses a [ScaffoldPrelayoutGeometry], which the [Scaffold] constructs
	/// during its layout phase after it has laid out every widget it can lay out
	/// except the [FloatingActionButton]. The [Scaffold] uses the [Offset]
	/// returned from this method to position the [FloatingActionButton] and
	/// complete its layout.
	Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry);

	@override
	String toString() => '${objectRuntimeType(this, 'FloatingActionButtonLocation')}';
	}

	double _leftOffset(ScaffoldPrelayoutGeometry scaffoldGeometry, { double offset = 0.0 }) {
	return kFloatingActionButtonMargin
	+ scaffoldGeometry.minInsets.left
	- offset;
	}

	double _rightOffset(ScaffoldPrelayoutGeometry scaffoldGeometry, { double offset = 0.0 }) {
	return scaffoldGeometry.scaffoldSize.width
	- kFloatingActionButtonMargin
	- scaffoldGeometry.minInsets.right
	- scaffoldGeometry.floatingActionButtonSize.width
	+ offset;
	}

	double _endOffset(ScaffoldPrelayoutGeometry scaffoldGeometry, { double offset = 0.0 }) {
	assert(scaffoldGeometry.textDirection != null);
	switch (scaffoldGeometry.textDirection) {
	case TextDirection.rtl:
	return _leftOffset(scaffoldGeometry, offset: offset);
	case TextDirection.ltr:
	return _rightOffset(scaffoldGeometry, offset: offset);
	}
	return null;
	}

	double _startOffset(ScaffoldPrelayoutGeometry scaffoldGeometry, { double offset = 0.0 }) {
	assert(scaffoldGeometry.textDirection != null);
	switch (scaffoldGeometry.textDirection) {
	case TextDirection.rtl:
	return _rightOffset(scaffoldGeometry, offset: offset);
	case TextDirection.ltr:
	return _leftOffset(scaffoldGeometry, offset: offset);
	}
	return null;
	}

	class _CenterFloatFloatingActionButtonLocation extends FloatingActionButtonLocation {
	const _CenterFloatFloatingActionButtonLocation();

	@override
	Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	// Compute the x-axis offset.
	final double fabX = (scaffoldGeometry.scaffoldSize.width - scaffoldGeometry.floatingActionButtonSize.width) / 2.0;

	// Compute the y-axis offset.
	final double contentBottom = scaffoldGeometry.contentBottom;
	final double bottomSheetHeight = scaffoldGeometry.bottomSheetSize.height;
	final double fabHeight = scaffoldGeometry.floatingActionButtonSize.height;
	final double snackBarHeight = scaffoldGeometry.snackBarSize.height;
	double fabY = contentBottom - fabHeight - kFloatingActionButtonMargin;
	if (snackBarHeight > 0.0)
	fabY = math.min(fabY, contentBottom - snackBarHeight - fabHeight - kFloatingActionButtonMargin);
	if (bottomSheetHeight > 0.0)
	fabY = math.min(fabY, contentBottom - bottomSheetHeight - fabHeight / 2.0);

	return Offset(fabX, fabY);
	}

	@override
	String toString() => 'FloatingActionButtonLocation.centerFloat';
	}

	class _EndFloatFloatingActionButtonLocation extends FloatingActionButtonLocation {
	const _EndFloatFloatingActionButtonLocation();

	@override
	Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	// Compute the x-axis offset.
	final double fabX = _endOffset(scaffoldGeometry);

	// Compute the y-axis offset.
	final double contentBottom = scaffoldGeometry.contentBottom;
	final double bottomSheetHeight = scaffoldGeometry.bottomSheetSize.height;
	final double fabHeight = scaffoldGeometry.floatingActionButtonSize.height;
	final double snackBarHeight = scaffoldGeometry.snackBarSize.height;

	double fabY = contentBottom - fabHeight - kFloatingActionButtonMargin;
	if (snackBarHeight > 0.0)
	fabY = math.min(fabY, contentBottom - snackBarHeight - fabHeight - kFloatingActionButtonMargin);
	if (bottomSheetHeight > 0.0)
	fabY = math.min(fabY, contentBottom - bottomSheetHeight - fabHeight / 2.0);

	return Offset(fabX, fabY);
	}

	@override
	String toString() => 'FloatingActionButtonLocation.endFloat';
	}

	// Provider of common logic for [FloatingActionButtonLocation]s that
	// dock to the [BottomAppBar].
	abstract class _DockedFloatingActionButtonLocation extends FloatingActionButtonLocation {
	const _DockedFloatingActionButtonLocation();

	// Positions the Y coordinate of the [FloatingActionButton] at a height
	// where it docks to the [BottomAppBar].
	@protected
	double getDockedY(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	final double contentBottom = scaffoldGeometry.contentBottom;
	final double bottomSheetHeight = scaffoldGeometry.bottomSheetSize.height;
	final double fabHeight = scaffoldGeometry.floatingActionButtonSize.height;
	final double snackBarHeight = scaffoldGeometry.snackBarSize.height;

	double fabY = contentBottom - fabHeight / 2.0;
	// The FAB should sit with a margin between it and the snack bar.
	if (snackBarHeight > 0.0)
	fabY = math.min(fabY, contentBottom - snackBarHeight - fabHeight - kFloatingActionButtonMargin);
	// The FAB should sit with its center in front of the top of the bottom sheet.
	if (bottomSheetHeight > 0.0)
	fabY = math.min(fabY, contentBottom - bottomSheetHeight - fabHeight / 2.0);

	final double maxFabY = scaffoldGeometry.scaffoldSize.height - fabHeight;
	return math.min(maxFabY, fabY);
	}
	}

	class _EndDockedFloatingActionButtonLocation extends _DockedFloatingActionButtonLocation {
	const _EndDockedFloatingActionButtonLocation();

	@override
	Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	final double fabX = _endOffset(scaffoldGeometry);
	return Offset(fabX, getDockedY(scaffoldGeometry));
	}

	@override
	String toString() => 'FloatingActionButtonLocation.endDocked';
	}

	class _CenterDockedFloatingActionButtonLocation extends _DockedFloatingActionButtonLocation {
	const _CenterDockedFloatingActionButtonLocation();

	@override
	Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	final double fabX = (scaffoldGeometry.scaffoldSize.width - scaffoldGeometry.floatingActionButtonSize.width) / 2.0;
	return Offset(fabX, getDockedY(scaffoldGeometry));
	}

	@override
	String toString() => 'FloatingActionButtonLocation.centerDocked';
	}

	double _straddleAppBar(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	final double fabHalfHeight = scaffoldGeometry.floatingActionButtonSize.height / 2.0;
	return scaffoldGeometry.contentTop - fabHalfHeight;
	}

	class _StartTopFloatingActionButtonLocation extends FloatingActionButtonLocation {
	const _StartTopFloatingActionButtonLocation();

	@override
	Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	return Offset(_startOffset(scaffoldGeometry), _straddleAppBar(scaffoldGeometry));
	}

	@override
	String toString() => 'FloatingActionButtonLocation.startTop';
	}

	class _MiniStartTopFloatingActionButtonLocation extends FloatingActionButtonLocation {
	const _MiniStartTopFloatingActionButtonLocation();

	@override
	Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	// We have to offset the FAB by four pixels because the FAB itself _adds_
	// four pixels in every direction in order to have a hit target area of 48
	// pixels in each dimension, despite being a circle of radius 40.
	return Offset(_startOffset(scaffoldGeometry, offset: 4.0), _straddleAppBar(scaffoldGeometry));
	}

	@override
	String toString() => 'FloatingActionButtonLocation.miniStartTop';
	}

	class _EndTopFloatingActionButtonLocation extends FloatingActionButtonLocation {
	const _EndTopFloatingActionButtonLocation();

	@override
	Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
	return Offset(_endOffset(scaffoldGeometry), _straddleAppBar(scaffoldGeometry));
	}

	@override
	String toString() => 'FloatingActionButtonLocation.endTop';
	}

	/// Provider of animations to move the [FloatingActionButton] between [FloatingActionButtonLocation]s.
	///
	/// The [Scaffold] uses [Scaffold.floatingActionButtonAnimator] to define:
	///
	/// * The [Offset] of the [FloatingActionButton] between the old and new
	/// [FloatingActionButtonLocation]s as part of the transition animation.
	/// * An [Animation] to scale the [FloatingActionButton] during the transition.
	/// * An [Animation] to rotate the [FloatingActionButton] during the transition.
	/// * Where to start a new animation from if an animation is interrupted.
	///
	/// See also:
	///
	/// * [FloatingActionButton], which is a circular button typically shown in the
	/// bottom right corner of the app.
	/// * [FloatingActionButtonLocation], which the [Scaffold] uses to place the
	/// [Scaffold.floatingActionButton] within the [Scaffold]'s layout.
	abstract class FloatingActionButtonAnimator {
	/// Abstract const constructor. This constructor enables subclasses to provide
	/// const constructors so that they can be used in const expressions.
	const FloatingActionButtonAnimator();

	/// Moves the [FloatingActionButton] by scaling out and then in at a new
	/// [FloatingActionButtonLocation].
	///
	/// This animator shrinks the [FloatingActionButton] down until it disappears, then
	/// grows it back to full size at its new [FloatingActionButtonLocation].
	///
	/// This is the default [FloatingActionButton] motion animation.
	static const FloatingActionButtonAnimator scaling = _ScalingFabMotionAnimator();

	/// Gets the [FloatingActionButton]'s position relative to the origin of the
	/// [Scaffold] based on [progress].
	///
	/// [begin] is the [Offset] provided by the previous
	/// [FloatingActionButtonLocation].
	///
	/// [end] is the [Offset] provided by the new
	/// [FloatingActionButtonLocation].
	///
	/// [progress] is the current progress of the transition animation.
	/// When [progress] is 0.0, the returned [Offset] should be equal to [begin].
	/// when [progress] is 1.0, the returned [Offset] should be equal to [end].
	Offset getOffset({ @required Offset begin, @required Offset end, @required double progress });

	/// Animates the scale of the [FloatingActionButton].
	///
	/// The animation should both start and end with a value of 1.0.
	///
	/// For example, to create an animation that linearly scales out and then back in,
	/// you could join animations that pass each other:
	///
	/// ```dart
	/// @override
	/// Animation<double> getScaleAnimation({@required Animation<double> parent}) {
	/// // The animations will cross at value 0, and the train will return to 1.0.
	/// return TrainHoppingAnimation(
	/// Tween<double>(begin: 1.0, end: -1.0).animate(parent),
	/// Tween<double>(begin: -1.0, end: 1.0).animate(parent),
	/// );
	/// }
	/// ```
	Animation<double> getScaleAnimation({ @required Animation<double> parent });

	/// Animates the rotation of [Scaffold.floatingActionButton].
	///
	/// The animation should both start and end with a value of 0.0 or 1.0.
	///
	/// The animation values are a fraction of a full circle, with 0.0 and 1.0
	/// corresponding to 0 and 360 degrees, while 0.5 corresponds to 180 degrees.
	///
	/// For example, to create a rotation animation that rotates the
	/// [FloatingActionButton] through a full circle:
	///
	/// ```dart
	/// @override
	/// Animation<double> getRotationAnimation({@required Animation<double> parent}) {
	/// return Tween<double>(begin: 0.0, end: 1.0).animate(parent);
	/// }
	/// ```
	Animation<double> getRotationAnimation({ @required Animation<double> parent });

	/// Gets the progress value to restart a motion animation from when the animation is interrupted.
	///
	/// [previousValue] is the value of the animation before it was interrupted.
	///
	/// The restart of the animation will affect all three parts of the motion animation:
	/// offset animation, scale animation, and rotation animation.
	///
	/// An interruption triggers if the [Scaffold] is given a new [FloatingActionButtonLocation]
	/// while it is still animating a transition between two previous [FloatingActionButtonLocation]s.
	///
	/// A sensible default is usually 0.0, which is the same as restarting
	/// the animation from the beginning, regardless of the original state of the animation.
	double getAnimationRestart(double previousValue) => 0.0;

	@override
	String toString() => '${objectRuntimeType(this, 'FloatingActionButtonAnimator')}';
	}

	class _ScalingFabMotionAnimator extends FloatingActionButtonAnimator {
	const _ScalingFabMotionAnimator();

	@override
	Offset getOffset({ Offset begin, Offset end, double progress }) {
	if (progress < 0.5) {
	return begin;
	} else {
	return end;
	}
	}

	@override
	Animation<double> getScaleAnimation({ Animation<double> parent }) {
	// Animate the scale down from 1 to 0 in the first half of the animation
	// then from 0 back to 1 in the second half.
	const Curve curve = Interval(0.5, 1.0, curve: Curves.ease);
	return _AnimationSwap<double>(
	ReverseAnimation(parent.drive(CurveTween(curve: curve.flipped))),
	parent.drive(CurveTween(curve: curve)),
	parent,
	0.5,
	);
	}

	// Because we only see the last half of the rotation tween,
	// it needs to go twice as far.
	static final Animatable<double> _rotationTween = Tween<double>(
	begin: 1.0 - kFloatingActionButtonTurnInterval * 2.0,
	end: 1.0,
	);

	static final Animatable<double> _thresholdCenterTween = CurveTween(curve: const Threshold(0.5));

	@override
	Animation<double> getRotationAnimation({ Animation<double> parent }) {
	// This rotation will turn on the way in, but not on the way out.
	return _AnimationSwap<double>(
	parent.drive(_rotationTween),
	ReverseAnimation(parent.drive(_thresholdCenterTween)),
	parent,
	0.5,
	);
	}

	// If the animation was just starting, we'll continue from where we left off.
	// If the animation was finishing, we'll treat it as if we were starting at that point in reverse.
	// This avoids a size jump during the animation.
	@override
	double getAnimationRestart(double previousValue) => math.min(1.0 - previousValue, previousValue);
	}

	/// An animation that swaps from one animation to the next when the [parent] passes [swapThreshold].
	///
	/// The [value] of this animation is the value of [first] when [parent.value] < [swapThreshold]
	/// and the value of [next] otherwise.
	class _AnimationSwap<T> extends CompoundAnimation<T> {
	/// Creates an [_AnimationSwap].
	///
	/// Both arguments must be non-null. Either can be an [_AnimationSwap] itself
	/// to combine multiple animations.
	_AnimationSwap(Animation<T> first, Animation<T> next, this.parent, this.swapThreshold) : super(first: first, next: next);

	final Animation<double> parent;
	final double swapThreshold;

	@override
	T get value => parent.value < swapThreshold ? first.value : next.value;
	}