// 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.

// @dart = 2.8

import 'dart:async';
import 'dart:math' as math;
import 'dart:ui' as ui;

import 'package:flutter/foundation.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/scheduler.dart' show timeDilation;
import 'package:flutter/widgets.dart';

import 'constants.dart';
import 'debug.dart';
import 'slider_theme.dart';
import 'theme.dart';

// Examples can assume:
// RangeValues _rangeValues = RangeValues(0.3, 0.7);
// RangeValues _dollarsRange = RangeValues(50, 100);
// void setState(VoidCallback fn) { }

/// [RangeSlider] uses this callback to paint the value indicator on the overlay.
/// Since the value indicator is painted on the Overlay; this method paints the
/// value indicator in a [RenderBox] that appears in the [Overlay].
typedef PaintRangeValueIndicator = void Function(PaintingContext context, Offset offset);

/// A Material Design range slider.
///
/// Used to select a range from a range of values.
///
/// {@tool dartpad --template=stateful_widget_scaffold}
///
/// ![A range slider widget, consisting of 5 divisions and showing the default
/// value indicator.](https://flutter.github.io/assets-for-api-docs/assets/material/range_slider.png)
///
/// This range values are in intervals of 20 because the Range Slider has 5
/// divisions, from 0 to 100. This means are values are split between 0, 20, 40,
/// 60, 80, and 100. The range values are initialized with 40 and 80 in this demo.
///
/// ```dart
/// RangeValues _currentRangeValues = const RangeValues(40, 80);
///
/// @override
/// Widget build(BuildContext context) {
///   return RangeSlider(
///     values: _currentRangeValues,
///     min: 0,
///     max: 100,
///     divisions: 5,
///     labels: RangeLabels(
///       _currentRangeValues.start.round().toString(),
///       _currentRangeValues.end.round().toString(),
///     ),
///     onChanged: (RangeValues values) {
///       setState(() {
///         _currentRangeValues = values;
///       });
///     },
///   );
/// }
/// ```
/// {@end-tool}
///
/// A range slider can be used to select from either a continuous or a discrete
/// set of values. The default is to use a continuous range of values from [min]
/// to [max]. To use discrete values, use a non-null value for [divisions], which
/// indicates the number of discrete intervals. For example, if [min] is 0.0 and
/// [max] is 50.0 and [divisions] is 5, then the slider can take on the
/// discrete values 0.0, 10.0, 20.0, 30.0, 40.0, and 50.0.
///
/// The terms for the parts of a slider are:
///
///  * The "thumbs", which are the shapes that slide horizontally when the user
///    drags them to change the selected range.
///  * The "track", which is the horizontal line that the thumbs can be dragged
///    along.
///  * The "tick marks", which mark the discrete values of a discrete slider.
///  * The "overlay", which is a highlight that's drawn over a thumb in response
///    to a user tap-down gesture.
///  * The "value indicators", which are the shapes that pop up when the user
///    is dragging a thumb to show the value being selected.
///  * The "active" segment of the slider is the segment between the two thumbs.
///  * The "inactive" slider segments are the two track intervals outside of the
///    slider's thumbs.
///
/// The range slider will be disabled if [onChanged] is null or if the range
/// given by [min]..[max] is empty (i.e. if [min] is equal to [max]).
///
/// The range slider widget itself does not maintain any state. Instead, when
/// the state of the slider changes, the widget calls the [onChanged] callback.
/// Most widgets that use a range slider will listen for the [onChanged] callback
/// and rebuild the slider with new [values] to update the visual appearance of
/// the slider. To know when the value starts to change, or when it is done
/// changing, set the optional callbacks [onChangeStart] and/or [onChangeEnd].
///
/// By default, a slider will be as wide as possible, centered vertically. When
/// given unbounded constraints, it will attempt to make the track 144 pixels
/// wide (including margins on each side) and will shrink-wrap vertically.
///
/// Requires one of its ancestors to be a [Material] widget. This is typically
/// provided by a [Scaffold] widget.
///
/// Requires one of its ancestors to be a [MediaQuery] widget. Typically, a
/// [MediaQuery] widget is introduced by the [MaterialApp] or [WidgetsApp]
/// widget at the top of your application widget tree.
///
/// To determine how it should be displayed (e.g. colors, thumb shape, etc.),
/// a slider uses the [SliderThemeData] available from either a [SliderTheme]
/// widget, or the [ThemeData.sliderTheme] inside a [Theme] widget above it in
/// the widget tree. You can also override some of the colors with the
/// [activeColor] and [inactiveColor] properties, although more fine-grained
/// control of the colors, and other visual properties is achieved using a
/// [SliderThemeData].
///
/// See also:
///
///  * [SliderTheme] and [SliderThemeData] for information about controlling
///    the visual appearance of the slider.
///  * [Slider], for a single-valued slider.
///  * [Radio], for selecting among a set of explicit values.
///  * [Checkbox] and [Switch], for toggling a particular value on or off.
///  * <https://material.io/design/components/sliders.html>
///  * [MediaQuery], from which the text scale factor is obtained.
class RangeSlider extends StatefulWidget {
  /// Creates a Material Design range slider.
  ///
  /// The range slider widget itself does not maintain any state. Instead, when
  /// the state of the slider changes, the widget calls the [onChanged] callback.
  /// Most widgets that use a range slider will listen for the [onChanged] callback
  /// and rebuild the slider with a new [value] to update the visual appearance of
  /// the slider. To know when the value starts to change, or when it is done
  /// changing, set the optional callbacks [onChangeStart] and/or [onChangeEnd].
  ///
  /// * [values], which  determines currently selected values for this range
  ///   slider.
  /// * [onChanged], which is called while the user is selecting a new value for
  ///   the range slider.
  /// * [onChangeStart], which is called when the user starts to select a new
  ///   value for the range slider.
  /// * [onChangeEnd], which is called when the user is done selecting a new
  ///   value for the range slider.
  ///
  /// You can override some of the colors with the [activeColor] and
  /// [inactiveColor] properties, although more fine-grained control of the
  /// appearance is achieved using a [SliderThemeData].
  ///
  /// The [values], [min], [max] must not be null. The [min] must be less than
  /// or equal to the [max]. [values.start] must be less than or equal to
  /// [values.end]. [values.start] and [values.end] must be greater than or
  /// equal to the [min] and less than or equal to the [max]. The [divisions]
  /// must be null or greater than 0.
  RangeSlider({
    Key key,
    @required this.values,
    @required this.onChanged,
    this.onChangeStart,
    this.onChangeEnd,
    this.min = 0.0,
    this.max = 1.0,
    this.divisions,
    this.labels,
    this.activeColor,
    this.inactiveColor,
    this.semanticFormatterCallback,
  }) : assert(values != null),
       assert(min != null),
       assert(max != null),
       assert(min <= max),
       assert(values.start <= values.end),
       assert(values.start >= min && values.start <= max),
       assert(values.end >= min && values.end <= max),
       assert(divisions == null || divisions > 0),
       super(key: key);

  /// The currently selected values for this range slider.
  ///
  /// The slider's thumbs are drawn at horizontal positions that corresponds to
  /// these values.
  final RangeValues values;

  /// Called when the user is selecting a new value for the slider by dragging.
  ///
  /// The slider passes the new values to the callback but does not actually
  /// change state until the parent widget rebuilds the slider with the new
  /// values.
  ///
  /// If null, the slider will be displayed as disabled.
  ///
  /// The callback provided to [onChanged] should update the state of the parent
  /// [StatefulWidget] using the [State.setState] method, so that the parent
  /// gets rebuilt; for example:
  ///
  /// {@tool snippet}
  ///
  /// ```dart
  /// RangeSlider(
  ///   values: _rangeValues,
  ///   min: 1.0,
  ///   max: 10.0,
  ///   onChanged: (RangeValues newValues) {
  ///     setState(() {
  ///       _rangeValues = newValues;
  ///     });
  ///   },
  /// )
  /// ```
  /// {@end-tool}
  ///
  /// See also:
  ///
  ///  * [onChangeStart], which  is called when the user starts  changing the
  ///    values.
  ///  * [onChangeEnd], which is called when the user stops changing the values.
  final ValueChanged<RangeValues> onChanged;

  /// Called when the user starts selecting new values for the slider.
  ///
  /// This callback shouldn't be used to update the slider [values] (use
  /// [onChanged] for that). Rather, it should be used to be notified when the
  /// user has started selecting a new value by starting a drag or with a tap.
  ///
  /// The values passed will be the last [values] that the slider had before the
  /// change began.
  ///
  /// {@tool snippet}
  ///
  /// ```dart
  /// RangeSlider(
  ///   values: _rangeValues,
  ///   min: 1.0,
  ///   max: 10.0,
  ///   onChanged: (RangeValues newValues) {
  ///     setState(() {
  ///       _rangeValues = newValues;
  ///     });
  ///   },
  ///   onChangeStart: (RangeValues startValues) {
  ///     print('Started change at $startValues');
  ///   },
  /// )
  /// ```
  /// {@end-tool}
  ///
  /// See also:
  ///
  ///  * [onChangeEnd] for a callback that is called when the value change is
  ///    complete.
  final ValueChanged<RangeValues> onChangeStart;

  /// Called when the user is done selecting new values for the slider.
  ///
  /// This differs from [onChanged] because it is only called once at the end
  /// of the interaction, while [onChanged] is called as the value is getting
  /// updated within the interaction.
  ///
  /// This callback shouldn't be used to update the slider [values] (use
  /// [onChanged] for that). Rather, it should be used to know when the user has
  /// completed selecting a new [values] by ending a drag or a click.
  ///
  /// {@tool snippet}
  ///
  /// ```dart
  /// RangeSlider(
  ///   values: _rangeValues,
  ///   min: 1.0,
  ///   max: 10.0,
  ///   onChanged: (RangeValues newValues) {
  ///     setState(() {
  ///       _rangeValues = newValues;
  ///     });
  ///   },
  ///   onChangeEnd: (RangeValues endValues) {
  ///     print('Ended change at $endValues');
  ///   },
  /// )
  /// ```
  /// {@end-tool}
  ///
  /// See also:
  ///
  ///  * [onChangeStart] for a callback that is called when a value change
  ///    begins.
  final ValueChanged<RangeValues> onChangeEnd;

  /// The minimum value the user can select.
  ///
  /// Defaults to 0.0. Must be less than or equal to [max].
  ///
  /// If the [max] is equal to the [min], then the slider is disabled.
  final double min;

  /// The maximum value the user can select.
  ///
  /// Defaults to 1.0. Must be greater than or equal to [min].
  ///
  /// If the [max] is equal to the [min], then the slider is disabled.
  final double max;

  /// The number of discrete divisions.
  ///
  /// Typically used with [labels] to show the current discrete values.
  ///
  /// If null, the slider is continuous.
  final int divisions;

  /// Labels to show as text in the [SliderThemeData.rangeValueIndicatorShape].
  ///
  /// There are two labels: one for the start thumb and one for the end thumb.
  ///
  /// Each label is rendered using the active [ThemeData]'s
  /// [TextTheme.bodyText1] text style, with the theme data's
  /// [ColorScheme.onPrimary] color. The label's text style can be overridden
  /// with [SliderThemeData.valueIndicatorTextStyle].
  ///
  /// If null, then the value indicator will not be displayed.
  ///
  /// See also:
  ///
  ///  * [RangeSliderValueIndicatorShape] for how to create a custom value
  ///    indicator shape.
  final RangeLabels labels;

  /// The color of the track's active segment, i.e. the span of track between
  /// the thumbs.
  ///
  /// Defaults to [ColorScheme.primary].
  ///
  /// Using a [SliderTheme] gives more fine-grained control over the
  /// appearance of various components of the slider.
  final Color activeColor;

  /// The color of the track's inactive segments, i.e. the span of tracks
  /// between the min and the start thumb, and the end thumb and the max.
  ///
  /// Defaults to [ColorScheme.primary] with 24% opacity.
  ///
  /// Using a [SliderTheme] gives more fine-grained control over the
  /// appearance of various components of the slider.
  final Color inactiveColor;

  /// The callback used to create a semantic value from the slider's values.
  ///
  /// Defaults to formatting values as a percentage.
  ///
  /// This is used by accessibility frameworks like TalkBack on Android to
  /// inform users what the currently selected value is with more context.
  ///
  /// {@tool snippet}
  ///
  /// In the example below, a slider for currency values is configured to
  /// announce a value with a currency label.
  ///
  /// ```dart
  /// RangeSlider(
  ///   values: _dollarsRange,
  ///   min: 20.0,
  ///   max: 330.0,
  ///   onChanged: (RangeValues newValues) {
  ///     setState(() {
  ///       _dollarsRange = newValues;
  ///     });
  ///   },
  ///   semanticFormatterCallback: (double newValue) {
  ///     return '${newValue.round()} dollars';
  ///   }
  ///  )
  /// ```
  /// {@end-tool}
  final SemanticFormatterCallback semanticFormatterCallback;

  // Touch width for the tap boundary of the slider thumbs.
  static const double _minTouchTargetWidth = kMinInteractiveDimension;

  @override
  _RangeSliderState createState() => _RangeSliderState();

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DoubleProperty('valueStart', values.start));
    properties.add(DoubleProperty('valueEnd', values.end));
    properties.add(ObjectFlagProperty<ValueChanged<RangeValues>>('onChanged', onChanged, ifNull: 'disabled'));
    properties.add(ObjectFlagProperty<ValueChanged<RangeValues>>.has('onChangeStart', onChangeStart));
    properties.add(ObjectFlagProperty<ValueChanged<RangeValues>>.has('onChangeEnd', onChangeEnd));
    properties.add(DoubleProperty('min', min));
    properties.add(DoubleProperty('max', max));
    properties.add(IntProperty('divisions', divisions));
    properties.add(StringProperty('labelStart', labels?.start));
    properties.add(StringProperty('labelEnd', labels?.end));
    properties.add(ColorProperty('activeColor', activeColor));
    properties.add(ColorProperty('inactiveColor', inactiveColor));
    properties.add(ObjectFlagProperty<ValueChanged<double>>.has('semanticFormatterCallback', semanticFormatterCallback));
  }
}

class _RangeSliderState extends State<RangeSlider> with TickerProviderStateMixin {
  static const Duration enableAnimationDuration = Duration(milliseconds: 75);
  static const Duration valueIndicatorAnimationDuration = Duration(milliseconds: 100);

  // Animation controller that is run when the overlay (a.k.a radial reaction)
  // changes visibility in response to user interaction.
  AnimationController overlayController;

  // Animation controller that is run when the value indicators change visibility.
  AnimationController valueIndicatorController;

  // Animation controller that is run when enabling/disabling the slider.
  AnimationController enableController;

  // Animation controllers that are run when transitioning between one value
  // and the next on a discrete slider.
  AnimationController startPositionController;
  AnimationController endPositionController;
  Timer interactionTimer;
  // Value Indicator paint Animation that appears on the Overlay.
  PaintRangeValueIndicator paintTopValueIndicator;
  PaintRangeValueIndicator paintBottomValueIndicator;


  @override
  void initState() {
    super.initState();
    overlayController = AnimationController(
      duration: kRadialReactionDuration,
      vsync: this,
    );
    valueIndicatorController = AnimationController(
      duration: valueIndicatorAnimationDuration,
      vsync: this,
    );
    enableController = AnimationController(
      duration: enableAnimationDuration,
      vsync: this,
      value: widget.onChanged != null ? 1.0 : 0.0,
    );
    startPositionController = AnimationController(
      duration: Duration.zero,
      vsync: this,
      value: _unlerp(widget.values.start),
    );
    endPositionController = AnimationController(
      duration: Duration.zero,
      vsync: this,
      value: _unlerp(widget.values.end),
    );
  }

  @override
  void didUpdateWidget(RangeSlider oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.onChanged == widget.onChanged)
      return;
    final bool wasEnabled = oldWidget.onChanged != null;
    final bool isEnabled = widget.onChanged != null;
    if (wasEnabled != isEnabled) {
      if (isEnabled) {
        enableController.forward();
      } else {
        enableController.reverse();
      }
    }
  }

  @override
  void dispose() {
    interactionTimer?.cancel();
    overlayController.dispose();
    valueIndicatorController.dispose();
    enableController.dispose();
    startPositionController.dispose();
    endPositionController.dispose();
    if (overlayEntry != null) {
      overlayEntry.remove();
      overlayEntry = null;
    }
    super.dispose();
  }

  void _handleChanged(RangeValues values) {
    assert(widget.onChanged != null);
    final RangeValues lerpValues = _lerpRangeValues(values);
    if (lerpValues != widget.values) {
      widget.onChanged(lerpValues);
    }
  }

  void _handleDragStart(RangeValues values) {
    assert(widget.onChangeStart != null);
    widget.onChangeStart(_lerpRangeValues(values));
  }

  void _handleDragEnd(RangeValues values) {
    assert(widget.onChangeEnd != null);
    widget.onChangeEnd(_lerpRangeValues(values));
  }

  // Returns a number between min and max, proportional to value, which must
  // be between 0.0 and 1.0.
  double _lerp(double value) => ui.lerpDouble(widget.min, widget.max, value);

  // Returns a new range value with the start and end lerped.
  RangeValues _lerpRangeValues(RangeValues values) {
    return RangeValues(_lerp(values.start), _lerp(values.end));
  }

  // Returns a number between 0.0 and 1.0, given a value between min and max.
  double _unlerp(double value) {
    assert(value <= widget.max);
    assert(value >= widget.min);
    return widget.max > widget.min ? (value - widget.min) / (widget.max - widget.min) : 0.0;
  }

  // Returns a new range value with the start and end unlerped.
  RangeValues _unlerpRangeValues(RangeValues values) {
    return RangeValues(_unlerp(values.start), _unlerp(values.end));
  }

  // Finds closest thumb. If the thumbs are close to each other, no thumb is
  // immediately selected while the drag displacement is zero. If the first
  // non-zero displacement is negative, then the left thumb is selected, and if its
  // positive, then the right thumb is selected.
  static final RangeThumbSelector _defaultRangeThumbSelector = (
    TextDirection textDirection,
    RangeValues values,
    double tapValue,
    Size thumbSize,
    Size trackSize,
    double dx, // The horizontal delta or displacement of the drag update.
  ) {
    final double touchRadius = math.max(thumbSize.width, RangeSlider._minTouchTargetWidth) / 2;
    final bool inStartTouchTarget = (tapValue - values.start).abs() * trackSize.width < touchRadius;
    final bool inEndTouchTarget = (tapValue - values.end).abs() * trackSize.width < touchRadius;

    // Use dx if the thumb touch targets overlap. If dx is 0 and the drag
    // position is in both touch targets, no thumb is selected because it is
    // ambiguous to which thumb should be selected. If the dx is non-zero, the
    // thumb selection is determined by the direction of the dx. The left thumb
    // is chosen for negative dx, and the right thumb is chosen for positive dx.
    if (inStartTouchTarget && inEndTouchTarget) {
      bool towardsStart;
      bool towardsEnd;
      switch (textDirection) {
        case TextDirection.ltr:
          towardsStart = dx < 0;
          towardsEnd = dx > 0;
          break;
        case TextDirection.rtl:
          towardsStart = dx > 0;
          towardsEnd = dx < 0;
          break;
      }
      if (towardsStart)
        return Thumb.start;
      if (towardsEnd)
        return Thumb.end;
    } else {
      // Snap position on the track if its in the inactive range.
      if (tapValue < values.start || inStartTouchTarget)
        return Thumb.start;
      if (tapValue > values.end || inEndTouchTarget)
        return Thumb.end;
    }
    return null;
  };


  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMaterial(context));
    assert(debugCheckHasMediaQuery(context));

    final ThemeData theme = Theme.of(context);
    SliderThemeData sliderTheme = SliderTheme.of(context);

    // If the widget has active or inactive colors specified, then we plug them
    // in to the slider theme as best we can. If the developer wants more
    // control than that, then they need to use a SliderTheme. The default
    // colors come from the ThemeData.colorScheme. These colors, along with
    // the default shapes and text styles are aligned to the Material
    // Guidelines.

    const double _defaultTrackHeight = 4;
    const RangeSliderTrackShape _defaultTrackShape = RoundedRectRangeSliderTrackShape();
    const RangeSliderTickMarkShape _defaultTickMarkShape = RoundRangeSliderTickMarkShape();
    const SliderComponentShape _defaultOverlayShape = RoundSliderOverlayShape();
    const RangeSliderThumbShape _defaultThumbShape = RoundRangeSliderThumbShape();
    const RangeSliderValueIndicatorShape _defaultValueIndicatorShape = RectangularRangeSliderValueIndicatorShape();
    const ShowValueIndicator _defaultShowValueIndicator = ShowValueIndicator.onlyForDiscrete;
    const double _defaultMinThumbSeparation = 8;

    // The value indicator's color is not the same as the thumb and active track
    // (which can be defined by activeColor) if the
    // RectangularSliderValueIndicatorShape is used. In all other cases, the
    // value indicator is assumed to be the same as the active color.
    final RangeSliderValueIndicatorShape valueIndicatorShape = sliderTheme.rangeValueIndicatorShape ?? _defaultValueIndicatorShape;
    Color valueIndicatorColor;
    if (valueIndicatorShape is RectangularRangeSliderValueIndicatorShape) {
      valueIndicatorColor = sliderTheme.valueIndicatorColor ?? Color.alphaBlend(theme.colorScheme.onSurface.withOpacity(0.60), theme.colorScheme.surface.withOpacity(0.90));
    } else {
      valueIndicatorColor = widget.activeColor ?? sliderTheme.valueIndicatorColor ?? theme.colorScheme.primary;
    }

    sliderTheme = sliderTheme.copyWith(
      trackHeight: sliderTheme.trackHeight ?? _defaultTrackHeight,
      activeTrackColor: widget.activeColor ?? sliderTheme.activeTrackColor ?? theme.colorScheme.primary,
      inactiveTrackColor: widget.inactiveColor ?? sliderTheme.inactiveTrackColor ?? theme.colorScheme.primary.withOpacity(0.24),
      disabledActiveTrackColor: sliderTheme.disabledActiveTrackColor ?? theme.colorScheme.onSurface.withOpacity(0.32),
      disabledInactiveTrackColor: sliderTheme.disabledInactiveTrackColor ?? theme.colorScheme.onSurface.withOpacity(0.12),
      activeTickMarkColor: widget.inactiveColor ?? sliderTheme.activeTickMarkColor ?? theme.colorScheme.onPrimary.withOpacity(0.54),
      inactiveTickMarkColor: widget.activeColor ?? sliderTheme.inactiveTickMarkColor ?? theme.colorScheme.primary.withOpacity(0.54),
      disabledActiveTickMarkColor: sliderTheme.disabledActiveTickMarkColor ?? theme.colorScheme.onPrimary.withOpacity(0.12),
      disabledInactiveTickMarkColor: sliderTheme.disabledInactiveTickMarkColor ?? theme.colorScheme.onSurface.withOpacity(0.12),
      thumbColor: widget.activeColor ?? sliderTheme.thumbColor ?? theme.colorScheme.primary,
      overlappingShapeStrokeColor: sliderTheme.overlappingShapeStrokeColor ?? theme.colorScheme.surface,
      disabledThumbColor: sliderTheme.disabledThumbColor ?? Color.alphaBlend(theme.colorScheme.onSurface.withOpacity(.38), theme.colorScheme.surface),
      overlayColor: widget.activeColor?.withOpacity(0.12) ?? sliderTheme.overlayColor ?? theme.colorScheme.primary.withOpacity(0.12),
      valueIndicatorColor: valueIndicatorColor,
      rangeTrackShape: sliderTheme.rangeTrackShape ?? _defaultTrackShape,
      rangeTickMarkShape: sliderTheme.rangeTickMarkShape ?? _defaultTickMarkShape,
      rangeThumbShape: sliderTheme.rangeThumbShape ?? _defaultThumbShape,
      overlayShape: sliderTheme.overlayShape ?? _defaultOverlayShape,
      rangeValueIndicatorShape: valueIndicatorShape,
      showValueIndicator: sliderTheme.showValueIndicator ?? _defaultShowValueIndicator,
      valueIndicatorTextStyle: sliderTheme.valueIndicatorTextStyle ?? theme.textTheme.bodyText1.copyWith(
        color: theme.colorScheme.onPrimary,
      ),
      minThumbSeparation: sliderTheme.minThumbSeparation ?? _defaultMinThumbSeparation,
      thumbSelector: sliderTheme.thumbSelector ?? _defaultRangeThumbSelector,
    );

    // This size is used as the max bounds for the painting of the value
    // indicators. It must be kept in sync with the function with the same name
    // in slider.dart.
    Size _screenSize() => MediaQuery.of(context).size;

    return CompositedTransformTarget(
      link: _layerLink,
      child: _RangeSliderRenderObjectWidget(
        values: _unlerpRangeValues(widget.values),
        divisions: widget.divisions,
        labels: widget.labels,
        sliderTheme: sliderTheme,
        textScaleFactor: MediaQuery.of(context).textScaleFactor,
        screenSize: _screenSize(),
        onChanged: (widget.onChanged != null) && (widget.max > widget.min) ? _handleChanged : null,
        onChangeStart: widget.onChangeStart != null ? _handleDragStart : null,
        onChangeEnd: widget.onChangeEnd != null ? _handleDragEnd : null,
        state: this,
        semanticFormatterCallback: widget.semanticFormatterCallback,
      ),
    );
  }

  final LayerLink _layerLink = LayerLink();

  OverlayEntry overlayEntry;

  void showValueIndicator() {
    if (overlayEntry == null) {
      overlayEntry = OverlayEntry(
        builder: (BuildContext context) {
          return CompositedTransformFollower(
            link: _layerLink,
            child: _ValueIndicatorRenderObjectWidget(
              state: this,
            ),
          );
        },
      );
      Overlay.of(context).insert(overlayEntry);
    }
  }
}

class _RangeSliderRenderObjectWidget extends LeafRenderObjectWidget {
  const _RangeSliderRenderObjectWidget({
    Key key,
    this.values,
    this.divisions,
    this.labels,
    this.sliderTheme,
    this.textScaleFactor,
    this.screenSize,
    this.onChanged,
    this.onChangeStart,
    this.onChangeEnd,
    this.state,
    this.semanticFormatterCallback,
  }) : super(key: key);

  final RangeValues values;
  final int divisions;
  final RangeLabels labels;
  final SliderThemeData sliderTheme;
  final double textScaleFactor;
  final Size screenSize;
  final ValueChanged<RangeValues> onChanged;
  final ValueChanged<RangeValues> onChangeStart;
  final ValueChanged<RangeValues> onChangeEnd;
  final SemanticFormatterCallback semanticFormatterCallback;
  final _RangeSliderState state;

  @override
  _RenderRangeSlider createRenderObject(BuildContext context) {
    return _RenderRangeSlider(
      values: values,
      divisions: divisions,
      labels: labels,
      sliderTheme: sliderTheme,
      theme: Theme.of(context),
      textScaleFactor: textScaleFactor,
      screenSize: screenSize,
      onChanged: onChanged,
      onChangeStart: onChangeStart,
      onChangeEnd: onChangeEnd,
      state: state,
      textDirection: Directionality.of(context),
      semanticFormatterCallback: semanticFormatterCallback,
      platform: Theme.of(context).platform,
    );
  }

  @override
  void updateRenderObject(BuildContext context, _RenderRangeSlider renderObject) {
    renderObject
      ..values = values
      ..divisions = divisions
      ..labels = labels
      ..sliderTheme = sliderTheme
      ..theme = Theme.of(context)
      ..textScaleFactor = textScaleFactor
      ..screenSize = screenSize
      ..onChanged = onChanged
      ..onChangeStart = onChangeStart
      ..onChangeEnd = onChangeEnd
      ..textDirection = Directionality.of(context)
      ..semanticFormatterCallback = semanticFormatterCallback
      ..platform = Theme.of(context).platform;
  }
}

class _RenderRangeSlider extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
  _RenderRangeSlider({
    @required RangeValues values,
    int divisions,
    RangeLabels labels,
    SliderThemeData sliderTheme,
    ThemeData theme,
    double textScaleFactor,
    Size screenSize,
    TargetPlatform platform,
    ValueChanged<RangeValues> onChanged,
    SemanticFormatterCallback semanticFormatterCallback,
    this.onChangeStart,
    this.onChangeEnd,
    @required _RangeSliderState state,
    @required TextDirection textDirection,
  })  : assert(values != null),
        assert(values.start >= 0.0 && values.start <= 1.0),
        assert(values.end >= 0.0 && values.end <= 1.0),
        assert(state != null),
        assert(textDirection != null),
        _platform = platform,
        _semanticFormatterCallback = semanticFormatterCallback,
        _labels = labels,
        _values = values,
        _divisions = divisions,
        _sliderTheme = sliderTheme,
        _theme = theme,
        _textScaleFactor = textScaleFactor,
        _screenSize = screenSize,
        _onChanged = onChanged,
        _state = state,
        _textDirection = textDirection {
    _updateLabelPainters();
    final GestureArenaTeam team = GestureArenaTeam();
    _drag = HorizontalDragGestureRecognizer()
      ..team = team
      ..onStart = _handleDragStart
      ..onUpdate = _handleDragUpdate
      ..onEnd = _handleDragEnd
      ..onCancel = _handleDragCancel;
    _tap = TapGestureRecognizer()
      ..team = team
      ..onTapDown = _handleTapDown
      ..onTapUp = _handleTapUp
      ..onTapCancel = _handleTapCancel;
    _overlayAnimation = CurvedAnimation(
      parent: _state.overlayController,
      curve: Curves.fastOutSlowIn,
    );
    _valueIndicatorAnimation = CurvedAnimation(
      parent: _state.valueIndicatorController,
      curve: Curves.fastOutSlowIn,
    )..addStatusListener((AnimationStatus status) {
      if (status == AnimationStatus.dismissed && _state.overlayEntry != null) {
        _state.overlayEntry.remove();
        _state.overlayEntry = null;
      }
    });
    _enableAnimation = CurvedAnimation(
      parent: _state.enableController,
      curve: Curves.easeInOut,
    );
  }

  // Keep track of the last selected thumb so they can be drawn in the
  // right order.
  Thumb _lastThumbSelection;

  static const Duration _positionAnimationDuration = Duration(milliseconds: 75);

  // This value is the touch target, 48, multiplied by 3.
  static const double _minPreferredTrackWidth = 144.0;

  // Compute the largest width and height needed to paint the slider shapes,
  // other than the track shape. It is assumed that these shapes are vertically
  // centered on the track.
  double get _maxSliderPartWidth => _sliderPartSizes.map((Size size) => size.width).reduce(math.max);
  double get _maxSliderPartHeight => _sliderPartSizes.map((Size size) => size.height).reduce(math.max);
  List<Size> get _sliderPartSizes => <Size>[
    _sliderTheme.overlayShape.getPreferredSize(isEnabled, isDiscrete),
    _sliderTheme.rangeThumbShape.getPreferredSize(isEnabled, isDiscrete),
    _sliderTheme.rangeTickMarkShape.getPreferredSize(isEnabled: isEnabled, sliderTheme: sliderTheme),
  ];
  double get _minPreferredTrackHeight => _sliderTheme.trackHeight;

  // This rect is used in gesture calculations, where the gesture coordinates
  // are relative to the sliders origin. Therefore, the offset is passed as
  // (0,0).
  Rect get _trackRect => _sliderTheme.rangeTrackShape.getPreferredRect(
    parentBox: this,
    offset: Offset.zero,
    sliderTheme: _sliderTheme,
    isDiscrete: false,
  );

  static const Duration _minimumInteractionTime = Duration(milliseconds: 500);

  final _RangeSliderState _state;
  Animation<double> _overlayAnimation;
  Animation<double> _valueIndicatorAnimation;
  Animation<double> _enableAnimation;
  final TextPainter _startLabelPainter = TextPainter();
  final TextPainter _endLabelPainter = TextPainter();
  HorizontalDragGestureRecognizer _drag;
  TapGestureRecognizer _tap;
  bool _active = false;
  RangeValues _newValues;

  bool get isEnabled => onChanged != null;

  bool get isDiscrete => divisions != null && divisions > 0;

  double get _minThumbSeparationValue => isDiscrete ? 0 : sliderTheme.minThumbSeparation / _trackRect.width;

  RangeValues get values => _values;
  RangeValues _values;
  set values(RangeValues newValues) {
    assert(newValues != null);
    assert(newValues.start != null && newValues.start >= 0.0 && newValues.start <= 1.0);
    assert(newValues.end != null && newValues.end >= 0.0 && newValues.end <= 1.0);
    assert(newValues.start <= newValues.end);
    final RangeValues convertedValues = isDiscrete ? _discretizeRangeValues(newValues) : newValues;
    if (convertedValues == _values) {
      return;
    }
    _values = convertedValues;
    if (isDiscrete) {
      // Reset the duration to match the distance that we're traveling, so that
      // whatever the distance, we still do it in _positionAnimationDuration,
      // and if we get re-targeted in the middle, it still takes that long to
      // get to the new location.
      final double startDistance = (_values.start -  _state.startPositionController.value).abs();
      _state.startPositionController.duration = startDistance != 0.0 ? _positionAnimationDuration * (1.0 / startDistance) : Duration.zero;
      _state.startPositionController.animateTo(_values.start, curve: Curves.easeInOut);
      final double endDistance = (_values.end -  _state.endPositionController.value).abs();
      _state.endPositionController.duration = endDistance != 0.0 ? _positionAnimationDuration * (1.0 / endDistance) : Duration.zero;
      _state.endPositionController.animateTo(_values.end, curve: Curves.easeInOut);
    } else {
      _state.startPositionController.value = convertedValues.start;
      _state.endPositionController.value = convertedValues.end;
    }
    markNeedsSemanticsUpdate();
  }

  TargetPlatform _platform;
  TargetPlatform get platform => _platform;
  set platform(TargetPlatform value) {
    if (_platform == value)
      return;
    _platform = value;
    markNeedsSemanticsUpdate();
  }

  SemanticFormatterCallback _semanticFormatterCallback;
  SemanticFormatterCallback get semanticFormatterCallback => _semanticFormatterCallback;
  set semanticFormatterCallback(SemanticFormatterCallback value) {
    if (_semanticFormatterCallback == value)
      return;
    _semanticFormatterCallback = value;
    markNeedsSemanticsUpdate();
  }

  int get divisions => _divisions;
  int _divisions;
  set divisions(int value) {
    if (value == _divisions) {
      return;
    }
    _divisions = value;
    markNeedsPaint();
  }

  RangeLabels get labels => _labels;
  RangeLabels _labels;
  set labels(RangeLabels labels) {
    if (labels == _labels)
      return;
    _labels = labels;
    _updateLabelPainters();
  }

  SliderThemeData get sliderTheme => _sliderTheme;
  SliderThemeData _sliderTheme;
  set sliderTheme(SliderThemeData value) {
    if (value == _sliderTheme)
      return;
    _sliderTheme = value;
    markNeedsPaint();
  }

  ThemeData get theme => _theme;
  ThemeData _theme;
  set theme(ThemeData value) {
    if (value == _theme)
      return;
    _theme = value;
    markNeedsPaint();
  }

  double get textScaleFactor => _textScaleFactor;
  double _textScaleFactor;
  set textScaleFactor(double value) {
    if (value == _textScaleFactor)
      return;
    _textScaleFactor = value;
    _updateLabelPainters();
  }

  Size get screenSize => _screenSize;
  Size _screenSize;
  set screenSize(Size value) {
    if (value == screenSize)
      return;
    _screenSize = value;
    markNeedsPaint();
  }

  ValueChanged<RangeValues> get onChanged => _onChanged;
  ValueChanged<RangeValues> _onChanged;
  set onChanged(ValueChanged<RangeValues> value) {
    if (value == _onChanged)
      return;
    final bool wasEnabled = isEnabled;
    _onChanged = value;
    if (wasEnabled != isEnabled) {
      markNeedsPaint();
      markNeedsSemanticsUpdate();
    }
  }

  ValueChanged<RangeValues> onChangeStart;
  ValueChanged<RangeValues> onChangeEnd;

  TextDirection get textDirection => _textDirection;
  TextDirection _textDirection;
  set textDirection(TextDirection value) {
    assert(value != null);
    if (value == _textDirection)
      return;
    _textDirection = value;
    _updateLabelPainters();
  }

  bool get showValueIndicator {
    bool showValueIndicator;
    switch (_sliderTheme.showValueIndicator) {
      case ShowValueIndicator.onlyForDiscrete:
        showValueIndicator = isDiscrete;
        break;
      case ShowValueIndicator.onlyForContinuous:
        showValueIndicator = !isDiscrete;
        break;
      case ShowValueIndicator.always:
        showValueIndicator = true;
        break;
      case ShowValueIndicator.never:
        showValueIndicator = false;
        break;
    }
    return showValueIndicator;
  }

  Size get _thumbSize => _sliderTheme.rangeThumbShape.getPreferredSize(isEnabled, isDiscrete);

  double get _adjustmentUnit {
    switch (_platform) {
      case TargetPlatform.iOS:
        // Matches iOS implementation of material slider.
        return 0.1;
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
      case TargetPlatform.linux:
      case TargetPlatform.macOS:
      case TargetPlatform.windows:
        // Matches Android implementation of material slider.
        return 0.05;
    }
    assert(false, 'Unhandled TargetPlatform $_platform');
    return 0.05;
  }

  void _updateLabelPainters() {
    _updateLabelPainter(Thumb.start);
    _updateLabelPainter(Thumb.end);
  }

  void _updateLabelPainter(Thumb thumb) {
    if (labels == null)
      return;

    String text;
    TextPainter labelPainter;
    switch (thumb) {
      case Thumb.start:
        text = labels.start;
        labelPainter = _startLabelPainter;
        break;
      case Thumb.end:
        text = labels.end;
        labelPainter = _endLabelPainter;
        break;
    }

    if (labels != null) {
      labelPainter
        ..text = TextSpan(
          style: _sliderTheme.valueIndicatorTextStyle,
          text: text,
        )
        ..textDirection = textDirection
        ..textScaleFactor = textScaleFactor
        ..layout();
    } else {
      labelPainter.text = null;
    }
    // Changing the textDirection can result in the layout changing, because the
    // bidi algorithm might line up the glyphs differently which can result in
    // different ligatures, different shapes, etc. So we always markNeedsLayout.
    markNeedsLayout();
  }

  @override
  void systemFontsDidChange() {
    super.systemFontsDidChange();
    _startLabelPainter.markNeedsLayout();
    _endLabelPainter.markNeedsLayout();
    _updateLabelPainters();
  }

  @override
  void attach(PipelineOwner owner) {
    super.attach(owner);
    _overlayAnimation.addListener(markNeedsPaint);
    _valueIndicatorAnimation.addListener(markNeedsPaint);
    _enableAnimation.addListener(markNeedsPaint);
    _state.startPositionController.addListener(markNeedsPaint);
    _state.endPositionController.addListener(markNeedsPaint);
  }

  @override
  void detach() {
    _overlayAnimation.removeListener(markNeedsPaint);
    _valueIndicatorAnimation.removeListener(markNeedsPaint);
    _enableAnimation.removeListener(markNeedsPaint);
    _state.startPositionController.removeListener(markNeedsPaint);
    _state.endPositionController.removeListener(markNeedsPaint);
    super.detach();
  }

  double _getValueFromVisualPosition(double visualPosition) {
    switch (textDirection) {
      case TextDirection.rtl:
        return 1.0 - visualPosition;
      case TextDirection.ltr:
        return visualPosition;
    }
    return null;
  }

  double _getValueFromGlobalPosition(Offset globalPosition) {
    final double visualPosition = (globalToLocal(globalPosition).dx - _trackRect.left) / _trackRect.width;
    return _getValueFromVisualPosition(visualPosition);
  }

  double _discretize(double value) {
    double result = value.clamp(0.0, 1.0) as double;
    if (isDiscrete) {
      result = (result * divisions).round() / divisions;
    }
    return result;
  }

  RangeValues _discretizeRangeValues(RangeValues values) {
    return RangeValues(_discretize(values.start), _discretize(values.end));
  }

  void _startInteraction(Offset globalPosition) {
    _state.showValueIndicator();
    final double tapValue = _getValueFromGlobalPosition(globalPosition).clamp(0.0, 1.0) as double;
    _lastThumbSelection = sliderTheme.thumbSelector(textDirection, values, tapValue, _thumbSize, size, 0);

    if (_lastThumbSelection != null) {
      _active = true;
      // We supply the *current* values as the start locations, so that if we have
      // a tap, it consists of a call to onChangeStart with the previous value and
      // a call to onChangeEnd with the new value.
      final RangeValues currentValues = _discretizeRangeValues(values);
      if (_lastThumbSelection == Thumb.start) {
        _newValues = RangeValues(tapValue, currentValues.end);
      } else if (_lastThumbSelection == Thumb.end) {
        _newValues = RangeValues(currentValues.start, tapValue);
      }
      _updateLabelPainter(_lastThumbSelection);

      if (onChangeStart != null) {
        onChangeStart(currentValues);
      }

      onChanged(_discretizeRangeValues(_newValues));

      _state.overlayController.forward();
      if (showValueIndicator) {
        _state.valueIndicatorController.forward();
        _state.interactionTimer?.cancel();
        _state.interactionTimer =
          Timer(_minimumInteractionTime * timeDilation, () {
            _state.interactionTimer = null;
            if (!_active && _state.valueIndicatorController.status == AnimationStatus.completed) {
              _state.valueIndicatorController.reverse();
            }
          });
      }
    }
  }

  void _handleDragUpdate(DragUpdateDetails details) {
    if (!_state.mounted) {
      return;
    }

    final double dragValue = _getValueFromGlobalPosition(details.globalPosition);

    // If no selection has been made yet, test for thumb selection again now
    // that the value of dx can be non-zero. If this is the first selection of
    // the interaction, then onChangeStart must be called.
    bool shouldCallOnChangeStart = false;
    if (_lastThumbSelection == null) {
      _lastThumbSelection = sliderTheme.thumbSelector(textDirection, values, dragValue, _thumbSize, size, details.delta.dx);
      if (_lastThumbSelection != null) {
        shouldCallOnChangeStart = true;
        _active = true;
        _state.overlayController.forward();
        if (showValueIndicator) {
          _state.valueIndicatorController.forward();
        }
      }
    }

    if (isEnabled && _lastThumbSelection != null) {
      final RangeValues currentValues = _discretizeRangeValues(values);
      if (onChangeStart != null && shouldCallOnChangeStart) {
        onChangeStart(currentValues);
      }
      final double currentDragValue = _discretize(dragValue);

      if (_lastThumbSelection == Thumb.start) {
        _newValues = RangeValues(math.min(currentDragValue, currentValues.end - _minThumbSeparationValue), currentValues.end);
      } else if (_lastThumbSelection == Thumb.end) {
        _newValues = RangeValues(currentValues.start, math.max(currentDragValue, currentValues.start + _minThumbSeparationValue));
      }
      onChanged(_newValues);
    }
  }

  void _endInteraction() {
    if (!_state.mounted) {
      return;
    }

    if (showValueIndicator && _state.interactionTimer == null) {
      _state.valueIndicatorController.reverse();
    }

    if (_active && _state.mounted && _lastThumbSelection != null) {
      final RangeValues discreteValues = _discretizeRangeValues(_newValues);
      if (onChangeEnd != null) {
        onChangeEnd(discreteValues);
      }
      _active = false;
    }
    _state.overlayController.reverse();
  }

  void _handleDragStart(DragStartDetails details) {
    _startInteraction(details.globalPosition);
  }

  void _handleDragEnd(DragEndDetails details) {
    _endInteraction();
  }

  void _handleDragCancel() {
    _endInteraction();
  }

  void _handleTapDown(TapDownDetails details) {
    _startInteraction(details.globalPosition);
  }

  void _handleTapUp(TapUpDetails details) {
    _endInteraction();
  }

  void _handleTapCancel() {
    _endInteraction();
  }

  @override
  bool hitTestSelf(Offset position) => true;

  @override
  void handleEvent(PointerEvent event, HitTestEntry entry) {
    assert(debugHandleEvent(event, entry));
    if (event is PointerDownEvent && isEnabled) {
      // We need to add the drag first so that it has priority.
      _drag.addPointer(event);
      _tap.addPointer(event);
    }
  }

  @override
  double computeMinIntrinsicWidth(double height) => _minPreferredTrackWidth + _maxSliderPartWidth;

  @override
  double computeMaxIntrinsicWidth(double height) => _minPreferredTrackWidth + _maxSliderPartWidth;

  @override
  double computeMinIntrinsicHeight(double width) => math.max(_minPreferredTrackHeight, _maxSliderPartHeight);

  @override
  double computeMaxIntrinsicHeight(double width) => math.max(_minPreferredTrackHeight, _maxSliderPartHeight);

  @override
  bool get sizedByParent => true;

  @override
  void performResize() {
    size = Size(
      constraints.hasBoundedWidth ? constraints.maxWidth : _minPreferredTrackWidth + _maxSliderPartWidth,
      constraints.hasBoundedHeight ? constraints.maxHeight : math.max(_minPreferredTrackHeight, _maxSliderPartHeight),
    );
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    final double startValue = _state.startPositionController.value;
    final double endValue = _state.endPositionController.value;

    // The visual position is the position of the thumb from 0 to 1 from left
    // to right. In left to right, this is the same as the value, but it is
    // reversed for right to left text.
    double startVisualPosition;
    double endVisualPosition;
    switch (textDirection) {
      case TextDirection.rtl:
        startVisualPosition = 1.0 - startValue;
        endVisualPosition = 1.0 - endValue;
        break;
      case TextDirection.ltr:
        startVisualPosition = startValue;
        endVisualPosition = endValue;
        break;
    }

    final Rect trackRect = _sliderTheme.rangeTrackShape.getPreferredRect(
        parentBox: this,
        offset: offset,
        sliderTheme: _sliderTheme,
        isDiscrete: isDiscrete,
    );
    final Offset startThumbCenter = Offset(trackRect.left + startVisualPosition * trackRect.width, trackRect.center.dy);
    final Offset endThumbCenter = Offset(trackRect.left + endVisualPosition * trackRect.width, trackRect.center.dy);

    _sliderTheme.rangeTrackShape.paint(
        context,
        offset,
        parentBox: this,
        sliderTheme: _sliderTheme,
        enableAnimation: _enableAnimation,
        textDirection: _textDirection,
        startThumbCenter: startThumbCenter,
        endThumbCenter: endThumbCenter,
        isDiscrete: isDiscrete,
        isEnabled: isEnabled,
    );

    final bool startThumbSelected = _lastThumbSelection == Thumb.start;
    final bool endThumbSelected = _lastThumbSelection == Thumb.end;

    if (!_overlayAnimation.isDismissed) {
      if (startThumbSelected) {
        _sliderTheme.overlayShape.paint(
          context,
          startThumbCenter,
          activationAnimation: _overlayAnimation,
          enableAnimation: _enableAnimation,
          isDiscrete: isDiscrete,
          labelPainter: _startLabelPainter,
          parentBox: this,
          sliderTheme: _sliderTheme,
          textDirection: _textDirection,
          value: startValue,
        );
      }
      if (endThumbSelected) {
        _sliderTheme.overlayShape.paint(
          context,
          endThumbCenter,
          activationAnimation: _overlayAnimation,
          enableAnimation: _enableAnimation,
          isDiscrete: isDiscrete,
          labelPainter: _endLabelPainter,
          parentBox: this,
          sliderTheme: _sliderTheme,
          textDirection: _textDirection,
          value: endValue,
        );
      }
    }

    if (isDiscrete) {
      final double tickMarkWidth = _sliderTheme.rangeTickMarkShape.getPreferredSize(
        isEnabled: isEnabled,
        sliderTheme: _sliderTheme,
      ).width;
      final double padding = trackRect.height;
      final double adjustedTrackWidth = trackRect.width - padding;
      // If the tick marks would be too dense, don't bother painting them.
      if (adjustedTrackWidth / divisions >= 3.0 * tickMarkWidth) {
        final double dy = trackRect.center.dy;
        for (int i = 0; i <= divisions; i++) {
          final double value = i / divisions;
          // The ticks are mapped to be within the track, so the tick mark width
          // must be subtracted from the track width.
          final double dx = trackRect.left + value * adjustedTrackWidth + padding / 2;
          final Offset tickMarkOffset = Offset(dx, dy);
          _sliderTheme.rangeTickMarkShape.paint(
            context,
            tickMarkOffset,
            parentBox: this,
            sliderTheme: _sliderTheme,
            enableAnimation: _enableAnimation,
            textDirection: _textDirection,
            startThumbCenter: startThumbCenter,
            endThumbCenter: endThumbCenter,
            isEnabled: isEnabled,
          );
        }
      }
    }

    final double thumbDelta = (endThumbCenter.dx - startThumbCenter.dx).abs();

    final bool isLastThumbStart = _lastThumbSelection == Thumb.start;
    final Thumb bottomThumb = isLastThumbStart ? Thumb.end : Thumb.start;
    final Thumb topThumb = isLastThumbStart ? Thumb.start : Thumb.end;
    final Offset bottomThumbCenter = isLastThumbStart ? endThumbCenter : startThumbCenter;
    final Offset topThumbCenter = isLastThumbStart ? startThumbCenter : endThumbCenter;
    final TextPainter bottomLabelPainter = isLastThumbStart ? _endLabelPainter : _startLabelPainter;
    final TextPainter topLabelPainter = isLastThumbStart ? _startLabelPainter : _endLabelPainter;
    final double bottomValue = isLastThumbStart ? endValue : startValue;
    final double topValue = isLastThumbStart ? startValue : endValue;
    final bool shouldPaintValueIndicators = isEnabled && labels != null && !_valueIndicatorAnimation.isDismissed && showValueIndicator;
    final Size resolvedscreenSize = screenSize.isEmpty ? size : screenSize;

    if (shouldPaintValueIndicators) {
      _state.paintBottomValueIndicator = (PaintingContext context, Offset offset) {
        if (attached) {
          _sliderTheme.rangeValueIndicatorShape.paint(
            context,
            bottomThumbCenter,
            activationAnimation: _valueIndicatorAnimation,
            enableAnimation: _enableAnimation,
            isDiscrete: isDiscrete,
            isOnTop: false,
            labelPainter: bottomLabelPainter,
            parentBox: this,
            sliderTheme: _sliderTheme,
            textDirection: _textDirection,
            thumb: bottomThumb,
            value: bottomValue,
            textScaleFactor: textScaleFactor,
            sizeWithOverflow: resolvedscreenSize,
          );
        }
      };
    }

    _sliderTheme.rangeThumbShape.paint(
      context,
      bottomThumbCenter,
      activationAnimation: _valueIndicatorAnimation,
      enableAnimation: _enableAnimation,
      isDiscrete: isDiscrete,
      isOnTop: false,
      textDirection: textDirection,
      sliderTheme: _sliderTheme,
      thumb: bottomThumb,
      isPressed: bottomThumb == Thumb.start ? startThumbSelected : endThumbSelected,
    );

    if (shouldPaintValueIndicators) {
      final double startOffset = sliderTheme.rangeValueIndicatorShape.getHorizontalShift(
        parentBox: this,
        center: startThumbCenter,
        labelPainter: _startLabelPainter,
        activationAnimation: _valueIndicatorAnimation,
        textScaleFactor: textScaleFactor,
        sizeWithOverflow: resolvedscreenSize,
      );
      final double endOffset = sliderTheme.rangeValueIndicatorShape.getHorizontalShift(
        parentBox: this,
        center: endThumbCenter,
        labelPainter: _endLabelPainter,
        activationAnimation: _valueIndicatorAnimation,
        textScaleFactor: textScaleFactor,
        sizeWithOverflow: resolvedscreenSize,
      );
      final double startHalfWidth = sliderTheme.rangeValueIndicatorShape.getPreferredSize(
        isEnabled,
        isDiscrete,
        labelPainter: _startLabelPainter,
        textScaleFactor: textScaleFactor,
      ).width / 2;
      final double endHalfWidth = sliderTheme.rangeValueIndicatorShape.getPreferredSize(
        isEnabled,
        isDiscrete,
        labelPainter: _endLabelPainter,
        textScaleFactor: textScaleFactor,
      ).width / 2;
      double innerOverflow = startHalfWidth + endHalfWidth;
      switch (textDirection) {
        case TextDirection.ltr:
          innerOverflow += startOffset;
          innerOverflow -= endOffset;
          break;
        case TextDirection.rtl:
          innerOverflow -= startOffset;
          innerOverflow += endOffset;
          break;
      }

      _state.paintTopValueIndicator = (PaintingContext context, Offset offset) {
        if (attached) {
          _sliderTheme.rangeValueIndicatorShape.paint(
            context,
            topThumbCenter,
            activationAnimation: _valueIndicatorAnimation,
            enableAnimation: _enableAnimation,
            isDiscrete: isDiscrete,
            isOnTop: thumbDelta < innerOverflow,
            labelPainter: topLabelPainter,
            parentBox: this,
            sliderTheme: _sliderTheme,
            textDirection: _textDirection,
            thumb: topThumb,
            value: topValue,
            textScaleFactor: textScaleFactor,
            sizeWithOverflow: resolvedscreenSize,
          );
        }
      };
    }

    _sliderTheme.rangeThumbShape.paint(
      context,
      topThumbCenter,
      activationAnimation: _overlayAnimation,
      enableAnimation: _enableAnimation,
      isDiscrete: isDiscrete,
      isOnTop: thumbDelta < sliderTheme.rangeThumbShape.getPreferredSize(isEnabled, isDiscrete).width,
      textDirection: textDirection,
      sliderTheme: _sliderTheme,
      thumb: topThumb,
      isPressed: topThumb == Thumb.start ? startThumbSelected : endThumbSelected,
    );
  }

  /// Describe the semantics of the start thumb.
  SemanticsNode _startSemanticsNode = SemanticsNode();

  /// Describe the semantics of the end thumb.
  SemanticsNode _endSemanticsNode = SemanticsNode();

  // Create the semantics configuration for a single value.
  SemanticsConfiguration _createSemanticsConfiguration(
      double value,
      double increasedValue,
      double decreasedValue,
      String label,
      VoidCallback increaseAction,
      VoidCallback decreaseAction,
  ) {
    final SemanticsConfiguration config = SemanticsConfiguration();
    config.isEnabled = isEnabled;
    config.textDirection = textDirection;
    if (isEnabled) {
      config.onIncrease = increaseAction;
      config.onDecrease = decreaseAction;
    }
    config.label = label ?? '';
    if (semanticFormatterCallback != null) {
      config.value = semanticFormatterCallback(_state._lerp(value));
      config.increasedValue = semanticFormatterCallback(_state._lerp(increasedValue));
      config.decreasedValue = semanticFormatterCallback(_state._lerp(decreasedValue));
    } else {
      config.value = '${(value * 100).round()}%';
      config.increasedValue = '${(increasedValue * 100).round()}%';
      config.decreasedValue = '${(decreasedValue * 100).round()}%';
    }

    return config;
  }

  @override
  void assembleSemanticsNode(
      SemanticsNode node,
      SemanticsConfiguration config,
      Iterable<SemanticsNode> children,
  ) {
    assert(children.isEmpty);

    final SemanticsConfiguration startSemanticsConfiguration = _createSemanticsConfiguration(
      values.start,
      _increasedStartValue,
      _decreasedStartValue,
      labels?.start,
      _increaseStartAction,
      _decreaseStartAction,
    );
    final SemanticsConfiguration endSemanticsConfiguration = _createSemanticsConfiguration(
      values.end,
      _increasedEndValue,
      _decreasedEndValue,
      labels?.end,
      _increaseEndAction,
      _decreaseEndAction,
    );

    // Split the semantics node area between the start and end nodes.
    final Rect leftRect = Rect.fromPoints(node.rect.topLeft, node.rect.bottomCenter);
    final Rect rightRect = Rect.fromPoints(node.rect.topCenter, node.rect.bottomRight);
    switch (textDirection) {
      case TextDirection.ltr:
        _startSemanticsNode.rect = leftRect;
        _endSemanticsNode.rect = rightRect;
        break;
      case TextDirection.rtl:
        _startSemanticsNode.rect = rightRect;
        _endSemanticsNode.rect = leftRect;
        break;
    }

    _startSemanticsNode.updateWith(config: startSemanticsConfiguration);
    _endSemanticsNode.updateWith(config: endSemanticsConfiguration);

    final List<SemanticsNode> finalChildren = <SemanticsNode>[
      _startSemanticsNode,
      _endSemanticsNode,
    ];

    node.updateWith(config: config, childrenInInversePaintOrder: finalChildren);
  }

  @override
  void clearSemantics() {
    super.clearSemantics();
    _startSemanticsNode = null;
    _endSemanticsNode = null;
  }

  @override
  void describeSemanticsConfiguration(SemanticsConfiguration config) {
    super.describeSemanticsConfiguration(config);
    config.isSemanticBoundary = true;
  }

  double get _semanticActionUnit => divisions != null ? 1.0 / divisions : _adjustmentUnit;

  void _increaseStartAction() {
    if (isEnabled) {
        onChanged(RangeValues(_increasedStartValue, values.end));
    }
  }

  void _decreaseStartAction() {
    if (isEnabled) {
      onChanged(RangeValues(_decreasedStartValue, values.end));
    }
  }

  void _increaseEndAction() {
    if (isEnabled) {
      onChanged(RangeValues(values.start, _increasedEndValue));
    }
  }

  void _decreaseEndAction() {
    if (isEnabled) {
      onChanged(RangeValues(values.start, _decreasedEndValue));
    }
  }

  double get _increasedStartValue {
    // Due to floating-point operations, this value can actually be greater than
    // expected (e.g. 0.4 + 0.2 = 0.600000000001), so we limit to 2 decimal points.
    final double increasedStartValue = double.parse((values.start + _semanticActionUnit).toStringAsFixed(2));
    return increasedStartValue <= values.end - _minThumbSeparationValue ? increasedStartValue : values.start;
  }

  double get _decreasedStartValue {
    return (values.start - _semanticActionUnit).clamp(0.0, 1.0) as double;
  }

  double get _increasedEndValue {
    return (values.end + _semanticActionUnit).clamp(0.0, 1.0) as double;
  }

  double get _decreasedEndValue {
    final double decreasedEndValue = values.end - _semanticActionUnit;
    return decreasedEndValue >= values.start + _minThumbSeparationValue ? decreasedEndValue : values.end;
  }
}


class _ValueIndicatorRenderObjectWidget extends LeafRenderObjectWidget {
  const _ValueIndicatorRenderObjectWidget({
    this.state,
  });

  final _RangeSliderState state;

  @override
  _RenderValueIndicator createRenderObject(BuildContext context) {
    return _RenderValueIndicator(
      state: state,
    );
  }
  @override
  void updateRenderObject(BuildContext context, _RenderValueIndicator renderObject) {
    renderObject._state = state;
  }
}

class _RenderValueIndicator extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
  _RenderValueIndicator({
    _RangeSliderState state,
  }) :_state = state {
    _valueIndicatorAnimation = CurvedAnimation(
      parent: _state.valueIndicatorController,
      curve: Curves.fastOutSlowIn,
    );
  }

  Animation<double> _valueIndicatorAnimation;
  _RangeSliderState _state;

  @override
  bool get sizedByParent => true;

  @override
  void attach(PipelineOwner owner) {
    super.attach(owner);
    _valueIndicatorAnimation.addListener(markNeedsPaint);
    _state.startPositionController.addListener(markNeedsPaint);
    _state.endPositionController.addListener(markNeedsPaint);
  }

  @override
  void detach() {
    _valueIndicatorAnimation.removeListener(markNeedsPaint);
    _state.startPositionController.removeListener(markNeedsPaint);
    _state.endPositionController.removeListener(markNeedsPaint);
    super.detach();
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if (_state.paintBottomValueIndicator != null) {
      _state.paintBottomValueIndicator(context, offset);
    }
    if (_state.paintTopValueIndicator != null) {
      _state.paintTopValueIndicator(context, offset);
    }
  }
}