packages/flutter/lib/src/material/checkbox.dart

// 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/gestures.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';

import 'constants.dart';
import 'debug.dart';
import 'material_state.dart';
import 'theme.dart';
import 'theme_data.dart';
import 'toggleable.dart';

/// A material design checkbox.
///
/// The checkbox itself does not maintain any state. Instead, when the state of
/// the checkbox changes, the widget calls the [onChanged] callback. Most
/// widgets that use a checkbox will listen for the [onChanged] callback and
/// rebuild the checkbox with a new [value] to update the visual appearance of
/// the checkbox.
///
/// The checkbox can optionally display three values - true, false, and null -
/// if [tristate] is true. When [value] is null a dash is displayed. By default
/// [tristate] is false and the checkbox's [value] must be true or false.
///
/// Requires one of its ancestors to be a [Material] widget.
///
/// See also:
///
///  * [CheckboxListTile], which combines this widget with a [ListTile] so that
///    you can give the checkbox a label.
///  * [Switch], a widget with semantics similar to [Checkbox].
///  * [Radio], for selecting among a set of explicit values.
///  * [Slider], for selecting a value in a range.
///  * <https://material.io/design/components/selection-controls.html#checkboxes>
///  * <https://material.io/design/components/lists.html#types>
class Checkbox extends StatefulWidget {
  /// Creates a material design checkbox.
  ///
  /// The checkbox itself does not maintain any state. Instead, when the state of
  /// the checkbox changes, the widget calls the [onChanged] callback. Most
  /// widgets that use a checkbox will listen for the [onChanged] callback and
  /// rebuild the checkbox with a new [value] to update the visual appearance of
  /// the checkbox.
  ///
  /// The following arguments are required:
  ///
  /// * [value], which determines whether the checkbox is checked. The [value]
  ///   can only be null if [tristate] is true.
  /// * [onChanged], which is called when the value of the checkbox should
  ///   change. It can be set to null to disable the checkbox.
  ///
  /// The values of [tristate] and [autofocus] must not be null.
  const Checkbox({
    Key? key,
    required this.value,
    this.tristate = false,
    required this.onChanged,
    this.mouseCursor,
    this.activeColor,
    this.fillColor,
    this.checkColor,
    this.focusColor,
    this.hoverColor,
    this.overlayColor,
    this.splashRadius,
    this.materialTapTargetSize,
    this.visualDensity,
    this.focusNode,
    this.autofocus = false,
  }) : assert(tristate != null),
       assert(tristate || value != null),
       assert(autofocus != null),
       super(key: key);

  /// Whether this checkbox is checked.
  ///
  /// This property must not be null.
  final bool? value;

  /// Called when the value of the checkbox should change.
  ///
  /// The checkbox passes the new value to the callback but does not actually
  /// change state until the parent widget rebuilds the checkbox with the new
  /// value.
  ///
  /// If this callback is null, the checkbox will be displayed as disabled
  /// and will not respond to input gestures.
  ///
  /// When the checkbox is tapped, if [tristate] is false (the default) then
  /// the [onChanged] callback will be applied to `!value`. If [tristate] is
  /// true this callback cycle from false to true to null.
  ///
  /// 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:
  ///
  /// ```dart
  /// Checkbox(
  ///   value: _throwShotAway,
  ///   onChanged: (bool newValue) {
  ///     setState(() {
  ///       _throwShotAway = newValue;
  ///     });
  ///   },
  /// )
  /// ```
  final ValueChanged<bool?>? onChanged;

  /// {@template flutter.material.checkbox.mouseCursor}
  /// The cursor for a mouse pointer when it enters or is hovering over the
  /// widget.
  ///
  /// If [mouseCursor] is a [MaterialStateProperty<MouseCursor>],
  /// [MaterialStateProperty.resolve] is used for the following [MaterialState]s:
  ///
  ///  * [MaterialState.selected].
  ///  * [MaterialState.hovered].
  ///  * [MaterialState.focused].
  ///  * [MaterialState.disabled].
  /// {@endtemplate}
  ///
  /// When [value] is null and [tristate] is true, [MaterialState.selected] is
  /// included as a state.
  ///
  /// If null, then the value of [CheckboxThemeData.mouseCursor] is used. If
  /// that is also null, then [MaterialStateMouseCursor.clickable] is used.
  ///
  /// See also:
  ///
  ///  * [MaterialStateMouseCursor], a [MouseCursor] that implements
  ///    `MaterialStateProperty` which is used in APIs that need to accept
  ///    either a [MouseCursor] or a [MaterialStateProperty<MouseCursor>].
  final MouseCursor? mouseCursor;

  /// The color to use when this checkbox is checked.
  ///
  /// Defaults to [ThemeData.toggleableActiveColor].
  ///
  /// If [fillColor] returns a non-null color in the [MaterialState.selected]
  /// state, it will be used instead of this color.
  final Color? activeColor;

  /// {@template flutter.material.checkbox.fillColor}
  /// The color that fills the checkbox, in all [MaterialState]s.
  ///
  /// Resolves in the following states:
  ///  * [MaterialState.selected].
  ///  * [MaterialState.hovered].
  ///  * [MaterialState.focused].
  ///  * [MaterialState.disabled].
  /// {@endtemplate}
  ///
  /// If null, then the value of [activeColor] is used in the selected
  /// state. If that is also null, the value of [CheckboxThemeData.fillColor]
  /// is used. If that is also null, then [ThemeData.disabledColor] is used in
  /// the disabled state, [ThemeData.toggleableActiveColor] is used in the
  /// selected state, and [ThemeData.unselectedWidgetColor] is used in the
  /// default state.
  final MaterialStateProperty<Color?>? fillColor;

  /// {@template flutter.material.checkbox.checkColor}
  /// The color to use for the check icon when this checkbox is checked.
  /// {@endtemplate}
  ///
  /// If null, then the value of [CheckboxThemeData.checkColor] is used. If
  /// that is also null, then Color(0xFFFFFFFF) is used.
  final Color? checkColor;

  /// If true the checkbox's [value] can be true, false, or null.
  ///
  /// Checkbox displays a dash when its value is null.
  ///
  /// When a tri-state checkbox ([tristate] is true) is tapped, its [onChanged]
  /// callback will be applied to true if the current value is false, to null if
  /// value is true, and to false if value is null (i.e. it cycles through false
  /// => true => null => false when tapped).
  ///
  /// If tristate is false (the default), [value] must not be null.
  final bool tristate;

  /// {@template flutter.material.checkbox.materialTapTargetSize}
  /// Configures the minimum size of the tap target.
  /// {@endtemplate}
  ///
  /// If null, then the value of [CheckboxThemeData.materialTapTargetSize] is
  /// used. If that is also null, then the value of
  /// [ThemeData.materialTapTargetSize] is used.
  ///
  /// See also:
  ///
  ///  * [MaterialTapTargetSize], for a description of how this affects tap targets.
  final MaterialTapTargetSize? materialTapTargetSize;

  /// {@template flutter.material.checkbox.visualDensity}
  /// Defines how compact the checkbox's layout will be.
  /// {@endtemplate}
  ///
  /// {@macro flutter.material.themedata.visualDensity}
  ///
  /// If null, then the value of [CheckboxThemeData.visualDensity] is used. If
  /// that is also null, then the value of [ThemeData.visualDensity] is used.
  ///
  /// See also:
  ///
  ///  * [ThemeData.visualDensity], which specifies the [visualDensity] for all
  ///    widgets within a [Theme].
  final VisualDensity? visualDensity;

  /// The color for the checkbox's [Material] when it has the input focus.
  ///
  /// If [overlayColor] returns a non-null color in the [MaterialState.focused]
  /// state, it will be used instead.
  ///
  /// If null, then the value of [CheckboxThemeData.overlayColor] is used in the
  /// focused state. If that is also null, then the value of
  /// [ThemeData.focusColor] is used.
  final Color? focusColor;

  /// The color for the checkbox's [Material] when a pointer is hovering over it.
  ///
  /// If [overlayColor] returns a non-null color in the [MaterialState.hovered]
  /// state, it will be used instead.
  ///
  /// If null, then the value of [CheckboxThemeData.overlayColor] is used in the
  /// hovered state. If that is also null, then the value of
  /// [ThemeData.hoverColor] is used.
  final Color? hoverColor;

  /// {@template flutter.material.checkbox.overlayColor}
  /// The color for the checkbox's [Material].
  ///
  /// Resolves in the following states:
  ///  * [MaterialState.pressed].
  ///  * [MaterialState.selected].
  ///  * [MaterialState.hovered].
  ///  * [MaterialState.focused].
  /// {@endtemplate}
  ///
  /// If null, then the value of [activeColor] with alpha
  /// [kRadialReactionAlpha], [focusColor] and [hoverColor] is used in the
  /// pressed, focused and hovered state. If that is also null,
  /// the value of [CheckboxThemeData.overlayColor] is used. If that is
  /// also null, then the value of [ThemeData.toggleableActiveColor] with alpha
  /// [kRadialReactionAlpha], [ThemeData.focusColor] and [ThemeData.hoverColor]
  /// is used in the pressed, focused and hovered state.
  final MaterialStateProperty<Color?>? overlayColor;

  /// {@template flutter.material.checkbox.splashRadius}
  /// The splash radius of the circular [Material] ink response.
  /// {@endtemplate}
  ///
  /// If null, then the value of [CheckboxThemeData.splashRadius] is used. If
  /// that is also null, then [kRadialReactionRadius] is used.
  final double? splashRadius;

  /// {@macro flutter.widgets.Focus.focusNode}
  final FocusNode? focusNode;

  /// {@macro flutter.widgets.Focus.autofocus}
  final bool autofocus;

  /// The width of a checkbox widget.
  static const double width = 18.0;

  @override
  _CheckboxState createState() => _CheckboxState();
}

class _CheckboxState extends State<Checkbox> with TickerProviderStateMixin {
  bool get enabled => widget.onChanged != null;
  late Map<Type, Action<Intent>> _actionMap;

  @override
  void initState() {
    super.initState();
    _actionMap = <Type, Action<Intent>>{
      ActivateIntent: CallbackAction<ActivateIntent>(onInvoke: _actionHandler),
    };
  }

  void _actionHandler(ActivateIntent intent) {
    if (widget.onChanged != null) {
      switch (widget.value) {
        case false:
          widget.onChanged!(true);
          break;
        case true:
          widget.onChanged!(widget.tristate ? null : false);
          break;
        case null:
          widget.onChanged!(false);
          break;
      }
    }
    final RenderObject renderObject = context.findRenderObject()!;
    renderObject.sendSemanticsEvent(const TapSemanticEvent());
  }

  bool _focused = false;
  void _handleFocusHighlightChanged(bool focused) {
    if (focused != _focused) {
      setState(() { _focused = focused; });
    }
  }

  bool _hovering = false;
  void _handleHoverChanged(bool hovering) {
    if (hovering != _hovering) {
      setState(() { _hovering = hovering; });
    }
  }

  Set<MaterialState> get _states => <MaterialState>{
    if (!enabled) MaterialState.disabled,
    if (_hovering) MaterialState.hovered,
    if (_focused) MaterialState.focused,
    if (widget.value == null || widget.value!) MaterialState.selected,
  };

  MaterialStateProperty<Color?> get _widgetFillColor {
    return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
      if (states.contains(MaterialState.disabled)) {
        return null;
      }
      if (states.contains(MaterialState.selected)) {
        return widget.activeColor;
      }
      return null;
    });
  }

  MaterialStateProperty<Color> get _defaultFillColor {
    final ThemeData themeData = Theme.of(context);
    return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
      if (states.contains(MaterialState.disabled)) {
        return themeData.disabledColor;
      }
      if (states.contains(MaterialState.selected)) {
        return themeData.toggleableActiveColor;
      }
      return themeData.unselectedWidgetColor;
    });
  }

  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMaterial(context));
    final ThemeData themeData = Theme.of(context);
    final MaterialTapTargetSize effectiveMaterialTapTargetSize = widget.materialTapTargetSize
      ?? themeData.checkboxTheme.materialTapTargetSize
      ?? themeData.materialTapTargetSize;
    final VisualDensity effectiveVisualDensity = widget.visualDensity
      ?? themeData.checkboxTheme.visualDensity
      ?? themeData.visualDensity;
    Size size;
    switch (effectiveMaterialTapTargetSize) {
      case MaterialTapTargetSize.padded:
        size = const Size(kMinInteractiveDimension, kMinInteractiveDimension);
        break;
      case MaterialTapTargetSize.shrinkWrap:
        size = const Size(kMinInteractiveDimension - 8.0, kMinInteractiveDimension - 8.0);
        break;
    }
    size += effectiveVisualDensity.baseSizeAdjustment;
    final BoxConstraints additionalConstraints = BoxConstraints.tight(size);
    final MouseCursor effectiveMouseCursor = MaterialStateProperty.resolveAs<MouseCursor?>(widget.mouseCursor, _states)
      ?? themeData.checkboxTheme.mouseCursor?.resolve(_states)
      ?? MaterialStateProperty.resolveAs<MouseCursor>(MaterialStateMouseCursor.clickable, _states);
    // Colors need to be resolved in selected and non selected states separately
    // so that they can be lerped between.
    final Set<MaterialState> activeStates = _states..add(MaterialState.selected);
    final Set<MaterialState> inactiveStates = _states..remove(MaterialState.selected);
    final Color effectiveActiveColor = widget.fillColor?.resolve(activeStates)
      ?? _widgetFillColor.resolve(activeStates)
      ?? themeData.checkboxTheme.fillColor?.resolve(activeStates)
      ?? _defaultFillColor.resolve(activeStates);
    final Color effectiveInactiveColor = widget.fillColor?.resolve(inactiveStates)
      ?? _widgetFillColor.resolve(inactiveStates)
      ?? themeData.checkboxTheme.fillColor?.resolve(inactiveStates)
      ?? _defaultFillColor.resolve(inactiveStates);

    final Set<MaterialState> focusedStates = _states..add(MaterialState.focused);
    final Color effectiveFocusOverlayColor = widget.overlayColor?.resolve(focusedStates)
      ?? widget.focusColor
      ?? themeData.checkboxTheme.overlayColor?.resolve(focusedStates)
      ?? themeData.focusColor;

    final Set<MaterialState> hoveredStates = _states..add(MaterialState.hovered);
    final Color effectiveHoverOverlayColor = widget.overlayColor?.resolve(hoveredStates)
        ?? widget.hoverColor
        ?? themeData.checkboxTheme.overlayColor?.resolve(hoveredStates)
        ?? themeData.hoverColor;

    final Set<MaterialState> activePressedStates = activeStates..add(MaterialState.pressed);
    final Color effectiveActivePressedOverlayColor = widget.overlayColor?.resolve(activePressedStates)
        ?? themeData.checkboxTheme.overlayColor?.resolve(activePressedStates)
        ?? effectiveActiveColor.withAlpha(kRadialReactionAlpha);

    final Set<MaterialState> inactivePressedStates = inactiveStates..add(MaterialState.pressed);
    final Color effectiveInactivePressedOverlayColor = widget.overlayColor?.resolve(inactivePressedStates)
        ?? themeData.checkboxTheme.overlayColor?.resolve(inactivePressedStates)
        ?? effectiveActiveColor.withAlpha(kRadialReactionAlpha);

    final Color effectiveCheckColor =  widget.checkColor
      ?? themeData.checkboxTheme.checkColor?.resolve(_states)
      ?? const Color(0xFFFFFFFF);

    return FocusableActionDetector(
      actions: _actionMap,
      focusNode: widget.focusNode,
      autofocus: widget.autofocus,
      enabled: enabled,
      onShowFocusHighlight: _handleFocusHighlightChanged,
      onShowHoverHighlight: _handleHoverChanged,
      mouseCursor: effectiveMouseCursor,
      child: Builder(
        builder: (BuildContext context) {
          return _CheckboxRenderObjectWidget(
            value: widget.value,
            tristate: widget.tristate,
            activeColor: effectiveActiveColor,
            checkColor: effectiveCheckColor,
            inactiveColor: effectiveInactiveColor,
            focusColor: effectiveFocusOverlayColor,
            hoverColor: effectiveHoverOverlayColor,
            reactionColor: effectiveActivePressedOverlayColor,
            inactiveReactionColor: effectiveInactivePressedOverlayColor,
            splashRadius: widget.splashRadius ?? themeData.checkboxTheme.splashRadius ?? kRadialReactionRadius,
            onChanged: widget.onChanged,
            additionalConstraints: additionalConstraints,
            vsync: this,
            hasFocus: _focused,
            hovering: _hovering,
          );
        },
      ),
    );
  }
}

class _CheckboxRenderObjectWidget extends LeafRenderObjectWidget {
  const _CheckboxRenderObjectWidget({
    Key? key,
    required this.value,
    required this.tristate,
    required this.activeColor,
    required this.checkColor,
    required this.inactiveColor,
    required this.focusColor,
    required this.hoverColor,
    required this.reactionColor,
    required this.inactiveReactionColor,
    required this.splashRadius,
    required this.onChanged,
    required this.vsync,
    required this.additionalConstraints,
    required this.hasFocus,
    required this.hovering,
  }) : assert(tristate != null),
       assert(tristate || value != null),
       assert(activeColor != null),
       assert(inactiveColor != null),
       assert(vsync != null),
       super(key: key);

  final bool? value;
  final bool tristate;
  final bool hasFocus;
  final bool hovering;
  final Color activeColor;
  final Color checkColor;
  final Color inactiveColor;
  final Color focusColor;
  final Color hoverColor;
  final Color reactionColor;
  final Color inactiveReactionColor;
  final double splashRadius;
  final ValueChanged<bool?>? onChanged;
  final TickerProvider vsync;
  final BoxConstraints additionalConstraints;

  @override
  _RenderCheckbox createRenderObject(BuildContext context) => _RenderCheckbox(
    value: value,
    tristate: tristate,
    activeColor: activeColor,
    checkColor: checkColor,
    inactiveColor: inactiveColor,
    focusColor: focusColor,
    hoverColor: hoverColor,
    reactionColor: reactionColor,
    inactiveReactionColor: inactiveReactionColor,
    splashRadius: splashRadius,
    onChanged: onChanged,
    vsync: vsync,
    additionalConstraints: additionalConstraints,
    hasFocus: hasFocus,
    hovering: hovering,
  );

  @override
  void updateRenderObject(BuildContext context, _RenderCheckbox renderObject) {
    renderObject
      // The `tristate` must be changed before `value` due to the assertion at
      // the beginning of `set value`.
      ..tristate = tristate
      ..value = value
      ..activeColor = activeColor
      ..checkColor = checkColor
      ..inactiveColor = inactiveColor
      ..focusColor = focusColor
      ..hoverColor = hoverColor
      ..reactionColor = reactionColor
      ..inactiveReactionColor = inactiveReactionColor
      ..splashRadius = splashRadius
      ..onChanged = onChanged
      ..additionalConstraints = additionalConstraints
      ..vsync = vsync
      ..hasFocus = hasFocus
      ..hovering = hovering;
  }
}

const double _kEdgeSize = Checkbox.width;
const Radius _kEdgeRadius = Radius.circular(1.0);
const double _kStrokeWidth = 2.0;

class _RenderCheckbox extends RenderToggleable {
  _RenderCheckbox({
    bool? value,
    required bool tristate,
    required Color activeColor,
    required this.checkColor,
    required Color inactiveColor,
    Color? focusColor,
    Color? hoverColor,
    Color? reactionColor,
    Color? inactiveReactionColor,
    required double splashRadius,
    required BoxConstraints additionalConstraints,
    ValueChanged<bool?>? onChanged,
    required bool hasFocus,
    required bool hovering,
    required TickerProvider vsync,
  }) : _oldValue = value,
       super(
         value: value,
         tristate: tristate,
         activeColor: activeColor,
         inactiveColor: inactiveColor,
         focusColor: focusColor,
         hoverColor: hoverColor,
         reactionColor: reactionColor,
         inactiveReactionColor: inactiveReactionColor,
         splashRadius: splashRadius,
         onChanged: onChanged,
         additionalConstraints: additionalConstraints,
         vsync: vsync,
         hasFocus: hasFocus,
         hovering: hovering,
       );

  bool? _oldValue;
  Color checkColor;

  @override
  set value(bool? newValue) {
    if (newValue == value)
      return;
    _oldValue = value;
    super.value = newValue;
  }

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

  // The square outer bounds of the checkbox at t, with the specified origin.
  // At t == 0.0, the outer rect's size is _kEdgeSize (Checkbox.width)
  // At t == 0.5, .. is _kEdgeSize - _kStrokeWidth
  // At t == 1.0, .. is _kEdgeSize
  RRect _outerRectAt(Offset origin, double t) {
    final double inset = 1.0 - (t - 0.5).abs() * 2.0;
    final double size = _kEdgeSize - inset * _kStrokeWidth;
    final Rect rect = Rect.fromLTWH(origin.dx + inset, origin.dy + inset, size, size);
    return RRect.fromRectAndRadius(rect, _kEdgeRadius);
  }

  // The checkbox's border color if value == false, or its fill color when
  // value == true or null.
  Color _colorAt(double t) {
    // As t goes from 0.0 to 0.25, animate from the inactiveColor to activeColor.
    return t >= 0.25 ? activeColor : Color.lerp(inactiveColor, activeColor, t * 4.0)!;
  }

  // White stroke used to paint the check and dash.
  Paint _createStrokePaint() {
    return Paint()
      ..color = checkColor
      ..style = PaintingStyle.stroke
      ..strokeWidth = _kStrokeWidth;
  }

  void _drawBorder(Canvas canvas, RRect outer, double t, Paint paint) {
    assert(t >= 0.0 && t <= 0.5);
    final double size = outer.width;
    // As t goes from 0.0 to 1.0, gradually fill the outer RRect.
    final RRect inner = outer.deflate(math.min(size / 2.0, _kStrokeWidth + size * t));
    canvas.drawDRRect(outer, inner, paint);
  }

  void _drawCheck(Canvas canvas, Offset origin, double t, Paint paint) {
    assert(t >= 0.0 && t <= 1.0);
    // As t goes from 0.0 to 1.0, animate the two check mark strokes from the
    // short side to the long side.
    final Path path = Path();
    const Offset start = Offset(_kEdgeSize * 0.15, _kEdgeSize * 0.45);
    const Offset mid = Offset(_kEdgeSize * 0.4, _kEdgeSize * 0.7);
    const Offset end = Offset(_kEdgeSize * 0.85, _kEdgeSize * 0.25);
    if (t < 0.5) {
      final double strokeT = t * 2.0;
      final Offset drawMid = Offset.lerp(start, mid, strokeT)!;
      path.moveTo(origin.dx + start.dx, origin.dy + start.dy);
      path.lineTo(origin.dx + drawMid.dx, origin.dy + drawMid.dy);
    } else {
      final double strokeT = (t - 0.5) * 2.0;
      final Offset drawEnd = Offset.lerp(mid, end, strokeT)!;
      path.moveTo(origin.dx + start.dx, origin.dy + start.dy);
      path.lineTo(origin.dx + mid.dx, origin.dy + mid.dy);
      path.lineTo(origin.dx + drawEnd.dx, origin.dy + drawEnd.dy);
    }
    canvas.drawPath(path, paint);
  }

  void _drawDash(Canvas canvas, Offset origin, double t, Paint paint) {
    assert(t >= 0.0 && t <= 1.0);
    // As t goes from 0.0 to 1.0, animate the horizontal line from the
    // mid point outwards.
    const Offset start = Offset(_kEdgeSize * 0.2, _kEdgeSize * 0.5);
    const Offset mid = Offset(_kEdgeSize * 0.5, _kEdgeSize * 0.5);
    const Offset end = Offset(_kEdgeSize * 0.8, _kEdgeSize * 0.5);
    final Offset drawStart = Offset.lerp(start, mid, 1.0 - t)!;
    final Offset drawEnd = Offset.lerp(mid, end, t)!;
    canvas.drawLine(origin + drawStart, origin + drawEnd, paint);
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    final Canvas canvas = context.canvas;
    paintRadialReaction(canvas, offset, size.center(Offset.zero));

    final Paint strokePaint = _createStrokePaint();
    final Offset origin = offset + (size / 2.0 - const Size.square(_kEdgeSize) / 2.0 as Offset);
    final AnimationStatus status = position.status;
    final double tNormalized = status == AnimationStatus.forward || status == AnimationStatus.completed
      ? position.value
      : 1.0 - position.value;

    // Four cases: false to null, false to true, null to false, true to false
    if (_oldValue == false || value == false) {
      final double t = value == false ? 1.0 - tNormalized : tNormalized;
      final RRect outer = _outerRectAt(origin, t);
      final Paint paint = Paint()..color = _colorAt(t);

      if (t <= 0.5) {
        _drawBorder(canvas, outer, t, paint);
      } else {
        canvas.drawRRect(outer, paint);

        final double tShrink = (t - 0.5) * 2.0;
        if (_oldValue == null || value == null)
          _drawDash(canvas, origin, tShrink, strokePaint);
        else
          _drawCheck(canvas, origin, tShrink, strokePaint);
      }
    } else { // Two cases: null to true, true to null
      final RRect outer = _outerRectAt(origin, 1.0);
      final Paint paint = Paint() ..color = _colorAt(1.0);
      canvas.drawRRect(outer, paint);

      if (tNormalized <= 0.5) {
        final double tShrink = 1.0 - tNormalized * 2.0;
        if (_oldValue == true)
          _drawCheck(canvas, origin, tShrink, strokePaint);
        else
          _drawDash(canvas, origin, tShrink, strokePaint);
      } else {
        final double tExpand = (tNormalized - 0.5) * 2.0;
        if (value == true)
          _drawCheck(canvas, origin, tExpand, strokePaint);
        else
          _drawDash(canvas, origin, tExpand, strokePaint);
      }
    }
  }
}