// 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 '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.
///
/// {@tool dartpad --template=stateful_widget_scaffold_center}
///
/// This example shows how you can override the default theme of
/// of a [Checkbox] with a [MaterialStateProperty].
/// In this example, the checkbox's color will be `Colors.blue` when the [Checkbox]
/// is being pressed, hovered, or focused. Otherwise, the checkbox's color will
/// be `Colors.red`.
///
/// ```dart
/// bool isChecked = false;
///
/// @override
/// Widget build(BuildContext context) {
///   Color getColor(Set<MaterialState> states) {
///     const Set<MaterialState> interactiveStates = <MaterialState>{
///       MaterialState.pressed,
///       MaterialState.hovered,
///       MaterialState.focused,
///     };
///     if (states.any(interactiveStates.contains)) {
///       return Colors.blue;
///     }
///     return Colors.red;
///   }
///   return Checkbox(
///     checkColor: Colors.white,
///     fillColor: MaterialStateProperty.resolveWith(getColor),
///     value: isChecked,
///     onChanged: (bool? value) {
///       setState(() {
///         isChecked = value!;
///       });
///     },
///   );
/// }
/// ```
/// {@end-tool}
///
/// 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,
    this.shape,
    this.side,
  }) : 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;

  /// {@template flutter.material.checkbox.shape}
  /// The shape of the checkbox's [Material].
  /// {@endtemplate}
  ///
  /// If this property is null then [CheckboxThemeData.shape] of [ThemeData.checkboxTheme]
  /// is used. If that's null then the shape will be a [RoundedRectangleBorder]
  /// with a circular corner radius of 1.0.
  final OutlinedBorder? shape;

  /// {@template flutter.material.checkbox.side}
  /// The color and width of the checkbox's border.
  ///
  /// This property can be a [MaterialStateBorderSide] that can
  /// specify different border color and widths depending on the
  /// checkbox's state.
  ///
  /// Resolves in the following states:
  ///  * [MaterialState.pressed].
  ///  * [MaterialState.selected].
  ///  * [MaterialState.hovered].
  ///  * [MaterialState.focused].
  ///  * [MaterialState.disabled].
  ///
  /// If this property is not a [MaterialStateBorderSide] and it is
  /// non-null, then it is only rendered when the checkbox's value is
  /// false. The difference in interpretation is for backwards
  /// compatibility.
  /// {@endtemplate}
  ///
  /// If this property is null then [CheckboxThemeData.side] of [ThemeData.checkboxTheme]
  /// is used. If that's null then the side will be width 2.
  final BorderSide? side;

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

  @override
  State<Checkbox> createState() => _CheckboxState();
}

class _CheckboxState extends State<Checkbox> with TickerProviderStateMixin, ToggleableStateMixin {
  final _CheckboxPainter _painter = _CheckboxPainter();
  bool? _previousValue;

  @override
  void initState() {
    super.initState();
    _previousValue = widget.value;
  }

  @override
  void didUpdateWidget(Checkbox oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.value != widget.value) {
      _previousValue = oldWidget.value;
      animateToValue();
    }
  }

  @override
  void dispose() {
    _painter.dispose();
    super.dispose();
  }

  @override
  ValueChanged<bool?>? get onChanged => widget.onChanged;

  @override
  bool get tristate => widget.tristate;

  @override
  bool? get value => widget.value;

  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;
    });
  }

  BorderSide? _resolveSide(BorderSide? side) {
    if (side is MaterialStateBorderSide)
      return MaterialStateProperty.resolveAs<BorderSide?>(side, states);
    if (!states.contains(MaterialState.selected))
      return side;
    return null;
  }

  @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 MaterialStateProperty<MouseCursor> effectiveMouseCursor = MaterialStateProperty.resolveWith<MouseCursor>((Set<MaterialState> states) {
      return MaterialStateProperty.resolveAs<MouseCursor?>(widget.mouseCursor, states)
        ?? themeData.checkboxTheme.mouseCursor?.resolve(states)
        ?? MaterialStateMouseCursor.clickable.resolve(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 Semantics(
      checked: widget.value == true,
      child: buildToggleable(
        mouseCursor: effectiveMouseCursor,
        focusNode: widget.focusNode,
        autofocus: widget.autofocus,
        size: size,
        painter: _painter
          ..position = position
          ..reaction = reaction
          ..reactionFocusFade = reactionFocusFade
          ..reactionHoverFade = reactionHoverFade
          ..inactiveReactionColor = effectiveInactivePressedOverlayColor
          ..reactionColor = effectiveActivePressedOverlayColor
          ..hoverColor = effectiveHoverOverlayColor
          ..focusColor = effectiveFocusOverlayColor
          ..splashRadius = widget.splashRadius ?? themeData.checkboxTheme.splashRadius ?? kRadialReactionRadius
          ..downPosition = downPosition
          ..isFocused = states.contains(MaterialState.focused)
          ..isHovered = states.contains(MaterialState.hovered)
          ..activeColor = effectiveActiveColor
          ..inactiveColor = effectiveInactiveColor
          ..checkColor = effectiveCheckColor
          ..value = value
          ..previousValue = _previousValue
          ..shape = widget.shape ?? themeData.checkboxTheme.shape ?? const RoundedRectangleBorder(
              borderRadius: BorderRadius.all(Radius.circular(1.0)),
          )
          ..side = _resolveSide(widget.side) ?? _resolveSide(themeData.checkboxTheme.side),
      ),
    );
  }
}

const double _kEdgeSize = Checkbox.width;
const double _kStrokeWidth = 2.0;

class _CheckboxPainter extends ToggleablePainter {
  Color get checkColor => _checkColor!;
  Color? _checkColor;
  set checkColor(Color value) {
    if (_checkColor == value) {
      return;
    }
    _checkColor = value;
    notifyListeners();
  }

  bool? get value => _value;
  bool? _value;
  set value(bool? value) {
    if (_value == value) {
      return;
    }
    _value = value;
    notifyListeners();
  }

  bool? get previousValue => _previousValue;
  bool? _previousValue;
  set previousValue(bool? value) {
    if (_previousValue == value) {
      return;
    }
    _previousValue = value;
    notifyListeners();
  }

  OutlinedBorder get shape => _shape!;
  OutlinedBorder? _shape;
  set shape(OutlinedBorder value) {
    if (_shape == value) {
      return;
    }
    _shape = value;
    notifyListeners();
  }

  BorderSide? get side => _side;
  BorderSide? _side;
  set side(BorderSide? value) {
    if (_side == value) {
      return;
    }
    _side = value;
    notifyListeners();
  }

  // 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
  Rect _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 rect;
  }

  // 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 _drawBox(Canvas canvas, Rect outer, Paint paint, BorderSide? side, bool fill) {
    if (fill) {
      canvas.drawPath(shape.getOuterPath(outer), paint);
    }
    if (side != null) {
      shape.copyWith(side: side).paint(canvas, outer);
    }
  }

  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(Canvas canvas, Size size) {
    paintRadialReaction(canvas: canvas, origin: size.center(Offset.zero));

    final Paint strokePaint = _createStrokePaint();
    final Offset origin = 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 (previousValue == false || value == false) {
      final double t = value == false ? 1.0 - tNormalized : tNormalized;
      final Rect outer = _outerRectAt(origin, t);
      final Paint paint = Paint()..color = _colorAt(t);

      if (t <= 0.5) {
        final BorderSide border = side ?? BorderSide(width: 2, color: paint.color);
        _drawBox(canvas, outer, paint, border, false); // only paint the border
      } else {
        _drawBox(canvas, outer, paint, side, true);
        final double tShrink = (t - 0.5) * 2.0;
        if (previousValue == null || value == null)
          _drawDash(canvas, origin, tShrink, strokePaint);
        else
          _drawCheck(canvas, origin, tShrink, strokePaint);
      }
    } else { // Two cases: null to true, true to null
      final Rect outer = _outerRectAt(origin, 1.0);
      final Paint paint = Paint() ..color = _colorAt(1.0);

      _drawBox(canvas, outer, paint, side, true);
      if (tNormalized <= 0.5) {
        final double tShrink = 1.0 - tNormalized * 2.0;
        if (previousValue == 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);
      }
    }
  }
}
