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

 // Copyright 2014 The Flutter Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

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

 import 'package:flutter/animation.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';

 // How close the begin and end points must be to an axis to be considered
 // vertical or horizontal.
 const double _kOnAxisDelta = 2.0;

 /// A [Tween] that interpolates an [Offset] along a circular arc.
 ///
 /// This class specializes the interpolation of [Tween<Offset>] so that instead
 /// of a straight line, the intermediate points follow the arc of a circle in a
 /// manner consistent with material design principles.
 ///
 /// The arc's radius is related to the bounding box that contains the [begin]
 /// and [end] points. If the bounding box is taller than it is wide, then the
 /// center of the circle will be horizontally aligned with the end point.
 /// Otherwise the center of the circle will be aligned with the begin point.
 /// The arc's sweep is always less than or equal to 90 degrees.
 ///
 /// See also:
 ///
 ///  * [Tween], for a discussion on how to use interpolation objects.
 ///  * [MaterialRectArcTween], which extends this concept to interpolating [Rect]s.
 class MaterialPointArcTween extends Tween<Offset> {
   /// Creates a [Tween] for animating [Offset]s along a circular arc.
   ///
   /// The [begin] and [end] properties must be non-null before the tween is
   /// first used, but the arguments can be null if the values are going to be
   /// filled in later.
   MaterialPointArcTween({
     Offset begin,
     Offset end,
   }) : super(begin: begin, end: end);

   bool _dirty = true;

   void _initialize() {
     assert(begin != null);
     assert(end != null);

     // An explanation with a diagram can be found at https://goo.gl/vMSdRg
     final Offset delta = end - begin;
     final double deltaX = delta.dx.abs();
     final double deltaY = delta.dy.abs();
     final double distanceFromAtoB = delta.distance;
     final Offset c = Offset(end.dx, begin.dy);

     double sweepAngle() => 2.0 * math.asin(distanceFromAtoB / (2.0 * _radius));

     if (deltaX > _kOnAxisDelta && deltaY > _kOnAxisDelta) {
       if (deltaX < deltaY) {
         _radius = distanceFromAtoB * distanceFromAtoB / (c - begin).distance / 2.0;
         _center = Offset(end.dx + _radius * (begin.dx - end.dx).sign, end.dy);
         if (begin.dx < end.dx) {
           _beginAngle = sweepAngle() * (begin.dy - end.dy).sign;
           _endAngle = 0.0;
         } else {
           _beginAngle = math.pi + sweepAngle() * (end.dy - begin.dy).sign;
           _endAngle = math.pi;
         }
       } else {
         _radius = distanceFromAtoB * distanceFromAtoB / (c - end).distance / 2.0;
         _center = Offset(begin.dx, begin.dy + (end.dy - begin.dy).sign * _radius);
         if (begin.dy < end.dy) {
           _beginAngle = -math.pi / 2.0;
           _endAngle = _beginAngle + sweepAngle() * (end.dx - begin.dx).sign;
         } else {
           _beginAngle = math.pi / 2.0;
           _endAngle = _beginAngle + sweepAngle() * (begin.dx - end.dx).sign;
         }
       }
       assert(_beginAngle != null);
       assert(_endAngle != null);
     } else {
       _beginAngle = null;
       _endAngle = null;
     }
     _dirty = false;
   }

   /// The center of the circular arc, null if [begin] and [end] are horizontally or
   /// vertically aligned, or if either is null.
   Offset get center {
     if (begin == null || end == null)
       return null;
     if (_dirty)
       _initialize();
     return _center;
   }
   Offset _center;

   /// The radius of the circular arc, null if [begin] and [end] are horizontally or
   /// vertically aligned, or if either is null.
   double get radius {
     if (begin == null || end == null)
       return null;
     if (_dirty)
       _initialize();
     return _radius;
   }
   double _radius;

   /// The beginning of the arc's sweep in radians, measured from the positive x
   /// axis. Positive angles turn clockwise.
   ///
   /// This will be null if [begin] and [end] are horizontally or vertically
   /// aligned, or if either is null.
   double get beginAngle {
     if (begin == null || end == null)
       return null;
     if (_dirty)
       _initialize();
     return _beginAngle;
   }
   double _beginAngle;

   /// The end of the arc's sweep in radians, measured from the positive x axis.
   /// Positive angles turn clockwise.
   ///
   /// This will be null if [begin] and [end] are horizontally or vertically
   /// aligned, or if either is null.
   double get endAngle {
     if (begin == null || end == null)
       return null;
     if (_dirty)
       _initialize();
     return _beginAngle;
   }
   double _endAngle;

   @override
   set begin(Offset value) {
     if (value != begin) {
       super.begin = value;
       _dirty = true;
     }
   }

   @override
   set end(Offset value) {
     if (value != end) {
       super.end = value;
       _dirty = true;
     }
   }

   @override
   Offset lerp(double t) {
     if (_dirty)
       _initialize();
     if (t == 0.0)
       return begin;
     if (t == 1.0)
       return end;
     if (_beginAngle == null || _endAngle == null)
       return Offset.lerp(begin, end, t);
     final double angle = lerpDouble(_beginAngle, _endAngle, t);
     final double x = math.cos(angle) * _radius;
     final double y = math.sin(angle) * _radius;
     return _center + Offset(x, y);
   }

   @override
   String toString() {
     return '${objectRuntimeType(this, 'MaterialPointArcTween')}($begin \u2192 $end; center=$center, radius=$radius, beginAngle=$beginAngle, endAngle=$endAngle)';
   }
 }

 enum _CornerId {
   topLeft,
   topRight,
   bottomLeft,
   bottomRight
 }

 class _Diagonal {
   const _Diagonal(this.beginId, this.endId);
   final _CornerId beginId;
   final _CornerId endId;
 }

 const List<_Diagonal> _allDiagonals = <_Diagonal>[
   _Diagonal(_CornerId.topLeft, _CornerId.bottomRight),
   _Diagonal(_CornerId.bottomRight, _CornerId.topLeft),
   _Diagonal(_CornerId.topRight, _CornerId.bottomLeft),
   _Diagonal(_CornerId.bottomLeft, _CornerId.topRight),
 ];

 typedef _KeyFunc<T> = double Function(T input);

 // Select the element for which the key function returns the maximum value.
 T _maxBy<T>(Iterable<T> input, _KeyFunc<T> keyFunc) {
   T maxValue;
   double maxKey;
   for (final T value in input) {
     final double key = keyFunc(value);
     if (maxKey == null || key > maxKey) {
       maxValue = value;
       maxKey = key;
     }
   }
   return maxValue;
 }

 /// A [Tween] that interpolates a [Rect] by having its opposite corners follow
 /// circular arcs.
 ///
 /// This class specializes the interpolation of [Tween<Rect>] so that instead of
 /// growing or shrinking linearly, opposite corners of the rectangle follow arcs
 /// in a manner consistent with material design principles.
 ///
 /// Specifically, the rectangle corners whose diagonals are closest to the overall
 /// direction of the animation follow arcs defined with [MaterialPointArcTween].
 ///
 /// See also:
 ///
 ///  * [MaterialRectCenterArcTween], which interpolates a rect along a circular
 ///    arc between the begin and end [Rect]'s centers.
 ///  * [Tween], for a discussion on how to use interpolation objects.
 ///  * [MaterialPointArcTween], the analog for [Offset] interpolation.
 ///  * [RectTween], which does a linear rectangle interpolation.
 ///  * [Hero.createRectTween], which can be used to specify the tween that defines
 ///    a hero's path.
 class MaterialRectArcTween extends RectTween {
   /// Creates a [Tween] for animating [Rect]s along a circular arc.
   ///
   /// The [begin] and [end] properties must be non-null before the tween is
   /// first used, but the arguments can be null if the values are going to be
   /// filled in later.
   MaterialRectArcTween({
     Rect begin,
     Rect end,
   }) : super(begin: begin, end: end);

   bool _dirty = true;

   void _initialize() {
     assert(begin != null);
     assert(end != null);
     final Offset centersVector = end.center - begin.center;
     final _Diagonal diagonal = _maxBy<_Diagonal>(_allDiagonals, (_Diagonal d) => _diagonalSupport(centersVector, d));
     _beginArc = MaterialPointArcTween(
       begin: _cornerFor(begin, diagonal.beginId),
       end: _cornerFor(end, diagonal.beginId),
     );
     _endArc = MaterialPointArcTween(
       begin: _cornerFor(begin, diagonal.endId),
       end: _cornerFor(end, diagonal.endId),
     );
     _dirty = false;
   }

   double _diagonalSupport(Offset centersVector, _Diagonal diagonal) {
     final Offset delta = _cornerFor(begin, diagonal.endId) - _cornerFor(begin, diagonal.beginId);
     final double length = delta.distance;
     return centersVector.dx * delta.dx / length + centersVector.dy * delta.dy / length;
   }

   Offset _cornerFor(Rect rect, _CornerId id) {
     switch (id) {
       case _CornerId.topLeft: return rect.topLeft;
       case _CornerId.topRight: return rect.topRight;
       case _CornerId.bottomLeft: return rect.bottomLeft;
       case _CornerId.bottomRight: return rect.bottomRight;
     }
     return Offset.zero;
   }

   /// The path of the corresponding [begin], [end] rectangle corners that lead
   /// the animation.
   MaterialPointArcTween get beginArc {
     if (begin == null)
       return null;
     if (_dirty)
       _initialize();
     return _beginArc;
   }
   MaterialPointArcTween _beginArc;

   /// The path of the corresponding [begin], [end] rectangle corners that trail
   /// the animation.
   MaterialPointArcTween get endArc {
     if (end == null)
       return null;
     if (_dirty)
       _initialize();
     return _endArc;
   }
   MaterialPointArcTween _endArc;

   @override
   set begin(Rect value) {
     if (value != begin) {
       super.begin = value;
       _dirty = true;
     }
   }

   @override
   set end(Rect value) {
     if (value != end) {
       super.end = value;
       _dirty = true;
     }
   }

   @override
   Rect lerp(double t) {
     if (_dirty)
       _initialize();
     if (t == 0.0)
       return begin;
     if (t == 1.0)
       return end;
     return Rect.fromPoints(_beginArc.lerp(t), _endArc.lerp(t));
   }

   @override
   String toString() {
     return '${objectRuntimeType(this, 'MaterialRectArcTween')}($begin \u2192 $end; beginArc=$beginArc, endArc=$endArc)';
   }
 }

 /// A [Tween] that interpolates a [Rect] by moving it along a circular arc from
 /// [begin]'s [Rect.center] to [end]'s [Rect.center] while interpolating the
 /// rectangle's width and height.
 ///
 /// The arc that defines that center of the interpolated rectangle as it morphs
 /// from [begin] to [end] is a [MaterialPointArcTween].
 ///
 /// See also:
 ///
 ///  * [MaterialRectArcTween], A [Tween] that interpolates a [Rect] by having
 ///    its opposite corners follow circular arcs.
 ///  * [Tween], for a discussion on how to use interpolation objects.
 ///  * [MaterialPointArcTween], the analog for [Offset] interpolation.
 ///  * [RectTween], which does a linear rectangle interpolation.
 ///  * [Hero.createRectTween], which can be used to specify the tween that defines
 ///    a hero's path.
 class MaterialRectCenterArcTween extends RectTween {
   /// Creates a [Tween] for animating [Rect]s along a circular arc.
   ///
   /// The [begin] and [end] properties must be non-null before the tween is
   /// first used, but the arguments can be null if the values are going to be
   /// filled in later.
   MaterialRectCenterArcTween({
     Rect begin,
     Rect end,
   }) : super(begin: begin, end: end);

   bool _dirty = true;

   void _initialize() {
     assert(begin != null);
     assert(end != null);
     _centerArc = MaterialPointArcTween(
       begin: begin.center,
       end: end.center,
     );
     _dirty = false;
   }

   /// If [begin] and [end] are non-null, returns a tween that interpolates along
   /// a circular arc between [begin]'s [Rect.center] and [end]'s [Rect.center].
   MaterialPointArcTween get centerArc {
     if (begin == null || end == null)
       return null;
     if (_dirty)
       _initialize();
     return _centerArc;
   }
   MaterialPointArcTween _centerArc;

   @override
   set begin(Rect value) {
     if (value != begin) {
       super.begin = value;
       _dirty = true;
     }
   }

   @override
   set end(Rect value) {
     if (value != end) {
       super.end = value;
       _dirty = true;
     }
   }

   @override
   Rect lerp(double t) {
     if (_dirty)
       _initialize();
     if (t == 0.0)
       return begin;
     if (t == 1.0)
       return end;
     final Offset center = _centerArc.lerp(t);
     final double width = lerpDouble(begin.width, end.width, t);
     final double height = lerpDouble(begin.height, end.height, t);
     return Rect.fromLTWH(center.dx - width / 2.0, center.dy - height / 2.0, width, height);
   }

   @override
   String toString() {
     return '${objectRuntimeType(this, 'MaterialRectCenterArcTween')}($begin \u2192 $end; centerArc=$centerArc)';
   }
 }
	// 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 'dart:ui' show lerpDouble;

	import 'package:flutter/animation.dart';
	import 'package:flutter/foundation.dart';
	import 'package:flutter/painting.dart';

	// How close the begin and end points must be to an axis to be considered
	// vertical or horizontal.
	const double _kOnAxisDelta = 2.0;

	/// A [Tween] that interpolates an [Offset] along a circular arc.
	///
	/// This class specializes the interpolation of [Tween<Offset>] so that instead
	/// of a straight line, the intermediate points follow the arc of a circle in a
	/// manner consistent with material design principles.
	///
	/// The arc's radius is related to the bounding box that contains the [begin]
	/// and [end] points. If the bounding box is taller than it is wide, then the
	/// center of the circle will be horizontally aligned with the end point.
	/// Otherwise the center of the circle will be aligned with the begin point.
	/// The arc's sweep is always less than or equal to 90 degrees.
	///
	/// See also:
	///
	/// * [Tween], for a discussion on how to use interpolation objects.
	/// * [MaterialRectArcTween], which extends this concept to interpolating [Rect]s.
	class MaterialPointArcTween extends Tween<Offset> {
	/// Creates a [Tween] for animating [Offset]s along a circular arc.
	///
	/// The [begin] and [end] properties must be non-null before the tween is
	/// first used, but the arguments can be null if the values are going to be
	/// filled in later.
	MaterialPointArcTween({
	Offset begin,
	Offset end,
	}) : super(begin: begin, end: end);

	bool _dirty = true;

	void _initialize() {
	assert(begin != null);
	assert(end != null);

	// An explanation with a diagram can be found at https://goo.gl/vMSdRg
	final Offset delta = end - begin;
	final double deltaX = delta.dx.abs();
	final double deltaY = delta.dy.abs();
	final double distanceFromAtoB = delta.distance;
	final Offset c = Offset(end.dx, begin.dy);

	double sweepAngle() => 2.0 * math.asin(distanceFromAtoB / (2.0 * _radius));

	if (deltaX > _kOnAxisDelta && deltaY > _kOnAxisDelta) {
	if (deltaX < deltaY) {
	_radius = distanceFromAtoB * distanceFromAtoB / (c - begin).distance / 2.0;
	_center = Offset(end.dx + _radius * (begin.dx - end.dx).sign, end.dy);
	if (begin.dx < end.dx) {
	_beginAngle = sweepAngle() * (begin.dy - end.dy).sign;
	_endAngle = 0.0;
	} else {
	_beginAngle = math.pi + sweepAngle() * (end.dy - begin.dy).sign;
	_endAngle = math.pi;
	}
	} else {
	_radius = distanceFromAtoB * distanceFromAtoB / (c - end).distance / 2.0;
	_center = Offset(begin.dx, begin.dy + (end.dy - begin.dy).sign * _radius);
	if (begin.dy < end.dy) {
	_beginAngle = -math.pi / 2.0;
	_endAngle = _beginAngle + sweepAngle() * (end.dx - begin.dx).sign;
	} else {
	_beginAngle = math.pi / 2.0;
	_endAngle = _beginAngle + sweepAngle() * (begin.dx - end.dx).sign;
	}
	}
	assert(_beginAngle != null);
	assert(_endAngle != null);
	} else {
	_beginAngle = null;
	_endAngle = null;
	}
	_dirty = false;
	}

	/// The center of the circular arc, null if [begin] and [end] are horizontally or
	/// vertically aligned, or if either is null.
	Offset get center {
	if (begin == null \|\| end == null)
	return null;
	if (_dirty)
	_initialize();
	return _center;
	}
	Offset _center;

	/// The radius of the circular arc, null if [begin] and [end] are horizontally or
	/// vertically aligned, or if either is null.
	double get radius {
	if (begin == null \|\| end == null)
	return null;
	if (_dirty)
	_initialize();
	return _radius;
	}
	double _radius;

	/// The beginning of the arc's sweep in radians, measured from the positive x
	/// axis. Positive angles turn clockwise.
	///
	/// This will be null if [begin] and [end] are horizontally or vertically
	/// aligned, or if either is null.
	double get beginAngle {
	if (begin == null \|\| end == null)
	return null;
	if (_dirty)
	_initialize();
	return _beginAngle;
	}
	double _beginAngle;

	/// The end of the arc's sweep in radians, measured from the positive x axis.
	/// Positive angles turn clockwise.
	///
	/// This will be null if [begin] and [end] are horizontally or vertically
	/// aligned, or if either is null.
	double get endAngle {
	if (begin == null \|\| end == null)
	return null;
	if (_dirty)
	_initialize();
	return _beginAngle;
	}
	double _endAngle;

	@override
	set begin(Offset value) {
	if (value != begin) {
	super.begin = value;
	_dirty = true;
	}
	}

	@override
	set end(Offset value) {
	if (value != end) {
	super.end = value;
	_dirty = true;
	}
	}

	@override
	Offset lerp(double t) {
	if (_dirty)
	_initialize();
	if (t == 0.0)
	return begin;
	if (t == 1.0)
	return end;
	if (_beginAngle == null \|\| _endAngle == null)
	return Offset.lerp(begin, end, t);
	final double angle = lerpDouble(_beginAngle, _endAngle, t);
	final double x = math.cos(angle) * _radius;
	final double y = math.sin(angle) * _radius;
	return _center + Offset(x, y);
	}

	@override
	String toString() {
	return '${objectRuntimeType(this, 'MaterialPointArcTween')}($begin \u2192 $end; center=$center, radius=$radius, beginAngle=$beginAngle, endAngle=$endAngle)';
	}
	}

	enum _CornerId {
	topLeft,
	topRight,
	bottomLeft,
	bottomRight
	}

	class _Diagonal {
	const _Diagonal(this.beginId, this.endId);
	final _CornerId beginId;
	final _CornerId endId;
	}

	const List<_Diagonal> _allDiagonals = <_Diagonal>[
	_Diagonal(_CornerId.topLeft, _CornerId.bottomRight),
	_Diagonal(_CornerId.bottomRight, _CornerId.topLeft),
	_Diagonal(_CornerId.topRight, _CornerId.bottomLeft),
	_Diagonal(_CornerId.bottomLeft, _CornerId.topRight),
	];

	typedef _KeyFunc<T> = double Function(T input);

	// Select the element for which the key function returns the maximum value.
	T _maxBy<T>(Iterable<T> input, _KeyFunc<T> keyFunc) {
	T maxValue;
	double maxKey;
	for (final T value in input) {
	final double key = keyFunc(value);
	if (maxKey == null \|\| key > maxKey) {
	maxValue = value;
	maxKey = key;
	}
	}
	return maxValue;
	}

	/// A [Tween] that interpolates a [Rect] by having its opposite corners follow
	/// circular arcs.
	///
	/// This class specializes the interpolation of [Tween<Rect>] so that instead of
	/// growing or shrinking linearly, opposite corners of the rectangle follow arcs
	/// in a manner consistent with material design principles.
	///
	/// Specifically, the rectangle corners whose diagonals are closest to the overall
	/// direction of the animation follow arcs defined with [MaterialPointArcTween].
	///
	/// See also:
	///
	/// * [MaterialRectCenterArcTween], which interpolates a rect along a circular
	/// arc between the begin and end [Rect]'s centers.
	/// * [Tween], for a discussion on how to use interpolation objects.
	/// * [MaterialPointArcTween], the analog for [Offset] interpolation.
	/// * [RectTween], which does a linear rectangle interpolation.
	/// * [Hero.createRectTween], which can be used to specify the tween that defines
	/// a hero's path.
	class MaterialRectArcTween extends RectTween {
	/// Creates a [Tween] for animating [Rect]s along a circular arc.
	///
	/// The [begin] and [end] properties must be non-null before the tween is
	/// first used, but the arguments can be null if the values are going to be
	/// filled in later.
	MaterialRectArcTween({
	Rect begin,
	Rect end,
	}) : super(begin: begin, end: end);

	bool _dirty = true;

	void _initialize() {
	assert(begin != null);
	assert(end != null);
	final Offset centersVector = end.center - begin.center;
	final _Diagonal diagonal = _maxBy<_Diagonal>(_allDiagonals, (_Diagonal d) => _diagonalSupport(centersVector, d));
	_beginArc = MaterialPointArcTween(
	begin: _cornerFor(begin, diagonal.beginId),
	end: _cornerFor(end, diagonal.beginId),
	);
	_endArc = MaterialPointArcTween(
	begin: _cornerFor(begin, diagonal.endId),
	end: _cornerFor(end, diagonal.endId),
	);
	_dirty = false;
	}

	double _diagonalSupport(Offset centersVector, _Diagonal diagonal) {
	final Offset delta = _cornerFor(begin, diagonal.endId) - _cornerFor(begin, diagonal.beginId);
	final double length = delta.distance;
	return centersVector.dx * delta.dx / length + centersVector.dy * delta.dy / length;
	}

	Offset _cornerFor(Rect rect, _CornerId id) {
	switch (id) {
	case _CornerId.topLeft: return rect.topLeft;
	case _CornerId.topRight: return rect.topRight;
	case _CornerId.bottomLeft: return rect.bottomLeft;
	case _CornerId.bottomRight: return rect.bottomRight;
	}
	return Offset.zero;
	}

	/// The path of the corresponding [begin], [end] rectangle corners that lead
	/// the animation.
	MaterialPointArcTween get beginArc {
	if (begin == null)
	return null;
	if (_dirty)
	_initialize();
	return _beginArc;
	}
	MaterialPointArcTween _beginArc;

	/// The path of the corresponding [begin], [end] rectangle corners that trail
	/// the animation.
	MaterialPointArcTween get endArc {
	if (end == null)
	return null;
	if (_dirty)
	_initialize();
	return _endArc;
	}
	MaterialPointArcTween _endArc;

	@override
	set begin(Rect value) {
	if (value != begin) {
	super.begin = value;
	_dirty = true;
	}
	}

	@override
	set end(Rect value) {
	if (value != end) {
	super.end = value;
	_dirty = true;
	}
	}

	@override
	Rect lerp(double t) {
	if (_dirty)
	_initialize();
	if (t == 0.0)
	return begin;
	if (t == 1.0)
	return end;
	return Rect.fromPoints(_beginArc.lerp(t), _endArc.lerp(t));
	}

	@override
	String toString() {
	return '${objectRuntimeType(this, 'MaterialRectArcTween')}($begin \u2192 $end; beginArc=$beginArc, endArc=$endArc)';
	}
	}

	/// A [Tween] that interpolates a [Rect] by moving it along a circular arc from
	/// [begin]'s [Rect.center] to [end]'s [Rect.center] while interpolating the
	/// rectangle's width and height.
	///
	/// The arc that defines that center of the interpolated rectangle as it morphs
	/// from [begin] to [end] is a [MaterialPointArcTween].
	///
	/// See also:
	///
	/// * [MaterialRectArcTween], A [Tween] that interpolates a [Rect] by having
	/// its opposite corners follow circular arcs.
	/// * [Tween], for a discussion on how to use interpolation objects.
	/// * [MaterialPointArcTween], the analog for [Offset] interpolation.
	/// * [RectTween], which does a linear rectangle interpolation.
	/// * [Hero.createRectTween], which can be used to specify the tween that defines
	/// a hero's path.
	class MaterialRectCenterArcTween extends RectTween {
	/// Creates a [Tween] for animating [Rect]s along a circular arc.
	///
	/// The [begin] and [end] properties must be non-null before the tween is
	/// first used, but the arguments can be null if the values are going to be
	/// filled in later.
	MaterialRectCenterArcTween({
	Rect begin,
	Rect end,
	}) : super(begin: begin, end: end);

	bool _dirty = true;

	void _initialize() {
	assert(begin != null);
	assert(end != null);
	_centerArc = MaterialPointArcTween(
	begin: begin.center,
	end: end.center,
	);
	_dirty = false;
	}

	/// If [begin] and [end] are non-null, returns a tween that interpolates along
	/// a circular arc between [begin]'s [Rect.center] and [end]'s [Rect.center].
	MaterialPointArcTween get centerArc {
	if (begin == null \|\| end == null)
	return null;
	if (_dirty)
	_initialize();
	return _centerArc;
	}
	MaterialPointArcTween _centerArc;

	@override
	set begin(Rect value) {
	if (value != begin) {
	super.begin = value;
	_dirty = true;
	}
	}

	@override
	set end(Rect value) {
	if (value != end) {
	super.end = value;
	_dirty = true;
	}
	}

	@override
	Rect lerp(double t) {
	if (_dirty)
	_initialize();
	if (t == 0.0)
	return begin;
	if (t == 1.0)
	return end;
	final Offset center = _centerArc.lerp(t);
	final double width = lerpDouble(begin.width, end.width, t);
	final double height = lerpDouble(begin.height, end.height, t);
	return Rect.fromLTWH(center.dx - width / 2.0, center.dy - height / 2.0, width, height);
	}

	@override
	String toString() {
	return '${objectRuntimeType(this, 'MaterialRectCenterArcTween')}($begin \u2192 $end; centerArc=$centerArc)';
	}
	}