Flutter | AnimatedCrossFade - 交叉淡入 Widget

const AnimatedCrossFade({
  Key key,
  @required this.firstChild,
  @required this.secondChild,
  this.firstCurve = Curves.linear,
  this.secondCurve = Curves.linear,
  this.sizeCurve = Curves.linear,
  this.alignment = Alignment.topCenter,
  @required this.crossFadeState,
  @required this.duration,
  this.reverseDuration,
  this.layoutBuilder = defaultLayoutBuilder,
}) : assert(firstChild != null),
assert(secondChild != null),
assert(firstCurve != null),
assert(secondCurve != null),
assert(sizeCurve != null),
assert(alignment != null),
assert(crossFadeState != null),
assert(duration != null),
assert(layoutBuilder != null),
super(key: key);

enum CrossFadeState {
  /// Show the first child ([AnimatedCrossFade.firstChild]) and hide the second
  /// ([AnimatedCrossFade.secondChild]]).
  showFirst,

  /// Show the second child ([AnimatedCrossFade.secondChild]) and hide the first
  /// ([AnimatedCrossFade.firstChild]).
  showSecond,

AnimatedCrossFade(
  duration: const Duration(seconds: 3),
  firstChild: const FlutterLogo(style: FlutterLogoStyle.horizontal, size: 100.0),
  secondChild: const FlutterLogo(style: FlutterLogoStyle.stacked, size: 100.0),
  crossFadeState: _first ? CrossFadeState.showFirst : CrossFadeState.showSecond,
)

/// The animation is controlled through the [crossFadeState] parameter.
/// [firstCurve] and [secondCurve] represent the opacity curves of the two
/// children. The [firstCurve] is inverted, i.e. it fades out when providing a
/// growing curve like [Curves.linear]. The [sizeCurve] is the curve used to
/// animate between the size of the fading-out child and the size of the
/// fading-in child.
///
/// This widget is intended to be used to fade a pair of widgets with the same
/// width. In the case where the two children have different heights, the
/// animation crops overflowing children during the animation by aligning their
/// top edge, which means that the bottom will be clipped.
///
/// The animation is automatically triggered when an existing
/// [AnimatedCrossFade] is rebuilt with a different value for the
/// [crossFadeState] property.


动画是通过[crossFadeState]参数控制的。[firstCurve]和[secondCurve]表示两个孩子的不透明度曲线。[firstCurve]是倒置的，即当提供诸如[Curves.linear]之类的增长曲线时，它会淡出。[sizeCurve]是用于在淡出子项的大小和淡入子项的大小之间进行动画处理的曲线。

此小部件用于淡化一对具有相同宽度的小部件。如果两个孩子的高度不同，则动画会在动画过程中通过对齐它们的顶部边缘来裁剪溢出的child，这意味着将裁剪底部。

当以不同的值重建现有的[AnimatedCrossFade]时，动画会自动触发。

@override
Widget build(BuildContext context) {
  // ...
  // 去掉于此无关代码
  return ClipRect(
    child: AnimatedSize(
      alignment: widget.alignment,
      duration: widget.duration,
      reverseDuration: widget.reverseDuration,
      curve: widget.sizeCurve,
      vsync: this,
      child: widget.layoutBuilder(topChild, topKey, bottomChild, bottomKey),
    ),
  );
}

// -----------------------------------------
//           defaultLayoutBuilder
// -----------------------------------------
static Widget defaultLayoutBuilder(Widget topChild, Key topChildKey, Widget bottomChild, Key bottomChildKey) {
  return Stack(
    overflow: Overflow.visible,
    children: <Widget>[
      Positioned(
        key: bottomChildKey,
        left: 0.0,
        top: 0.0,
        right: 0.0,
        child: bottomChild,
      ),
      Positioned(
        key: topChildKey,
        child: topChild,
      ),
    ],
  );
}

layoutBuilder: (topChild, topChildKey, bottomChild, bottomChildKey) {
  return Stack(
    overflow: Overflow.visible,
    alignment: Alignment.center,
    children: <Widget>[
      Positioned(
        key: topChildKey,
        child: topChild,
      ),
      Positioned(
        key: bottomChildKey,
        top:0,
        child: bottomChild,
      ),
    ],
  );
}