AnimationController的持续时间不影响PositionedTransition

Question

代码实验室上有一个很好的例子，关于Flutter中的Material Advanced Components。但是，无论我为 AnimationController 的 duration 更改了什么类型和数量，它都不会影响 PositionedTransition 。 我曾经与 CurvedAnimation 一起工作，并且持续时间很好。 我在这里没有发现任何问题，请给我一些建议。

我尝试了什么：

_controller = AnimationController(
        duration: Duration(seconds: 5), value: 1.0, vsync: this);

代码段：

 class _BackdropState extends State<Backdrop>
    with SingleTickerProviderStateMixin {
  final double _kFlingVelocity = 2.0;
  AnimationController _controller;
  final GlobalKey _backdropKey = GlobalKey(debugLabel: 'Backdrop');

  @override
  void didUpdateWidget(Backdrop oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.currentCategory != oldWidget.currentCategory) {
      _toggleBackdropLayerVisibility();
    } else if (!_frontLayerVisible) {
      _controller.fling(velocity: _kFlingVelocity);
    }
  }

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
        duration: Duration(microseconds: 300), value: 1.0, vsync: this);
  }

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

  bool get _frontLayerVisible {
    final AnimationStatus animationStatus = _controller.status;
    return animationStatus == AnimationStatus.completed ||
        animationStatus == AnimationStatus.forward;
  }

  void _toggleBackdropLayerVisibility() {
    _controller.fling(
        velocity: _frontLayerVisible ? -_kFlingVelocity : _kFlingVelocity);
  }

  Widget _buildStack(BuildContext context, BoxConstraints constraints) {
    const double layerTitleHeight = 48.0;
    final Size layerSize = constraints.biggest;
    final double layerTop = layerSize.height - layerTitleHeight;

    Animation<RelativeRect> layerAnimation = RelativeRectTween(
            begin: RelativeRect.fromLTRB(
                0.0, layerTop, 0.0, layerTop - layerSize.height),
            end: RelativeRect.fromLTRB(0, 0, 0, 0))
        .animate(_controller.view);

    return Stack(
      key: _backdropKey,
      children: <Widget>[
        ExcludeSemantics(
          child: widget.backLayer,
          excluding: _frontLayerVisible,
        ),
        PositionedTransition(
          rect: layerAnimation,
          child: _FrontLayer(
            onTap: _toggleBackdropLayerVisibility,
            child: widget.frontLayer,
          ),
        )
      ],
    );
  }

  @override
  Widget build(BuildContext context) {
    var appBar = AppBar(
      brightness: Brightness.light,
      elevation: 0.0,
      titleSpacing: 0.0,
    /*  leading: IconButton(
          icon: Icon(Icons.menu), onPressed: _toggleBackdropLayerVisibility),*/
      title: _BackdropTitle(
        listenable: _controller.view,
          onPress: _toggleBackdropLayerVisibility,
          frontTitle: widget.frontTitle,
          backTitle: widget.backTitle),
      actions: <Widget>[
        IconButton(
          icon: Icon(
            Icons.search,
            semanticLabel: 'login',
          ),
          onPressed: () {
            Navigator.push(
                context,
                MaterialPageRoute(builder: (BuildContext context) => LoginPage()));
          },
        ),
        IconButton(
          icon: Icon(
            Icons.tune,
            semanticLabel: 'login',
          ),
          onPressed: () {
            Navigator.push(
                context,
                MaterialPageRoute(builder: (BuildContext context) => LoginPage()));
          },
        )
      ],
    );

    return Scaffold(
      appBar: appBar,
      body: LayoutBuilder(builder: _buildStack),
    );
  }
}