Dart 如何用 Flutter 在同屏路由上制作英雄风格的动画

发布于09月25日

我正在try 创建上面的样机中描述的动态动画.我需要的是:

代表化身的圆形(例如用户图片)；
在它下面居中的文字.
在它们(使用半个屏幕)下面是一个可滚动的PageView.

动画应如下所示:

Begin: In a Stack both centered at the begin.
Animation: Scaling down and sliding the text (with variable lenght) that must be to the right of the avatar.
End: As the second image in the mockup. Side-to-side while the content below keeps scrolling.

想在SliverPersistentHeader和CustomMultiChildLayout结合起来，但问题是文本开始居中，结束时向左对齐，我可以动态地制作动画.我试着go 掉最后居中文本的偏移量，但感觉不对劲.

任何帮助或样品只与此动画将不胜感激.谢谢.

推荐答案

您需要一个小条来根据滚动偏移设置布局动画.更具体地说，你的情况是SliverPersistentHeader美元.

虽然CustomMultiChildLayout不是必需的，但是您可以使用补间和对齐/填充/填充来达到相同的效果.但是如果你的布局开始变得太复杂，你可以试一试.

诀窍是使用SliverPersistentHeader给出的滚动偏移量来计算当前进度.然后用这个级数将元素定位在原始位置和最终位置之间.

以下是一个原始示例:

class TransitionAppBar extends StatelessWidget {
  final Widget avatar;
  final Widget title;

  const TransitionAppBar({this.avatar, this.title, Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return SliverPersistentHeader(
      pinned: true,
      delegate: _TransitionAppBarDelegate(
        avatar: avatar,
        title: title,
      ),
    );
  }
}

class _TransitionAppBarDelegate extends SliverPersistentHeaderDelegate {
  final _avatarTween =
      SizeTween(begin: Size(150.0, 150.0), end: Size(50.0, 50.0));
  final _avatarMarginTween =
      EdgeInsetsTween(begin: EdgeInsets.zero, end: EdgeInsets.only(left: 10.0));
  final _avatarAlignTween =
      AlignmentTween(begin: Alignment.topCenter, end: Alignment.centerLeft);

  final _titleMarginTween = EdgeInsetsTween(
      begin: EdgeInsets.only(top: 150.0 + 5.0),
      end: EdgeInsets.only(left: 10.0 + 50.0 + 5.0));
  final _titleAlignTween =
      AlignmentTween(begin: Alignment.center, end: Alignment.centerLeft);

  final Widget avatar;
  final Widget title;

  _TransitionAppBarDelegate({this.avatar, this.title})
      : assert(avatar != null),
        assert(title != null);

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    final progress = shrinkOffset / 200.0;

    final avatarSize = _avatarTween.lerp(progress);
    final avatarMargin = _avatarMarginTween.lerp(progress);
    final avatarAlign = _avatarAlignTween.lerp(progress);

    final titleMargin = _titleMarginTween.lerp(progress);
    final titleAlign = _titleAlignTween.lerp(progress);
    return Stack(
      fit: StackFit.expand,
      children: <Widget>[
        Padding(
          padding: avatarMargin,
          child: Align(
            alignment: avatarAlign,
            child: SizedBox.fromSize(size: avatarSize, child: avatar),
          ),
        ),
        Padding(
          padding: titleMargin,
          child: Align(
            alignment: titleAlign,
            child: DefaultTextStyle(
                style: Theme.of(context).textTheme.title, child: title),
          ),
        )
      ],
    );
  }

  @override
  double get maxExtent => 200.0;

  @override
  double get minExtent => 100.0;

  @override
  bool shouldRebuild(_TransitionAppBarDelegate oldDelegate) {
    return avatar != oldDelegate.avatar || title != oldDelegate.title;
  }
}

可与CustomScrollView一起使用:

Scaffold(
  body: CustomScrollView(
    slivers: <Widget>[
      TransitionAppBar(
        avatar: Material(
          color: Colors.blue,
          elevation: 3.0,
        ),
        title: Text("Hello World"),
      ),
      SliverList(
        delegate: SliverChildBuilderDelegate((context, index) {
          return ListTile(
            title: Text('$index'),
          );
        }),
      )
    ],
  ),
);