Flutter 1.17 新 Material motion 规范的预构建动画

老孟导读：在 Flutter 1.17 发布大会上，Flutter 团队还发布了新的 Animations 软件包，该软件包提供了实现新的 Material motion 规范的预构建动画。

软件包 pub 地址：https://pub.dev/packages/animations

Material motion 规范：https://material.io/design/motion/the-motion-system.html

引入插件，版本号请到 pub 上查看最新版本号：

animations: ^1.1.1

Container transform

容器转换模式设计用于包含容器的UI元素之间的转换。此模式在两个UI元素之间创建可见连接。

案例：构建GridView，点击其中一项时跳转到期详情页面：

GridView.builder(
  padding: EdgeInsets.all(8),
  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
      crossAxisCount: 2, crossAxisSpacing: 2, mainAxisSpacing: 4),
  itemBuilder: (context, index) {
    return OpenContainer(
      transitionDuration: _duration,
      closedBuilder: (BuildContext _, VoidCallback openContainer) {
        return Container(
          child: Image.asset(
            'assets/images/b.jpg',
            fit: BoxFit.fitWidth,
          ),
        );
      },
      openBuilder: (BuildContext context, VoidCallback _) {
        return _DetailPage();
      },
    );
  },
  itemCount: 50,
)

使用 OpenContainer 组件，closedBuilder 表示关闭状态时到组件，在这里表示 GridView Item，openBuilder 表示点击要跳转的页面，这里表示详情页面。

详情页面代码如下：

class _DetailPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Container(
        width: double.infinity,
        height: double.infinity,
        child: Image.asset(
          'assets/images/b.jpg',
          fit: BoxFit.cover,
        ),
      ),
    );
  }
}

构建ListView

ListView.builder(
  itemBuilder: (context, index) {
    return OpenContainer(
      transitionDuration: _duration,
      closedBuilder: (BuildContext _, VoidCallback openContainer) {
        return Card(
          child: Container(
            height: 45,
            alignment: Alignment.center,
            child: Text('$index'),
          ),
        );
      },
      openBuilder: (BuildContext context, VoidCallback _) {
        return _DetailPage();
      },
    );
  },
  itemCount: 50,
)

也可以是一个按钮，比如 floatingActionButton

Scaffold(
  body: _buildListView(),
  floatingActionButton: OpenContainer(
    openBuilder: (BuildContext context, VoidCallback _) {
      return _DetailPage();
    },
    transitionDuration: _duration,
    closedElevation: 6.0,
    closedShape: const RoundedRectangleBorder(
      borderRadius: BorderRadius.all(
        Radius.circular(50),
      ),
    ),
    closedColor: Theme.of(context).colorScheme.secondary,
    closedBuilder: (BuildContext context, VoidCallback openContainer) {
      return SizedBox(
        height: 50,
        width: 50,
        child: Center(
          child: Icon(
            Icons.add,
            color: Theme.of(context).colorScheme.onSecondary,
          ),
        ),
      );
    },
  ),
)

顶部输入框

Scaffold(
  appBar: AppBar(
    title: OpenContainer(
      transitionDuration: _duration,
      closedBuilder: (BuildContext _, VoidCallback openContainer) {
        return Container(
          width: 300,
          height: 45,
          padding: EdgeInsets.only(left: 5),
          decoration: BoxDecoration(
              border: Border.all(color: Colors.grey.withOpacity(.5))),
          alignment: Alignment.centerLeft,
          child: Icon(Icons.search,color: Colors.black,),
        );
      },
      openBuilder: (BuildContext context, VoidCallback _) {
        return _DetailPage();
      },
    ),
  ),
)

Shared axis

共享轴模式用于具有空间或导航关系的UI元素之间的过渡。此模式在x，y或z轴上使用共享的变换来加强元素之间的关系。

底部导航案例：

@override
Widget build(BuildContext context) {
  Widget _child = _OnePage();
  switch (_currentIndex) {
    case 1:
      _child = _TwoPage();
      break;
  }
  return Scaffold(
    body: PageTransitionSwitcher(
      duration: const Duration(milliseconds: 1500),
      reverse: false,
      transitionBuilder: (
        Widget child,
        Animation<double> animation,
        Animation<double> secondaryAnimation,
      ) {
        return SharedAxisTransition(
          child: child,
          animation: animation,
          transitionType: SharedAxisTransitionType.horizontal,
          secondaryAnimation: secondaryAnimation,
        );
      },
      child: _child,
    ),
    bottomNavigationBar: BottomNavigationBar(
      onTap: (int index) {
        setState(() {
          _currentIndex = index;
        });
      },
      currentIndex: _currentIndex,
      items: <BottomNavigationBarItem>[
        BottomNavigationBarItem(title: Text('首页'), icon: Icon(Icons.home)),
        BottomNavigationBarItem(
            title: Text('我的'), icon: Icon(Icons.perm_identity)),
      ],
    ),
  );
}

类型为 y 轴：

transitionType: SharedAxisTransitionType.vertical,

类型为 z 轴：

transitionType: SharedAxisTransitionType.scaled,

Fade through

淡入模式用于彼此之间没有密切关系的UI元素之间的过渡。

下面案例来源于官方Demo：

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(title: const Text('Fade through')),
    body: PageTransitionSwitcher(
      transitionBuilder: (
          Widget child,
          Animation<double> animation,
          Animation<double> secondaryAnimation,
          ) {
        return FadeThroughTransition(
          animation: animation,
          secondaryAnimation: secondaryAnimation,
          child: child,
        );
      },
      child: pageList[pageIndex],
    ),
    bottomNavigationBar: BottomNavigationBar(
      currentIndex: pageIndex,
      onTap: (int newValue) {
        setState(() {
          pageIndex = newValue;
        });
      },
      items: const <BottomNavigationBarItem>[
        BottomNavigationBarItem(
          icon: Icon(Icons.photo_library),
          title: Text('Albums'),
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.photo),
          title: Text('Photos'),
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.search),
          title: Text('Search'),
        ),
      ],
    ),
  );
}

效果适用于：

底部导航切换。
刷新列表。
切换器。

Fade

淡入淡出模式用于在屏幕范围内进入或退出的UI元素，例如在屏幕中央淡入淡出的对话框。

弹出对话框案例：

Scaffold(
  body: Center(
    child: RaisedButton(
      onPressed: () {
        showModal<void>(
          context: context,
          builder: (BuildContext context) {
            return AlertDialog(
              content: const Text('对话框'),
              actions: <Widget>[
                FlatButton(
                  onPressed: () {
                    Navigator.of(context).pop();
                  },
                  child: const Text('取消'),
                ),
                FlatButton(
                  onPressed: () {
                    Navigator.of(context).pop();
                  },
                  child: const Text('确定'),
                ),
              ],
            );
          },
        );
      },
      color: Theme.of(context).colorScheme.primary,
      textColor: Theme.of(context).colorScheme.onPrimary,
      child: const Text('弹出对话框'),
    ),
  ),
)

适用场景：

dialog
menu
snackbar
FloatingActionButton

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