创建交错菜单动画

单个应用屏幕可能包含多个动画。同时播放所有动画可能会让人应接不暇。逐个播放动画可能会花费太长时间。更好的选择是交错播放动画。每个动画在不同的时间开始，但动画会重叠以创建更短的持续时间。在本教程中，你将构建一个带有动画内容的分抽屉菜单，该菜单具有交错效果，并且底部有一个弹出按钮。

以下动画展示了应用程序的行为

分抽屉菜单显示一个标题列表，然后在菜单底部显示一个“开始使用”按钮。

定义一个有状态的 Widget，名为 Menu，它在静态位置显示列表和按钮。

class Menu extends StatefulWidget {
  const Menu({super.key});
​
  @override
  State<Menu> createState() => _MenuState();
}
​
class _MenuState extends State<Menu> {
  static const _menuTitles = [
    'Declarative Style',
    'Premade Widgets',
    'Stateful Hot Reload',
    'Native Performance',
    'Great Community',
  ];
​
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      child: Stack(
        fit: StackFit.expand,
        children: [_buildFlutterLogo(), _buildContent()],
      ),
    );
  }
​
  Widget _buildFlutterLogo() {
    // TODO: We'll implement this later.
    return Container();
  }
​
  Widget _buildContent() {
    return Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: [
        const SizedBox(height: 16),
        ..._buildListItems(),
        const Spacer(),
        _buildGetStartedButton(),
      ],
    );
  }
​
  List<Widget> _buildListItems() {
    final listItems = <Widget>[];
    for (var i = 0; i < _menuTitles.length; ++i) {
      listItems.add(
        Padding(
          padding: const EdgeInsets.symmetric(horizontal: 36, vertical: 16),
          child: Text(
            _menuTitles[i],
            textAlign: TextAlign.left,
            style: const TextStyle(fontSize: 24, fontWeight: FontWeight.w500),
          ),
        ),
      );
    }
    return listItems;
  }
​
  Widget _buildGetStartedButton() {
    return SizedBox(
      width: double.infinity,
      child: Padding(
        padding: const EdgeInsets.all(24),
        child: ElevatedButton(
          style: ElevatedButton.styleFrom(
            shape: const StadiumBorder(),
            backgroundColor: Colors.blue,
            padding: const EdgeInsets.symmetric(horizontal: 48, vertical: 14),
          ),
          onPressed: () {},
          child: const Text(
            'Get Started',
            style: TextStyle(color: Colors.white, fontSize: 22),
          ),
        ),
      ),
    );
  }
}

动画时机的控制需要一个 AnimationController。

将 SingleTickerProviderStateMixin 添加到 MenuState 类。然后，声明并实例化一个 AnimationController。

class _MenuState extends State<Menu> with SingleTickerProviderStateMixin {
  late AnimationController _staggeredController;
​
  @override
  void initState() {
    super.initState();
​
    _staggeredController = AnimationController(vsync: this);
  }
​
  @override
  void dispose() {
    _staggeredController.dispose();
    super.dispose();
  }
}

每次动画前的延迟长度由你决定。定义动画延迟、单个动画持续时间和总动画持续时间。

class _MenuState extends State<Menu> with SingleTickerProviderStateMixin {
  static const _initialDelayTime = Duration(milliseconds: 50);
  static const _itemSlideTime = Duration(milliseconds: 250);
  static const _staggerTime = Duration(milliseconds: 50);
  static const _buttonDelayTime = Duration(milliseconds: 150);
  static const _buttonTime = Duration(milliseconds: 500);
  final _animationDuration =
      _initialDelayTime +
      (_staggerTime * _menuTitles.length) +
      _buttonDelayTime +
      _buttonTime;
}

在本例中，所有动画都延迟 50 毫秒。之后，列表项开始出现。每个列表项的出现都延迟了 50 毫秒，在之前的列表项开始滑动之后。每个列表项滑动从右到左需要 250 毫秒。在最后一个列表项开始滑动后，底部的按钮等待另外 150 毫秒才能弹出。按钮动画需要 500 毫秒。

定义了每个延迟和动画持续时间后，将计算总持续时间，以便可以用来计算单个动画时间。

所需的动画时间如图所示

为了在较大动画的子部分内动画化一个值，Flutter 提供了 Interval 类。一个 Interval 接受开始时间百分比和结束时间百分比。然后，该 Interval 可以用来在这些开始和结束时间之间动画化一个值，而不是使用整个动画的开始和结束时间。例如，给定一个持续 1 秒的动画，从 0.2 到 0.5 的区间将在 200 毫秒 (20%) 处开始，并在 500 毫秒 (50%) 处结束。

声明并计算每个列表项的 Interval 和底部的按钮 Interval。

class _MenuState extends State<Menu> with SingleTickerProviderStateMixin {
  final List<Interval> _itemSlideIntervals = [];
  late Interval _buttonInterval;
​
  @override
  void initState() {
    super.initState();
​
    _createAnimationIntervals();
​
    _staggeredController = AnimationController(
      vsync: this,
      duration: _animationDuration,
    );
  }
​
  void _createAnimationIntervals() {
    for (var i = 0; i < _menuTitles.length; ++i) {
      final startTime = _initialDelayTime + (_staggerTime * i);
      final endTime = startTime + _itemSlideTime;
      _itemSlideIntervals.add(
        Interval(
          startTime.inMilliseconds / _animationDuration.inMilliseconds,
          endTime.inMilliseconds / _animationDuration.inMilliseconds,
        ),
      );
    }
​
    final buttonStartTime =
        Duration(milliseconds: _menuTitles.length * 50) + _buttonDelayTime;
    final buttonEndTime = buttonStartTime + _buttonTime;
    _buttonInterval = Interval(
      buttonStartTime.inMilliseconds / _animationDuration.inMilliseconds,
      buttonEndTime.inMilliseconds / _animationDuration.inMilliseconds,
    );
  }
}

交错动画在菜单可见时立即播放。

在 initState() 中启动动画。

@override
void initState() {
  super.initState();
​
  _createAnimationIntervals();
​
  _staggeredController = AnimationController(
    vsync: this,
    duration: _animationDuration,
  )..forward();
}

每个列表项从右向左滑动，并同时淡入。

使用列表项的 Interval 和一个 easeOut 曲线来动画化每个列表项的不透明度和转换值。

List<Widget> _buildListItems() {
  final listItems = <Widget>[];
  for (var i = 0; i < _menuTitles.length; ++i) {
    listItems.add(
      AnimatedBuilder(
        animation: _staggeredController,
        builder: (context, child) {
          final animationPercent = Curves.easeOut.transform(
            _itemSlideIntervals[i].transform(_staggeredController.value),
          );
          final opacity = animationPercent;
          final slideDistance = (1.0 - animationPercent) * 150;
​
          return Opacity(
            opacity: opacity,
            child: Transform.translate(
              offset: Offset(slideDistance, 0),
              child: child,
            ),
          );
        },
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 36, vertical: 16),
          child: Text(
            _menuTitles[i],
            textAlign: TextAlign.left,
            style: const TextStyle(fontSize: 24, fontWeight: FontWeight.w500),
          ),
        ),
      ),
    );
  }
  return listItems;
}

使用相同的方法来动画化底部按钮的不透明度和缩放。这次，使用一个 elasticOut 曲线，为按钮提供弹簧效果。

Widget _buildGetStartedButton() {
  return SizedBox(
    width: double.infinity,
    child: Padding(
      padding: const EdgeInsets.all(24),
      child: AnimatedBuilder(
        animation: _staggeredController,
        builder: (context, child) {
          final animationPercent = Curves.elasticOut.transform(
            _buttonInterval.transform(_staggeredController.value),
          );
          final opacity = animationPercent.clamp(0.0, 1.0);
          final scale = (animationPercent * 0.5) + 0.5;
​
          return Opacity(
            opacity: opacity,
            child: Transform.scale(scale: scale, child: child),
          );
        },
        child: ElevatedButton(
          style: ElevatedButton.styleFrom(
            shape: const StadiumBorder(),
            backgroundColor: Colors.blue,
            padding: const EdgeInsets.symmetric(horizontal: 48, vertical: 14),
          ),
          onPressed: () {},
          child: const Text(
            'Get Started',
            style: TextStyle(color: Colors.white, fontSize: 22),
          ),
        ),
      ),
    ),
  );
}

恭喜！你已经创建了一个动画菜单，其中每个列表项的出现都交错进行，然后底部按钮弹出到位。

import 'package:flutter/material.dart';

void main() {
  runApp(
    const MaterialApp(
      home: ExampleStaggeredAnimations(),
      debugShowCheckedModeBanner: false,
    ),
  );
}

class ExampleStaggeredAnimations extends StatefulWidget {
  const ExampleStaggeredAnimations({super.key});

  @override
  State<ExampleStaggeredAnimations> createState() =>
      _ExampleStaggeredAnimationsState();
}

class _ExampleStaggeredAnimationsState extends State<ExampleStaggeredAnimations>
    with SingleTickerProviderStateMixin {
  late AnimationController _drawerSlideController;

  @override
  void initState() {
    super.initState();

    _drawerSlideController = AnimationController(
      vsync: this,
      duration: const Duration(milliseconds: 150),
    );
  }

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

  bool _isDrawerOpen() {
    return _drawerSlideController.value == 1.0;
  }

  bool _isDrawerOpening() {
    return _drawerSlideController.status == AnimationStatus.forward;
  }

  bool _isDrawerClosed() {
    return _drawerSlideController.value == 0.0;
  }

  void _toggleDrawer() {
    if (_isDrawerOpen() || _isDrawerOpening()) {
      _drawerSlideController.reverse();
    } else {
      _drawerSlideController.forward();
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: _buildAppBar(),
      body: Stack(children: [_buildContent(), _buildDrawer()]),
    );
  }

  PreferredSizeWidget _buildAppBar() {
    return AppBar(
      title: const Text('Flutter Menu', style: TextStyle(color: Colors.black)),
      backgroundColor: Colors.transparent,
      elevation: 0.0,
      automaticallyImplyLeading: false,
      actions: [
        AnimatedBuilder(
          animation: _drawerSlideController,
          builder: (context, child) {
            return IconButton(
              onPressed: _toggleDrawer,
              icon: _isDrawerOpen() || _isDrawerOpening()
                  ? const Icon(Icons.clear, color: Colors.black)
                  : const Icon(Icons.menu, color: Colors.black),
            );
          },
        ),
      ],
    );
  }

  Widget _buildContent() {
    // Put page content here.
    return const SizedBox();
  }

  Widget _buildDrawer() {
    return AnimatedBuilder(
      animation: _drawerSlideController,
      builder: (context, child) {
        return FractionalTranslation(
          translation: Offset(1.0 - _drawerSlideController.value, 0.0),
          child: _isDrawerClosed() ? const SizedBox() : const Menu(),
        );
      },
    );
  }
}

class Menu extends StatefulWidget {
  const Menu({super.key});

  @override
  State<Menu> createState() => _MenuState();
}

class _MenuState extends State<Menu> with SingleTickerProviderStateMixin {
  static const _menuTitles = [
    'Declarative style',
    'Premade widgets',
    'Stateful hot reload',
    'Native performance',
    'Great community',
  ];

  static const _initialDelayTime = Duration(milliseconds: 50);
  static const _itemSlideTime = Duration(milliseconds: 250);
  static const _staggerTime = Duration(milliseconds: 50);
  static const _buttonDelayTime = Duration(milliseconds: 150);
  static const _buttonTime = Duration(milliseconds: 500);
  final _animationDuration =
      _initialDelayTime +
      (_staggerTime * _menuTitles.length) +
      _buttonDelayTime +
      _buttonTime;

  late AnimationController _staggeredController;
  final List<Interval> _itemSlideIntervals = [];
  late Interval _buttonInterval;

  @override
  void initState() {
    super.initState();

    _createAnimationIntervals();

    _staggeredController = AnimationController(
      vsync: this,
      duration: _animationDuration,
    )..forward();
  }

  void _createAnimationIntervals() {
    for (var i = 0; i < _menuTitles.length; ++i) {
      final startTime = _initialDelayTime + (_staggerTime * i);
      final endTime = startTime + _itemSlideTime;
      _itemSlideIntervals.add(
        Interval(
          startTime.inMilliseconds / _animationDuration.inMilliseconds,
          endTime.inMilliseconds / _animationDuration.inMilliseconds,
        ),
      );
    }

    final buttonStartTime =
        Duration(milliseconds: _menuTitles.length * 50) + _buttonDelayTime;
    final buttonEndTime = buttonStartTime + _buttonTime;
    _buttonInterval = Interval(
      buttonStartTime.inMilliseconds / _animationDuration.inMilliseconds,
      buttonEndTime.inMilliseconds / _animationDuration.inMilliseconds,
    );
  }

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

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      child: Stack(
        fit: StackFit.expand,
        children: [_buildFlutterLogo(), _buildContent()],
      ),
    );
  }

  Widget _buildFlutterLogo() {
    return const Positioned(
      right: -100,
      bottom: -30,
      child: Opacity(opacity: 0.2, child: FlutterLogo(size: 400)),
    );
  }

  Widget _buildContent() {
    return Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: [
        const SizedBox(height: 16),
        ..._buildListItems(),
        const Spacer(),
        _buildGetStartedButton(),
      ],
    );
  }

  List<Widget> _buildListItems() {
    final listItems = <Widget>[];
    for (var i = 0; i < _menuTitles.length; ++i) {
      listItems.add(
        AnimatedBuilder(
          animation: _staggeredController,
          builder: (context, child) {
            final animationPercent = Curves.easeOut.transform(
              _itemSlideIntervals[i].transform(_staggeredController.value),
            );
            final opacity = animationPercent;
            final slideDistance = (1.0 - animationPercent) * 150;

            return Opacity(
              opacity: opacity,
              child: Transform.translate(
                offset: Offset(slideDistance, 0),
                child: child,
              ),
            );
          },
          child: Padding(
            padding: const EdgeInsets.symmetric(horizontal: 36, vertical: 16),
            child: Text(
              _menuTitles[i],
              textAlign: TextAlign.left,
              style: const TextStyle(fontSize: 24, fontWeight: FontWeight.w500),
            ),
          ),
        ),
      );
    }
    return listItems;
  }

  Widget _buildGetStartedButton() {
    return SizedBox(
      width: double.infinity,
      child: Padding(
        padding: const EdgeInsets.all(24),
        child: AnimatedBuilder(
          animation: _staggeredController,
          builder: (context, child) {
            final animationPercent = Curves.elasticOut.transform(
              _buttonInterval.transform(_staggeredController.value),
            );
            final opacity = animationPercent.clamp(0.0, 1.0);
            final scale = (animationPercent * 0.5) + 0.5;

            return Opacity(
              opacity: opacity,
              child: Transform.scale(scale: scale, child: child),
            );
          },
          child: ElevatedButton(
            style: ElevatedButton.styleFrom(
              shape: const StadiumBorder(),
              backgroundColor: Colors.blue,
              padding: const EdgeInsets.symmetric(horizontal: 48, vertical: 14),
            ),
            onPressed: () {},
            child: const Text(
              'Get started',
              style: TextStyle(color: Colors.white, fontSize: 22),
            ),
          ),
        ),
      ),
    );
  }
}