建立階梯式選單動畫

一個單一的應用程式螢幕可能包含多個動畫（Animation）。 如果同時播放所有動畫，畫面會顯得過於繁雜；而若一個接一個依序播放，則可能花費太多時間。更好的做法是採用階梯式（staggered）動畫：每個動畫在不同的時間點開始，但動畫彼此重疊，總體持續時間較短。 在本教學中，你將建立一個帶有階梯式動畫內容的抽屜選單（drawer menu），並在底部加入一個彈出（pop-in）按鈕。

下方動畫展示了應用程式的行為：

抽屜選單會顯示一個標題清單，並在選單底部顯示一個「Get started」按鈕。

請定義一個名為 Menu 的 StatefulWidget（有狀態元件），用於在固定位置顯示清單與按鈕。

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),
          ),
        ),
      ),
    );
  }
}

要控制動畫 (Animation) 的時序，需要一個AnimationController。

將SingleTickerProviderStateMixin新增到MenuState類別中。然後，宣告並實例化一個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();
  }
}

每個動畫開始前的延遲時間長短取決於你的需求。你可以自訂動畫的延遲時間、各自的動畫持續時間，以及整體動畫的總持續時間。

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;
}

在這個範例中，所有動畫都會延遲 50 毫秒後才開始。 接著，清單項目會依序出現。 每個清單項目的出現，會在前一個清單項目開始滑入後延遲 50 毫秒。 每個清單項目從右側滑入到左側所需的時間為 250 毫秒。 當最後一個清單項目開始滑入後， 底部的按鈕會再等待 150 毫秒才彈出。 按鈕的動畫時間為 500 毫秒。

在定義好每個延遲與動畫持續時間後， 會計算總動畫時間，這樣就能用來 計算各個動畫的起止時間。

下圖顯示了預期的動畫時序：

若要在一個較長動畫的子區段內對某個值進行動畫， Flutter 提供了 Interval 類別。 Interval 需要傳入起始時間百分比與結束時間百分比。 然後可以使用該 Interval， 在指定的起始與結束時間之間對值進行動畫， 而不是使用整個動畫的起始與結束時間。例如，假設一個動畫總長為 1 秒， 若設定區間為 0.2 到 0.5，則會從 200 毫秒（20%）開始， 在 500 毫秒（50%）結束。

宣告並計算每個清單項目的 Interval 以及 底部按鈕的 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,
    );
  }
}

當選單變為可見時，會立即播放階層式（staggered）動畫。

在 initState() 中啟動動畫。

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

  _createAnimationIntervals();

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

每個清單項目會從右側滑入至左側，並同時淡入顯示。

使用清單項目的 Interval 以及 easeOut 曲線，為每個清單項目的透明度與平移值添加動畫效果。

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;
}

使用相同的方法來為底部按鈕的透明度（opacity）和縮放（scale）製作動畫。這次，請使用elasticOut曲線，讓按鈕呈現彈跳（springy）的效果。

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！ 你已經完成了一個帶有動畫效果的選單，每個 清單項目的出現都有間隔，接著底部的按鈕 會以彈出的方式顯示。

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),
            ),
          ),
        ),
      ),
    );
  }
}