建立交錯式選單動畫

單個應用程式畫面可能包含多個動畫。同時播放所有動畫可能會讓人眼花撩亂。依序播放動畫可能需要太長時間。更好的選擇是錯開動畫。每個動畫在不同的時間開始，但動畫會重疊，以縮短持續時間。在本食譜中，您將建立一個具有動畫內容的抽屜選單，該內容是錯開的，並且在底部有一個彈出的按鈕。

以下動畫顯示了應用程式的行為

抽屜選單會顯示標題列表，接著在選單底部顯示「開始使用」按鈕。

定義一個名為 Menu 的有狀態 Widget，在靜態位置顯示列表和按鈕。

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

動畫計時的控制需要一個 AnimationController。

將 SingleTickerProviderStateMixin 加入到 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();
  }
}

每個動畫之前的延遲長度由您決定。定義動畫延遲、個別動畫持續時間和總動畫持續時間。

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;
}

在此案例中，所有動畫都延遲 50 毫秒。之後，列表項目開始顯示。每個列表項目的顯示都比前一個列表項目開始滑入延遲 50 毫秒。每個列表項目從右向左滑入需要 250 毫秒。在最後一個列表項目開始滑入後，底部的按鈕會再等待 150 毫秒才彈出。按鈕動畫需要 500 毫秒。

定義了每個延遲和動畫持續時間後，會計算總持續時間，以便可用於計算個別動畫時間。

所需的動畫時間如下圖所示

為了在較大動畫的子區段期間為值加入動畫，Flutter 提供了 Interval 類別。Interval 採用開始時間百分比和結束時間百分比。然後，可以使用該 Interval 在這些開始和結束時間之間為值加入動畫，而不是使用整個動畫的開始和結束時間。例如，如果給定一個需要 1 秒的動畫，則從 0.2 到 0.5 的間隔將在 200 毫秒 (20%) 時開始，並在 500 毫秒 (50%) 時結束。

宣告並計算每個列表項目的 Interval 和底部按鈕的 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,
    );
  }
}

錯開的動畫會在選單可見時立即播放。

在 initState() 中啟動動畫。

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

  _createAnimationIntervals();

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

每個列表項目都從右向左滑入，同時淡入。

使用列表項目的 Interval 和 easeOut 曲線來為每個列表項目的不透明度和轉換值加入動畫。

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不透明度和縮放加入動畫。這次，使用 elasticOut 曲線來讓按鈕產生彈跳效果。

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！您現在有一個動畫選單，其中每個列表項目的顯示是錯開的，然後底部按鈕會彈出到位。

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

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