Flutter 动画 缩放图片、动画简化

上海大哥2024-03-27 16:55

图片放大缩小

scss 复制代码 
class ScaleAnimationRoute extends StatefulWidget {
  const ScaleAnimationRoute({Key? key}) : super(key: key);

  @override
  _ScaleAnimationRouteState createState() => _ScaleAnimationRouteState();
}

//需要继承TickerProvider，如果有多个AnimationController，则应该使用TickerProviderStateMixin。
class _ScaleAnimationRouteState extends State<ScaleAnimationRoute>
    with SingleTickerProviderStateMixin {
  late Animation<double> animation;
  late AnimationController controller;

  @override
  initState() {
    super.initState();
    controller = AnimationController(
      duration: const Duration(seconds: 2),
      vsync: this,
    );

    //匀速
    //图片宽高从0变到300
    animation = Tween(begin: 0.0, end: 300.0).animate(controller)
      ..addListener(() {
        setState(() => {});
      });

    controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
          child: GestureDetector(
        onTap: () {
          //启动动画(正向执行)
          controller.reset();
          controller.forward();
        },
        child: Image.asset(
          "assets/imgs/avatar.png",
          width: animation.value,
          height: animation.value,
        ),
      )),
    );
  }

  @override
  dispose() {
    //路由销毁时需要释放动画资源
    controller.dispose();
    super.dispose();
  }
}

addListener()函数调用了setState()，所以每次动画生成一个新的数字时，当前帧被标记为脏(dirty)，这会导致widget的build()方法再次被调用，而在build()中，改变Image的宽高，因为它的高度和宽度现在使用的是animation.value ，所以就会逐渐放大。值得注意的是动画完成时要释放控制器(调用dispose()方法)以防止内存泄漏。

实现点击下陷的效果

scss 复制代码 
class ScaleAnimationRoute extends StatefulWidget {
  const ScaleAnimationRoute({Key? key}) : super(key: key);

  @override
  _ScaleAnimationRouteState createState() => _ScaleAnimationRouteState();
}

//需要继承TickerProvider，如果有多个AnimationController，则应该使用TickerProviderStateMixin。
class _ScaleAnimationRouteState extends State<ScaleAnimationRoute>
    with SingleTickerProviderStateMixin {
  late Animation<double> animation;
  late AnimationController controller;

  @override
  initState() {
    super.initState();
    controller = AnimationController(
      duration: const Duration(seconds: 2),
      vsync: this,
    );

    //匀速
    //图片宽高从0变到300
    animation = Tween(begin: 0.0, end: 300.0).animate(controller)
      ..addListener(() {
        setState(() => {});
      });

    controller.forward(from: 0.9);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
          child: Container(
              width: 330,
              color: Colors.orange,
              height: 180,
              alignment: Alignment.center,
              child: GestureDetector(
                onTap: () {
                  //启动动画(正向执行)
                  controller.reset();
                  controller.forward(from: 0.9);
                },
                child: Container(
                  color: Colors.blue,
                  width: animation.value,
                  height: animation.value * 0.5,
                ),
              ))),
    );
  }

  @override
  dispose() {
    //路由销毁时需要释放动画资源
    controller.dispose();
    super.dispose();
  }
}

AnimatedWidget简化

通过addListener()和setState() 来更新UI这一步其实是通用的，如果每个动画中都加这么一句是比较繁琐的。AnimatedWidget类封装了调用setState()的细节，并允许我们将 widget 分离出来，重构后的代码如下：

scala 复制代码 
class ScaleAnimationRoute1 extends StatefulWidget {
  const ScaleAnimationRoute1({Key? key}) : super(key: key);

  @override
  _ScaleAnimationRouteState createState() => _ScaleAnimationRouteState();
}

class _ScaleAnimationRouteState extends State<ScaleAnimationRoute1>
    with SingleTickerProviderStateMixin {
  late Animation<double> animation;
  late AnimationController controller;

  @override
  initState() {
    super.initState();
    controller =
        AnimationController(duration: const Duration(seconds: 2), vsync: this);
    //图片宽高从0变到300
    animation = Tween(begin: 0.0, end: 300.0).animate(controller);
    //启动动画
    controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: GestureDetector(
      onTap: () {
        controller.reset();
        controller.forward();
      },
      child: AnimatedImage(
        animation: animation,
      ),
    ));
  }

  @override
  dispose() {
    //路由销毁时需要释放动画资源
    controller.dispose();
    super.dispose();
  }
}

class AnimatedImage extends AnimatedWidget {
  const AnimatedImage({
    Key? key,
    required Animation<double> animation,
  }) : super(key: key, listenable: animation);

  @override
  Widget build(BuildContext context) {
    final animation = listenable as Animation<double>;
    return Center(
      child: Image.asset(
        "assets/imgs/avatar.png",
        width: animation.value,
        height: animation.value,
      ),
    );
  }
}

使用AnimatedBuilder

scala 复制代码 
class ScaleAnimationRoute1 extends StatefulWidget {
  const ScaleAnimationRoute1({Key? key}) : super(key: key);

  @override
  _ScaleAnimationRouteState createState() => _ScaleAnimationRouteState();
}

class _ScaleAnimationRouteState extends State<ScaleAnimationRoute1>
    with SingleTickerProviderStateMixin {
  late Animation<double> animation;
  late AnimationController controller;

  @override
  initState() {
    super.initState();
    controller =
        AnimationController(duration: const Duration(seconds: 2), vsync: this);
    //图片宽高从0变到300
    animation = Tween(begin: 0.0, end: 300.0).animate(controller);
    //启动动画
    controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: GestureDetector(
            onTap: () {
              controller.reset();
              controller.forward();
            },
            child: GrowTransition(
              animation: animation,
              child: Image.asset("assets/imgs/avatar.png"),
            )));
  }

  @override
  dispose() {
    //路由销毁时需要释放动画资源
    controller.dispose();
    super.dispose();
  }
}

class GrowTransition extends StatelessWidget {
  const GrowTransition({
    Key? key,
    required this.animation,
    this.child,
  }) : super(key: key);

  final Widget? child;
  final Animation<double> animation;

  @override
  Widget build(BuildContext context) {
    return Center(
      child: AnimatedBuilder(
        animation: animation,
        builder: (BuildContext context, child) {
          return SizedBox(
            height: animation.value,
            width: animation.value,
            child: child,
          );
        },
        child: child,
      ),
    );
  }
}

用AnimatedWidget 可以从动画中分离出 widget，而动画的渲染过程（即设置宽高）仍然在AnimatedWidget 中，假设如果我们再添加一个 widget 透明度变化的动画，那么我们需要再实现一个AnimatedWidget，这样不是很优雅，如果我们能把渲染过程也抽象出来，那就会好很多，而AnimatedBuilder正是将渲染逻辑分离出来。

上面的代码中有一个迷惑的问题是，child看起来像被指定了两次。但实际发生的事情是：将外部引用child传递给AnimatedBuilder后，AnimatedBuilder再将其传递给匿名构造器， 然后将该对象用作其子对象。最终的结果是AnimatedBuilder返回的对象插入到 widget 树中。

其实它会带来三个好处：

  1. 不用显式的去添加帧监听器，然后再调用setState() 了，这个好处和AnimatedWidget是一样的。
  2. 更好的性能：因为动画每一帧需要构建的 widget 的范围缩小了，如果没有builder，setState()将会在父组件上下文中调用，这将会导致父组件的build方法重新调用；而有了builder之后，只会导致动画widget自身的build重新调用，避免不必要的rebuild。
  3. 通过AnimatedBuilder可以封装常见的过渡效果来复用动画。通过封装一个GrowTransition来说明。

FadeTransition 实现透明度变化效果

Flutter中正是通过这种方式封装了很多动画，如：FadeTransition、ScaleTransition、SizeTransition等，很多时候都可以复用这些预置的过渡类。

scala 复制代码 
class FadeTranstionScreen extends StatefulWidget {
  FadeTranstionScreen({Key? key}) : super(key: key);

  @override
  _FadeTranstionScreenState createState() => _FadeTranstionScreenState();
}

class _FadeTranstionScreenState extends State<FadeTranstionScreen>
    with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  late Animation<double> _animation;

  @override
  void initState() {
    super.initState();
    _controller =
        AnimationController(vsync: this, duration: const Duration(seconds: 2));
    _animation = Tween(begin: 0.0, end: 1.0).animate(_controller);
  }

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

  @override
  Widget build(BuildContext context) {
    _controller.forward();
    return Scaffold(
        appBar: AppBar(title: const Text("FadeTranstion")),
        body: GestureDetector(
          onTap: () {
            _controller.reset();
            _controller.forward();
          },
          child: Container(
            alignment: Alignment.center,
            child: FadeTransition(
              opacity: _animation,
              child: Container(
                width: 100,
                height: 100,
                color: Colors.blue,
              ),
            ),
          ),
        ));
  }
}

ScaleTransition 实现缩小放大效果

scala 复制代码 
class FadeTranstionScreen extends StatefulWidget {
  FadeTranstionScreen({Key? key}) : super(key: key);

  @override
  _FadeTranstionScreenState createState() => _FadeTranstionScreenState();
}

class _FadeTranstionScreenState extends State<FadeTranstionScreen>
    with SingleTickerProviderStateMixin {

  late final AnimationController _controller = AnimationController(
    duration: const Duration(seconds: 2),
    vsync: this,
  )..repeat(reverse: true);

  late final Animation<double> _animation = CurvedAnimation(
    parent: _controller,
    curve: Curves.fastOutSlowIn,
  );

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
          child: GestureDetector(
        onTap: () {
          _controller.reset();
          _controller.forward();
        },
        child: ScaleTransition(
          scale: _animation,
          child: const Padding(
            padding: EdgeInsets.all(8.0),
            child: FlutterLogo(size: 150.0),
          ),
        ),
      )),
    );
  }
}

SizeTransition 实现文字横向飞入效果

scala 复制代码 
class DemoSizeTransition extends StatefulWidget {
  const DemoSizeTransition({super.key});

  @override
  _DemoSizeTransitionState createState() => _DemoSizeTransitionState();
}

class _DemoSizeTransitionState extends State<DemoSizeTransition>
    with SingleTickerProviderStateMixin {
  late AnimationController _animationController;

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

    _animationController =
        AnimationController(vsync: this, duration: const Duration(seconds: 1));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text("SizeTransition")),
      body: SizeTransition(
        //偏移量
        axisAlignment: 0.0,
        //动画控制
        sizeFactor: _animationController,
        axis: Axis.horizontal,
        child: buildContainer(),
      ),
      floatingActionButton: FloatingActionButton(
        child: const Text("开始"),
        onPressed: () {
          _animationController.forward(from: 0);
        },
      ),
    );
  }

  Container buildContainer() {
    return Container(
      //子Widget 对齐方式
      alignment: Alignment.center,
      width: 300,
      //高度
      height: 200,
      //背景颜色
      color: Colors.blue,
      child: const Text(
        "此处文字需要有一些动画效果",
        style: TextStyle(color: Colors.white),
      ),
    );
  }
}

案例 切换到分支flutter_animation

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