Flutter 实现酷炫的3D效果

老孟导读：此文讲解3个酷炫的3D动画效果。

下面是要实现的效果：

Flutter 中3D效果是通过 Transform 组件实现的，没有变换效果的实现：

class TransformDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('3D 变换Demo'),
      ),
      body: Container(
        alignment: Alignment.center,
        color: Colors.white,
        child: Text('3D 变换Demo'),
      ),
    );
  }
}

通过 GestureDetector 组件添加滑动事件监听：

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text('3D 变换Demo'),
    ),
    body: GestureDetector(
      onPanUpdate: (details) {
        print('$details');
      },
      child: Container(
        alignment: Alignment.center,
        color: Colors.white,
        child: Text('3D 变换Demo'),
      ),
    ),
  );
}

添加 Transform 对组件进入旋转：

@override
Widget build(BuildContext context) {
  return Transform(
      transform: Matrix4.identity()
        ..setEntry(3, 2, 0.001)
        ..rotateX(pi/6)
        ..rotateY(pi/6),
      alignment: Alignment.center,
      child: Scaffold(
        appBar: AppBar(
          title: Text('3D 变换Demo'),
        ),
        body: GestureDetector(
          onPanUpdate: (details) {
          },
          child: Container(
            alignment: Alignment.center,
            color: Colors.white,
            child: Text('3D 变换Demo'),
          ),
        ),
      ));
}

将滑动的偏移和旋转进行关联：

class TransformDemo extends StatefulWidget {
  @override
  _TransformDemoState createState() => _TransformDemoState();
}

class _TransformDemoState extends State<TransformDemo> {
  double _rotateX = .0;
  double _rotateY = .0;

  @override
  Widget build(BuildContext context) {
    return Transform(
        transform: Matrix4.identity()
          ..rotateX(_rotateX)
          ..rotateY(_rotateY),
        alignment: Alignment.center,
        child: Scaffold(
          appBar: AppBar(
            title: Text('3D 变换Demo'),
          ),
          body: GestureDetector(
            onPanUpdate: (details) {
              setState(() {
                _rotateX += details.delta.dy * .01;
                _rotateY += details.delta.dx * -.01;
              });
            },
            child: Container(
              alignment: Alignment.center,
              color: Colors.white,
              child: Text('3D 变换Demo'),
            ),
          ),
        ));
  }
}

基本已经实现了3D效果，但效果比较生硬，尤其垂直方向旋转的时候远点和近点在屏幕上的宽度是一样，

添加近大远小的效果：

Transform(
    transform: Matrix4.identity()
      ..setEntry(3, 2, 0.001)
      ..rotateX(_rotateX)
      ..rotateY(_rotateY),
  ...

翻书效果

上面的效果类似于翻书的效果。

实现的原理：

将图片左右切割为两部分，两张图片共分割为4个新的组件，如下图，分别为1、2、3、4

代码实现：

_child1 = ClipRect(
  child: Align(
    alignment: Alignment.centerLeft,
    widthFactor: 0.5,
    child: child1,
  ),
);
_child2 = ClipRect(
  child: Align(
    alignment: Alignment.centerRight,
    widthFactor: 0.5,
    child: child1,
  ),
);

_child3 = ClipRect(
  child: Align(
    alignment: Alignment.centerLeft,
    widthFactor: 0.5,
    child: child2,
  ),
);

_child4 = ClipRect(
  child: Align(
    alignment: Alignment.centerRight,
    widthFactor: 0.5,
    child: child2,
  ),
);

将第一张图片放在第二种图片的上面，先旋转 组件2 从 0度到 90度，然后再旋转 组件3 从 -90度到0度，代码实现：

Row(
  mainAxisAlignment: MainAxisAlignment.center,
  children: <Widget>[
    Stack(
      children: [
        _child1,
        Transform(
          alignment: Alignment.centerRight,
          transform: Matrix4.identity()
            ..setEntry(3, 2, 0.001)
            ..rotateY(_animation1.value),
          child: _child3,
        ),
      ],
    ),
    Container(
      width: 3,
      color: Colors.white,
    ),
    Stack(
      children: [
        _child4,
        Transform(
          alignment: Alignment.centerLeft,
          transform: Matrix4.identity()
            ..setEntry(3, 2, 0.001)
            ..rotateY(_animation.value),
          child: _child2,
        )
      ],
    )
  ],
)

动画控制器设置：

@override
void initState() {
  init();
  _controller =
      AnimationController(vsync: this, duration: Duration(seconds: 5))
        ..addListener(() {
          setState(() {});
        });
  _animation = Tween(begin: .0, end: pi / 2)
      .animate(CurvedAnimation(parent: _controller, curve: Interval(.0, .5)));
  _animation1 = Tween(begin: -pi / 2, end: 0.0).animate(
      CurvedAnimation(parent: _controller, curve: Interval(.5, 1.0)));
  _controller.forward();
  super.initState();
}

其中 child1, child2为两种图片，代码如下：

_FlipUpDemoState(
    Container(
      width: 300,
      height: 400,
      child: Image.asset(
        'assets/images/b.jpg',
        fit: BoxFit.cover,
      ),
    ),
    Container(
      width: 300,
      height: 400,
      child: Image.asset(
        'assets/images/c.jpeg',
        fit: BoxFit.cover,
      ),
    ))

最后生成的效果就是开始的翻书效果。

上面是左右翻页效果，同理换成上下翻页效果：

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(),
    body: Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Stack(
          children: [
            _upperChild1,
            Transform(
              alignment: Alignment.bottomCenter,
              transform: Matrix4.identity()
                ..setEntry(3, 2, 0.003)
                ..rotateX(_animation1.value),
              child: _upperChild2,
            ),
          ],
        ),
        SizedBox(
          height: 2,
        ),
        Stack(
          children: [
            _lowerChild2,
            Transform(
              alignment: Alignment.topCenter,
              transform: Matrix4.identity()
                ..setEntry(3, 2, 0.003)
                ..rotateX(_animation.value),
              child: _lowerChild1,
            )
          ],
        )
      ],
    ),
  );
}

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