Widget 的物理模拟动画效果

物理模拟能够让应用富有真实感和更好的交互性。例如，你可能会为一个 widget 添加动画，让它看起来就像安着弹簧，或是在随重力下落。

这个指南演示了如何将 widget 从拖动的点移回到中心，并使用弹簧模拟效果。

这个演示将进行下面几步：

1. 创建一个动画控制器

2. 使用手势移动 widget

3. 对 widget 进行动画

4. 计算速度以模拟弹跳运动

首先，创建一个叫做 DraggableCard 的 stateful widget：

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(home: PhysicsCardDragDemo()));
}

class PhysicsCardDragDemo extends StatelessWidget {
  const PhysicsCardDragDemo({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: const DraggableCard(
        child: FlutterLogo(
          size: 128,
        ),
      ),
    );
  }
}

class DraggableCard extends StatefulWidget {
  const DraggableCard({required this.child, super.key});

  final Widget child;

  @override
  State<DraggableCard> createState() => _DraggableCardState();
}

class _DraggableCardState extends State<DraggableCard> {
  @override
  void initState() {
    super.initState();
  }

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

  @override
  Widget build(BuildContext context) {
    return Align(
      child: Card(
        child: widget.child,
      ),
    );
  }
}

让 _DraggableCardState 类继承至 SingleTickerProviderStateMixin。然后在 initState 中构造一个 AnimationController，并将其 vsync 属性设为 this。

class _DraggableCardState extends State<DraggableCard> {
class _DraggableCardState extends State<DraggableCard>
    with SingleTickerProviderStateMixin {
  late AnimationController _controller;

  @override
  void initState() {
    super.initState();
    _controller =
        AnimationController(vsync: this, duration: const Duration(seconds: 1));
  }

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

让 widget 可以被拖拽，并为 _DraggableCardState 类添加一个 Alignment 范围。

class _DraggableCardState extends State<DraggableCard>
    with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  Alignment _dragAlignment = Alignment.center;

添加一个 GestureDetector 来捕获 onPanDown、onPanUpdate，以及 onPanEnd 回调。为了调整对齐方式，请使用 MediaQuery 来获得 widget 的大小，然后除以 2。（这会将「拖动的像素」单位转为 Align 使用的坐标。）然后，将 Align widget 的 alignmnt 属性设为 _dragAlignment。

@override
Widget build(BuildContext context) {
  return Align(
    child: Card(
      child: widget.child,
  var size = MediaQuery.of(context).size;
  return GestureDetector(
    onPanDown: (details) {},
    onPanUpdate: (details) {
      setState(() {
        _dragAlignment += Alignment(
          details.delta.dx / (size.width / 2),
          details.delta.dy / (size.height / 2),
        );
      });
    },
    onPanEnd: (details) {},
    child: Align(
      alignment: _dragAlignment,
      child: Card(
        child: widget.child,
      ),
    ),
  );
}

当一个 widget 被释放，它应该就会弹回中心。

添加一个 Animation<Alignment>，以及 _runAnimation 方法。此方法定义了一个 Tween，它在 widget 被拖动到的点之间插入到中心点。

class _DraggableCardState extends State<DraggableCard>
    with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  late Animation<Alignment> _animation;
  Alignment _dragAlignment = Alignment.center;

void _runAnimation() {
  _animation = _controller.drive(
    AlignmentTween(
      begin: _dragAlignment,
      end: Alignment.center,
    ),
  );
  _controller.reset();
  _controller.forward();
}

接下来，当 AnimationController 产生一个值时，更新 _dragAlignment：

@override
void initState() {
  super.initState();
  _controller =
      AnimationController(vsync: this, duration: const Duration(seconds: 1));
  _controller.addListener(() {
    setState(() {
      _dragAlignment = _animation.value;
    });
  });
}

下一步，让 Align widget 使用 _dragAlignment 字段：

child: Align(
  alignment: _dragAlignment,
  child: Card(
    child: widget.child,
  ),
),

最后，更新 GestureDetector 来管理动画控制器：

return GestureDetector(
  onPanDown: (details) {},
  onPanDown: (details) {
    _controller.stop();
  },
  onPanUpdate: (details) {
    // ...
  },
  onPanEnd: (details) {},
  onPanEnd: (details) {
    _runAnimation();
  },
  child: Align(

最后一步时做一些简单的数学计算，计算小部件被拖动完成之后的速度。这样小部件在被快速恢复之前实际上以该速度继续运动。（_runAnimation 方法已经通过设置动画的开始和结束对齐方式来设置方向。）

首先，引入 physics 这个 package:

import 'package:flutter/physics.dart';

onPanEnd 回调提供了一个 DragEndDetails 对象。此对象提供指针停止接触屏幕时的速度。速度以每秒像素为单位，但 Align widget 不使用像素。它使用 [-1.0，-1.0] 和 [1.0,1.0] 之间的坐标值，其中 [0.0,0.0] 表示中心。在步骤 2 中计算的 size 用于将像素转换为该范围内的坐标值。

最后，AnimationController 有一个 animateWith() 方法可以产生 SpringSimulation:

/// Calculates and runs a [SpringSimulation].
void _runAnimation(Offset pixelsPerSecond, Size size) {
  _animation = _controller.drive(
    AlignmentTween(
      begin: _dragAlignment,
      end: Alignment.center,
    ),
  );
  // Calculate the velocity relative to the unit interval, [0,1],
  // used by the animation controller.
  final unitsPerSecondX = pixelsPerSecond.dx / size.width;
  final unitsPerSecondY = pixelsPerSecond.dy / size.height;
  final unitsPerSecond = Offset(unitsPerSecondX, unitsPerSecondY);
  final unitVelocity = unitsPerSecond.distance;

  const spring = SpringDescription(
    mass: 30,
    stiffness: 1,
    damping: 1,
  );

  final simulation = SpringSimulation(spring, 0, 1, -unitVelocity);

  _controller.animateWith(simulation);
}

不要忘记调用 _runAnimation()，并传入速度和大小：

onPanEnd: (details) {
  _runAnimation(details.velocity.pixelsPerSecond, size);
},

import 'package:flutter/material.dart';
import 'package:flutter/physics.dart';

void main() {
  runApp(const MaterialApp(home: PhysicsCardDragDemo()));
}

class PhysicsCardDragDemo extends StatelessWidget {
  const PhysicsCardDragDemo({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: const DraggableCard(
        child: FlutterLogo(
          size: 128,
        ),
      ),
    );
  }
}

/// A draggable card that moves back to [Alignment.center] when it's
/// released.
class DraggableCard extends StatefulWidget {
  const DraggableCard({required this.child, super.key});

  final Widget child;

  @override
  State<DraggableCard> createState() => _DraggableCardState();
}

class _DraggableCardState extends State<DraggableCard>
    with SingleTickerProviderStateMixin {
  late AnimationController _controller;

  /// The alignment of the card as it is dragged or being animated.
  ///
  /// While the card is being dragged, this value is set to the values computed
  /// in the GestureDetector onPanUpdate callback. If the animation is running,
  /// this value is set to the value of the [_animation].
  Alignment _dragAlignment = Alignment.center;

  late Animation<Alignment> _animation;

  /// Calculates and runs a [SpringSimulation].
  void _runAnimation(Offset pixelsPerSecond, Size size) {
    _animation = _controller.drive(
      AlignmentTween(
        begin: _dragAlignment,
        end: Alignment.center,
      ),
    );
    // Calculate the velocity relative to the unit interval, [0,1],
    // used by the animation controller.
    final unitsPerSecondX = pixelsPerSecond.dx / size.width;
    final unitsPerSecondY = pixelsPerSecond.dy / size.height;
    final unitsPerSecond = Offset(unitsPerSecondX, unitsPerSecondY);
    final unitVelocity = unitsPerSecond.distance;

    const spring = SpringDescription(
      mass: 30,
      stiffness: 1,
      damping: 1,
    );

    final simulation = SpringSimulation(spring, 0, 1, -unitVelocity);

    _controller.animateWith(simulation);
  }

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(vsync: this);

    _controller.addListener(() {
      setState(() {
        _dragAlignment = _animation.value;
      });
    });
  }

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

  @override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    return GestureDetector(
      onPanDown: (details) {
        _controller.stop();
      },
      onPanUpdate: (details) {
        setState(() {
          _dragAlignment += Alignment(
            details.delta.dx / (size.width / 2),
            details.delta.dy / (size.height / 2),
          );
        });
      },
      onPanEnd: (details) {
        _runAnimation(details.velocity.pixelsPerSecond, size);
      },
      child: Align(
        alignment: _dragAlignment,
        child: Card(
          child: widget.child,
        ),
      ),
    );
  }
}

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