使用物理模擬為元件 (Widget) 建立動畫

物理模擬可以讓應用程式的互動感覺更真實且具互動性。 舉例來說，你可能會希望動畫化一個元件 (Widget)，讓它看起來像是被彈簧連接，或是在重力作用下下落。

本教學將示範如何將一個元件從拖曳的位置移動回中心，並使用彈簧模擬來實現。

本教學包含以下步驟：

1. 建立動畫控制器
2. 透過手勢移動元件
3. 動畫化元件
4. 計算速度以模擬彈簧運動

首先建立一個名為 DraggableCard 的 stateful 元件 (StatefulWidget)：

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));
  }
}

讓 _DraggableCardState 類別繼承自 SingleTickerProviderStateMixin。 然後在 initState 中建立一個 AnimationController， 並將 vsync 設為 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();
  }

讓元件在被拖曳時能夠移動，並在 _DraggableCardState 類別中新增一個 Alignment 欄位：

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

新增一個 GestureDetector，用來處理 onPanDown、onPanUpdate 和 onPanEnd 回呼（callback）。為了調整對齊方式，可以使用 MediaQuery 來取得元件的尺寸，並除以 2。（這會將「拖曳的像素」單位轉換為 Align 使用的座標。）然後，將 Align 元件的 alignment 設為 _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,
      ),
    ),
  );
}

當元件被釋放時，應該會以彈簧效果回到中心位置。

新增一個 Animation<Alignment> 欄位和一個 _runAnimation 方法。這個方法會定義一個 Tween，用來在元件被拖曳到的位置與中心點之間進行插值（interpolate）。

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();
}

接下來，當 AnimationController 產生數值時，請更新 _dragAlignment：

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

接下來，讓 Align 元件使用 _dragAlignment 欄位：

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

最後，更新 GestureDetector 以管理動畫控制器（AnimationController）：

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

最後一步需要進行一些數學運算，以計算元件在拖曳結束後的速度。這樣可以讓元件以更真實的方式，先依照該速度繼續移動，然後再被拉回來。（_runAnimation 方法已經透過設定動畫的起始與結束對齊方式來決定方向。）

首先，匯入 physics 套件：

import 'package:flutter/physics.dart';

onPanEnd 回呼會提供一個 DragEndDetails 物件。這個物件會提供指標停止接觸螢幕時的速度。該速度的單位是每秒像素（pixels per second），但 Align 元件並不是以像素為單位，而是使用介於 [-1.0, -1.0] 到 [1.0, 1.0] 的座標值，其中 [0.0, 0.0] 代表中心點。在步驟 2 計算出的 size，則用來將像素轉換為這個範圍內的座標值。

最後，AnimationController 有一個 animateWith() 方法，可以傳入一個 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: 1, stiffness: 1, damping: 1);
​
  final simulation = SpringSimulation(spring, 0, 1, -unitVelocity);
​
  _controller.animateWith(simulation);
}

別忘了以 velocity（速度）和 size（大小）來呼叫 _runAnimation()：

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: 1, 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),
      ),
    );
  }
}