Flutter - DragBox 反馈动画到原始位置

Question

我想在 DropTarget 不接受时显示可拖动的反馈动画。Flutter 不显示反馈。有什么方法可以展示或控制它。像这个例子，我想实现这个效果。我以某种方式实现了这种效果，但返回原始偏移量是不正确的。它正朝着原来的位置前进。

我想要的动画效果。

这是我的代码，当我将它提升到某个位置并将他从那里离开时，我有一个拖动框，它应该动画回到原始位置，但它正在返回到其他一些像这样的偏移。

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

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(body: DragBox()),
    );
  }
}

class DragBox extends StatefulWidget {
  DragBox({
    Key key,
  }) : super(key: key);

  @override
  State<StatefulWidget> createState() {
    return new _MyDragBox();
  }
}

class _MyDragBox extends State<DragBox> with TickerProviderStateMixin {
  GlobalKey _globalKey = new GlobalKey();
  AnimationController _controller;
  Offset begin;
  Offset cancelledOffset;
  Offset _offsetOfWidget;
  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback((s) {
      _afeteLayout();
    });
    super.initState();
    _controller = AnimationController(
      vsync: this,
      duration: Duration(milliseconds: 1000),
    );
  }

  void _afeteLayout() {
    final RenderBox box = _globalKey.currentContext.findRenderObject();
    Offset offset = -box.globalToLocal(Offset(0.0, 0.0));
    _offsetOfWidget = offset;
  }

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

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        Center(
          child: Draggable(
            key: _globalKey,
            onDraggableCanceled: (v, o) {
              setState(() {
                cancelledOffset = o;
              });
              _controller.forward();
            },
            feedback: Container(
              color: Colors.red,
              height: 50,
              width: 50,
            ),
            child: Container(
              color: Colors.red,
              height: 50,
              width: 50,
            ),
          ),
        ),
        _controller.isAnimating
            ? SlideTransition(
                position: Tween<Offset>(
                        begin: cancelledOffset * 0.01,
                        end: _offsetOfWidget * 0.01)
                    .animate(_controller)
                      ..addStatusListener((status) {
                        if (status == AnimationStatus.completed) {
                          _controller.stop();
                        } else {
                          _controller.reverse();
                        }
                      }),
                child: Container(
                  color: Colors.red,
                  height: 50,
                  width: 50,
                ),
              )
            : Container(
                child: Text('data'),
              )
      ],
    );
  }
}

Answer 1

我认为，有关“使用物理模拟对小部件进行动画处理”的文档是最适合您想要实现的目标的示例。

本节演示如何使用弹簧模拟将小部件从拖动点移回中心。

要实际了解它，请看一下示例：

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

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

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

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

  @override
  _DraggableCardState 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,
        ),
      ),
    );
  }
}

示例输出：