Animate a widget using a physics simulation

Animation을 이용하여 widget이 스프링처럼 움직이도록 하는 방법에 대해서 알아보겠습니다.

Set up an animation controller

DraggableCard 라는 widget을 만들고, AnimationController 를 생성합니다.

import 'package:flutter/material.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,
        ),
      ),
    );
  }
}

class DraggableCard extends StatefulWidget {
  final Widget child;
  DraggableCard({required this.child});

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

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

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

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

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

Move the widget using gestures

• Alignment를 추가합니다.
• Drag를 했을때, widget이 움직이도록 합니다.
• GestureDetector 의 onPanDown, onPanUpdate, onPanEnd 를 이용합니다.

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

@override
  Widget build(BuildContext context) {
    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,
        ),
      ),
    );
  }

Animate the widget

• Animation을 이용해서 widget이 스프링처럼 움직이도록 합니다.
• Animation< Alignment > 와 _runAnimation 함수를 추가합니다.

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

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

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

Calculate the velocity to simulate a springing motion

• 스프링 모션을 위한 속도를 계산합니다.
• AnimationController 는 animateWith() 함수를 가지고있고, 여기서 SpringSimulation 을 사용할 수 있습니다.

content_copy

/// 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);
  }

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

Example

Reference

[Cookbook - Animate a widget using a physics simulation]