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_runAnimation(details.velocity.pixelsPerSecond, size);
},
<code_end>
info note
now that the animation controller uses a simulation it’s duration argument
is no longer required.
<topic_end>
<topic_start>
interactive example
<code_start>
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) {