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and the CurvedAnimation computes the curve
that replaces the controller’s default linear motion.
Like widgets, animations in Flutter work with composition.When building the widget tree you assign the Animation to an animated
property of a widget, such as the opacity of a FadeTransition,
and tell the controller to start the animation.The following example shows how to write a FadeTransition that
fades the widget into a logo when you press the FloatingActionButton:
<code_start>import 'package:flutter/material.dart';
class SampleApp extends StatelessWidget {
// This widget is the root of your application.
const SampleApp({super.key});
@override
Widget build(BuildContext context) {
return const MaterialApp(
title: 'Fade Demo',
home: MyFadeTest(title: 'Fade Demo'),
);
}
}
class MyFadeTest extends StatefulWidget {
const MyFadeTest({super.key, required this.title});
final String title;
@override
State<MyFadeTest> createState() => _MyFadeTest();
}
class _MyFadeTest extends State<MyFadeTest>
with SingleTickerProviderStateMixin {
late AnimationController controller;
late CurvedAnimation curve;
@override
void initState() {
super.initState();
controller = AnimationController(
duration: const Duration(milliseconds: 2000),
vsync: this,
);
curve = CurvedAnimation(
parent: controller,
curve: Curves.easeIn,
);
}
@override
void dispose() {
controller.dispose();
super.dispose();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text(widget.title)),
body: Center(
child: FadeTransition(
opacity: curve,
child: const FlutterLogo(size: 100),
),
),
floatingActionButton: FloatingActionButton(
onPressed: () {
controller.forward();
},
tooltip: 'Fade',
child: const Icon(Icons.brush),
),
);
}
}<code_end>
For more information, see Animation & Motion widgets,
the Animations tutorial, and the Animations overview.<topic_end>
<topic_start>
Drawing on the screen
In UIKit, you use CoreGraphics to draw lines and shapes to the
screen. Flutter has a different API based on the Canvas class,
with two other classes that help you draw: CustomPaint and CustomPainter,
the latter of which implements your algorithm to draw to the canvas.To learn how to implement a signature painter in Flutter,
see Collin’s answer on StackOverflow.
<code_start>import 'package:flutter/material.dart';
void main() => runApp(const MaterialApp(home: DemoApp()));
class DemoApp extends StatelessWidget {
const DemoApp({super.key});
@override
Widget build(BuildContext context) => const Scaffold(body: Signature());
}
class Signature extends StatefulWidget {
const Signature({super.key});
@override
State<Signature> createState() => SignatureState();
}
class SignatureState extends State<Signature> {
List<Offset?> _points = <Offset?>[];
@override
Widget build(BuildContext context) {
return GestureDetector(
onPanUpdate: (details) {
setState(() {
RenderBox? referenceBox = context.findRenderObject() as RenderBox;
Offset localPosition =
referenceBox.globalToLocal(details.globalPosition);
_points = List.from(_points)..add(localPosition);
});
},
onPanEnd: (details) => _points.add(null),