I have a simple app that draws via a CustomPainter
a red or green circle on a canvas, depending on which button is pressed in the AppBar
:
The class ColorCircle
extends CustomPainter
and is responsible for drawing the colored circle:
class ColorCircle extends CustomPainter {
MaterialColor myColor;
ColorCircle({@required this.myColor});
@override
void paint(Canvas canvas, Size size) {
debugPrint('ColorCircle.paint, ${DateTime.now()}');
final paint = Paint()..color = myColor;
canvas.drawCircle(Offset(size.width / 2, size.height / 2), 100, paint);
}
@override
bool shouldRepaint(CustomPainter oldDelegate) => false;
}
The drawing of the different colors works fine, but when I click (only once!) or hover over one of the buttons, the paint
method gets called several times:
Further implementation details:
I use a StatefulWidget
for storing the actualColor
. In the build method actualColor
is passed to the ColorCircle
constructor:
class _MyHomePageState extends State<MyHomePage> {
MaterialColor actualColor = Colors.red;
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
actions: <Widget>[
OutlinedButton(
onPressed: () => setState(() => actualColor = Colors.red),
child: Text('RedCircle'),
),
OutlinedButton(
onPressed: () => setState(() => actualColor = Colors.green),
child: Text('GreenCircle'),
),
],
),
body: Center(
child: CustomPaint(
size: Size(300, 300),
painter: ColorCircle(myColor: actualColor),
),
),
);
}
}
The complete source code with a running example can be found here: CustonPainter Demo
So why is paint
called several times instead of only once? (And how could you implement it so that paint
is called only once?).
A poor solution might be to add a RepaintBoundary
around the hover Widgets:
class _MyHomePageState extends State<MyHomePage> {
MaterialColor actualColor = Colors.red;
@override
Widget build(BuildContext context) {
print('Rebuilding with $actualColor');
return Scaffold(
appBar: AppBar(
title: Text('CustomPainter Demo'),
actions: <Widget>[
RepaintBoundary(
child: OutlinedButton(
style: ButtonStyle(
foregroundColor: MaterialStateProperty.all(Colors.black)),
onPressed: () {
setState(() => actualColor = Colors.red);
},
child: Text('RedCircle')),
),
RepaintBoundary(
child: OutlinedButton(
style: ButtonStyle(
foregroundColor: MaterialStateProperty.all(Colors.black)),
onPressed: () {
setState(() => actualColor = Colors.green);
},
child: Text('GreenCircle')),
),
],
),
body: Center(
child: CustomPaint(
size: Size(300, 300),
painter: ColorCircle(myColor: actualColor),
),
),
);
}
}
And then, to properly define the shouldRepaint
method of the ColorCircle
(currently returning false
):
@override
bool shouldRepaint(CustomPainter oldDelegate) {
return (oldDelegate as ColorCircle).myColor != myColor;
}
This seems to be a really poor solution. I would be interested to know of a better, more sustainable answer.
RepaintBoundary
workaroundimport 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
title: 'CustomPainter Demo',
home: MyHomePage(),
);
}
}
class ColorCirle extends CustomPainter {
MaterialColor myColor;
ColorCirle({@required this.myColor});
@override
void paint(Canvas canvas, Size size) {
debugPrint('ColorCircle.paint, ${DateTime.now()}');
final paint = Paint()..color = myColor;
canvas.drawCircle(Offset(size.width / 2, size.height / 2), 100, paint);
}
@override
bool shouldRepaint(CustomPainter oldDelegate) {
return (oldDelegate as ColorCirle).myColor != myColor;
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({Key key}) : super(key: key);
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
MaterialColor actualColor = Colors.red;
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('CustomPainter Demo'),
actions: <Widget>[
RepaintBoundary(
child: OutlinedButton(
style: ButtonStyle(
foregroundColor: MaterialStateProperty.all(Colors.black)),
onPressed: () {
setState(() => actualColor = Colors.red);
},
child: Text('RedCircle')),
),
RepaintBoundary(
child: OutlinedButton(
style: ButtonStyle(
foregroundColor: MaterialStateProperty.all(Colors.black)),
onPressed: () {
setState(() => actualColor = Colors.green);
},
child: Text('GreenCircle')),
),
],
),
body: Center(
child: CustomPaint(
size: Size(300, 300),
painter: ColorCirle(myColor: actualColor),
),
),
);
}
}