Arc gradients in Flutter? - dart

Question

I have a Paint object and I'm trying to use it to paint an Arc Gradient using canvas.drawArc, but the only way to do this (at least according to my research) is to use a Shader, but to get a Shader from a Gradient object, you have to use Gradient.createShader(Rect rect), which takes a rectangle. My question is, is there any way to create a shader for an Arc and not a Rectangle? Here's what I have so far for reference:
Paint paint = new Paint()
..color = bgColor
..strokeCap = StrokeCap.round
..strokeWidth = 3.0
..style = PaintingStyle.stroke
..shader = new Gradient.radial(size.width / 2.0, size.height / 2.0, size.height / 3.0, Colors.transparent, timerColor, TileMode.mirror).createShader(/* I don't have a rect object */);
canvas.drawArc(..., paint);

Answer 1

The Rectangle that you need is actually a square into which the circle that you are drawing would fit. The arc is just a slice of pie from that circle swept through so many radians. Create this square using Rect.fromCircle, using the centre and radius. You then use this square when creating the gradient and drawing the arc.
Here's an example
import 'dart:math';
import 'package:flutter/material.dart';
class X1Painter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
// create a bounding square, based on the centre and radius of the arc
Rect rect = new Rect.fromCircle(
center: new Offset(165.0, 55.0),
radius: 180.0,
);
// a fancy rainbow gradient
final Gradient gradient = new RadialGradient(
colors: <Color>[
Colors.green.withOpacity(1.0),
Colors.green.withOpacity(0.3),
Colors.yellow.withOpacity(0.2),
Colors.red.withOpacity(0.1),
Colors.red.withOpacity(0.0),
],
stops: [
0.0,
0.5,
0.7,
0.9,
1.0,
],
);
// create the Shader from the gradient and the bounding square
final Paint paint = new Paint()..shader = gradient.createShader(rect);
// and draw an arc
canvas.drawArc(rect, pi / 4, pi * 3 / 4, true, paint);
}
#override
bool shouldRepaint(X1Painter oldDelegate) {
return true;
}
}
class X1Demo extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(title: const Text('Arcs etc')),
body: new CustomPaint(
painter: new X1Painter(),
),
);
}
}
void main() {
runApp(
new MaterialApp(
theme: new ThemeData.dark(),
home: new X1Demo(),
),
);
}

Answer 2

My SweepGradient version.
Complete example:
import 'dart:math' as math;
import 'package:flutter/material.dart';
class GradientArcPainterDemo extends StatefulWidget {
const GradientArcPainterDemo({
Key key,
}) : super(key: key);
#override
GradientArcPainterDemoState createState() => GradientArcPainterDemoState();
}
class GradientArcPainterDemoState extends State<GradientArcPainterDemo> {
double _progress = 0.9;
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: AppBar(title: const Text('GradientArcPainter Demo')),
body: GestureDetector(
onTap: () {
setState(() {
_progress += 0.1;
});
},
child: Center(
child: SizedBox(
width: 200.0,
height: 200.0,
child: CustomPaint(
painter: GradientArcPainter(
progress: _progress,
startColor: Colors.blue,
endColor: Colors.red,
width: 8.0,
),
child: Center(child: Text('$_progress')),
),
),
),
),
);
}
}
class GradientArcPainter extends CustomPainter {
const GradientArcPainter({
#required this.progress,
#required this.startColor,
#required this.endColor,
#required this.width,
}) : assert(progress != null),
assert(startColor != null),
assert(endColor != null),
assert(width != null),
super();
final double progress;
final Color startColor;
final Color endColor;
final double width;
#override
void paint(Canvas canvas, Size size) {
final rect = new Rect.fromLTWH(0.0, 0.0, size.width, size.height);
final gradient = new SweepGradient(
startAngle: 3 * math.pi / 2,
endAngle: 7 * math.pi / 2,
tileMode: TileMode.repeated,
colors: [startColor, endColor],
);
final paint = new Paint()
..shader = gradient.createShader(rect)
..strokeCap = StrokeCap.butt // StrokeCap.round is not recommended.
..style = PaintingStyle.stroke
..strokeWidth = width;
final center = new Offset(size.width / 2, size.height / 2);
final radius = math.min(size.width / 2, size.height / 2) - (width / 2);
final startAngle = -math.pi / 2;
final sweepAngle = 2 * math.pi * progress;
canvas.drawArc(new Rect.fromCircle(center: center, radius: radius),
startAngle, sweepAngle, false, paint);
}
#override
bool shouldRepaint(CustomPainter oldDelegate) => false;
}
Result: