Exemple de Widget consommant beaucoup de ressources

expensive_widget.dart

import 'dart:math';

import 'package:flutter/material.dart';

class ExpensiveWidget extends StatefulWidget {
  const ExpensiveWidget({super.key});

  @override
  State<StatefulWidget> createState() => ExpensiveWidgetState();
}

class ExpensiveWidgetState extends State<ExpensiveWidget> {
  final GlobalKey _paintKey = GlobalKey();
  Offset _offset = Offset.zero;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        fit: StackFit.expand,
        children: <Widget>[
          RepaintBoundary(
            child: CustomPaint(
              painter: _ExpensivePainter(),
              isComplex: false,
              willChange: true,
            ),
          ),
          Listener(
            onPointerDown: _updateOffset,
            onPointerMove: _updateOffset,
            child: CustomPaint(
              key: _paintKey,
              painter: MyCustomPainter(_offset),
              child: const SizedBox.expand(),
            ),
          ),
        ],
      ),
    );
  }

  _updateOffset(PointerEvent event) {
    RenderBox? referenceBox =
        _paintKey.currentContext?.findRenderObject() as RenderBox?;
    Offset? offset = referenceBox?.globalToLocal(event.position);

    if (offset != null) {
      setState(() => _offset = offset);
    }
  }
}

class _ExpensivePainter extends CustomPainter {
  final Random rand = Random(12345);
  final List<Color> colors = [
    Colors.red,
    Colors.blue,
    Colors.yellow,
    Colors.green,
    Colors.white,
  ];

  @override
  void paint(Canvas canvas, Size size) {
    print("Paint rainbow");

    for (int i = 0; i < 100000; i++) {
      canvas.drawCircle(
        Offset(rand.nextDouble() * size.width, rand.nextDouble() * size.height),
        10 + rand.nextDouble() * 20,
        Paint()..color = colors[rand.nextInt(colors.length)].withOpacity(0.2),
      );
    }
  }

  @override
  bool shouldRepaint(_ExpensivePainter oldDelegate) => false;
}

class MyCustomPainter extends CustomPainter {
  MyCustomPainter(this._offset);

  final Offset _offset;
  final Paint _paint = Paint()..color = Colors.black;

  @override
  void paint(Canvas canvas, Size size) {
    print("Paint cursor");
    canvas.drawCircle(_offset, 10.0, _paint);
  }

  @override
  bool shouldRepaint(MyCustomPainter oldDelegate) =>
      oldDelegate._offset != _offset;
}