import 'package:flutter/material.dart';

void main() {
  runApp(CustomPaint(painter: Painter()));
}

class Painter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final Paint paint = Paint();
    paint.shader = const LinearGradient(
      colors: <Color>[Color(0xFF00FF00), Color(0xFF0000FF)],
      tileMode: TileMode.repeated
    ).createShader(const Rect.fromLTRB(0, 0, 300, 300), textDirection: TextDirection.ltr);
    canvas.drawRect(Rect.fromLTWH(0, 0, 300, 300), paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => true;
}