samples/simple_shader/lib/main.dart

import 'dart:ui' as ui;

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Simple Shader Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        useMaterial3: true,
      ),
      home: const MyHomePage(),
    );
  }
}

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

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  late Future<ui.FragmentProgram> program;

  @override
  void initState() {
    super.initState();
    program = ui.FragmentProgram.fromAsset('shaders/simple.frag');
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Simple Shader'),
      ),
      body: FutureBuilder<ui.FragmentProgram>(
        future: program,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            return CustomPaint(
              size: MediaQuery.of(context).size,
              painter: ShaderPainter(
                shader: snapshot.data!.fragmentShader(),
              ),
            );
          }
          if (snapshot.hasError) {
            return Center(child: Text('${snapshot.error}'));
          }
          return const Center(child: CircularProgressIndicator());
        },
      ),
    );
  }
}

class ShaderPainter extends CustomPainter {
  ShaderPainter({required this.shader});
  ui.FragmentShader shader;

  @override
  void paint(Canvas canvas, Size size) {
    shader.setFloat(0, size.width);
    shader.setFloat(1, size.height);

    final paint = Paint()..shader = shader;
    canvas.drawRect(
      Rect.fromLTWH(0, 0, size.width, size.height),
      paint,
    );
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return false;
  }
}