custom_paint.dart

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(primarySwatch: Colors.red),
      title: 'Flutter Demo',
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  const HomePage({Key key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage>
    with SingleTickerProviderStateMixin {
  AnimationController _animation;
  Animation<double> _radiusAnimation;
  @override
  void initState() {
    super.initState();
    _animation = AnimationController(
      vsync: this,
      duration: const Duration(milliseconds: 1000),
    );

    _radiusAnimation = _animation.drive(Tween<double>(
      begin: 0.2,
      end: 0.5,
    ));

    _animation.repeat(reverse: true);
  }

  @override
  void dispose() {
    _animation.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Title'),
      ),
      body: AnimatedBuilder(
        animation: _radiusAnimation,
        builder: (context, child) => CustomPaint(
              painter: Sky(radius: _radiusAnimation.value),
              child: child,
            ),
        child: SizedBox.expand(),
      ),
    );
  }
}

class Sky extends CustomPainter {
  const Sky({
    @required this.radius,
  }) : super();
  final double radius;

  @override
  void paint(Canvas canvas, Size size) {
    var rect = Offset.zero & size;
    var gradient = RadialGradient(
      center: Alignment.center,
      radius: radius,
      colors: const [Color(0xFFFF0000), Color(0xFFFFCCFF)],
      stops: [0.2, 1.0],
    );
    canvas.drawRect(
      rect,
      Paint()..shader = gradient.createShader(rect),
    );
  }

  @override
  bool shouldRepaint(Sky oldDelegate) => oldDelegate.radius != radius;
}