Hello, Flutter World

main.dart

import 'package:flutter/material.dart';

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext ctx) {
    return MaterialApp(home: new Home());
  }
}

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  @override
  Widget build(BuildContext ctx) {
    return Scaffold(
        appBar: AppBar(title: Text("Hello, Flutter World")),
        body: Center(
            child: SizedBox(
                width: 400,
                height: 400,
                child: CustomPaint(painter: Drawing()))));
  }
}

class Drawing extends CustomPainter {
  @override
  bool shouldRepaint(Drawing prev) {
    return false;
  }

  @override
  void paint(Canvas canvas, Size size) {
    var center = size.width / 2;
    var paint = Paint()..color = Colors.blue;
    canvas.drawCircle(Offset(center, center), 100, paint);
  }
}