main.dart

import 'package:flutter/material.dart';
import 'dart:math' as math;

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter4Fun.com',
      theme: ThemeData(
        primarySwatch: Colors.yellow,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: HomePage(),
    );
  }
}

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

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

class _HomePageState extends State<HomePage> {
  double degree = 0.0;

  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color(0xFF191636),
      body: Center(
        child: Column(
          children: [
            Expanded(child: Container()),
            CustomPaint(
              painter: _OrbitPainter(degree),
              size: Size(300, 100),
            ),
            Expanded(child: Container()),
            Text('${degree.toInt().toString()}°', style: TextStyle(color: Colors.white),),
            Slider(
              value: degree,
              min: 0.0,
              max: 360.0,
              onChanged: (newValue) {
                setState(() {
                  degree = newValue;
                });
              },
            )
          ],
        ),
      ),
    );
  }
}

class _OrbitPainter extends CustomPainter {
  final double electronDegree;

  _OrbitPainter(this.electronDegree);

  @override
  void paint(Canvas canvas, Size size) {
    final center = Offset(size.width / 2, size.height / 2);
    final rect = Rect.fromCenter(center: center, width: size.width, height: size.height);
    canvas.drawOval(
      rect,
      Paint()
        ..color = Colors.white
        ..style = PaintingStyle.stroke
        ..strokeWidth = 1,
    );

    final degree = degreesToRads(electronDegree);
    final electronPos = center +
        Offset(
          math.cos(degree) * (size.width / 2),
          math.sin(degree) * (size.height / 2),
        );
    canvas.drawCircle(electronPos, 8, Paint()..color = Colors.yellowAccent);
  }

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

num degreesToRads(num deg) {
  return (deg * math.pi) / 180.0;
}