Loading Animation with a Custom Painter

main.dart

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

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark()
      .copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget>
    with SingleTickerProviderStateMixin {
  AnimationController animation;
  double turn = 0;

  @override
  void initState() {
    super.initState();

    // create the animation controller
    animation = AnimationController(
      vsync: this,
      duration: Duration(seconds: 20),
    )
      ..addListener(() {
        setState(() => turn = animation.value);
      })..forward();

    
  }

  @override
  void dispose() {
    animation.dispose();
    super.dispose();
  }
  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      size: Size(200, 200),
      painter: ArcPainter(angle: turn),
    );
  }
}

class ArcPainter extends CustomPainter {
  final double angle;

  ArcPainter({this.angle});

  @override
  bool shouldRepaint(ArcPainter _) {
    return angle != _.angle;
  }

  @override
  void paint(Canvas canvas, Size size) {
    var p = Paint()
      ..color = Colors.blueGrey
      ..strokeWidth = 10.0
      ..style = PaintingStyle.stroke;

    var rect = Rect.fromLTWH(0.0, 0.0, size.width / 2, size.height / 2,);
    canvas.drawArc(
        rect,
        2 * pi,
        angle * (2 * pi),
        false,
        p);
  }

  degToRad(double degrees) => degrees * pi / 180.0;
}