Flutter simple circle canvas animation (DartPad link https://dartpad.dev/905d84f36ebd0ee4b9bab2552564bf44)

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 StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
        width: 250,
        height: 250,
        padding: EdgeInsets.all(32.0),
        child: Circle());
  }
}

class CirclePainter extends CustomPainter {
  Paint _whiteArc = Paint()
    ..color = Colors.white
    ..style = PaintingStyle.stroke
    ..strokeCap = StrokeCap.round
    ..strokeWidth = 2.0;

  double _completed = 0.0;

  CirclePainter(this._completed);

  @override
  void paint(Canvas canvas, Size size) {
    double centerX = size.width / 2.0;
    double centerY = size.height / 2.0;
    Offset centerOffset = Offset(centerX, centerY);
    double radius = min(centerX, centerY);
    double arcAngle = 2 * pi * (_completed / 100);

    canvas.drawArc(Rect.fromCircle(center: centerOffset, radius: radius),
        -pi / 2, arcAngle, false, _whiteArc);
  }

  @override
  bool shouldRepaint(CirclePainter oldDelegate) {
    return oldDelegate._completed != _completed;
  }
}

class Circle extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _CircleState();
}

class _CircleState extends State<Circle> with SingleTickerProviderStateMixin {
  double _completed = 0;
  Animation<double> _animation;

  @override
  void initState() {
    super.initState();
    var controller =
        AnimationController(duration: Duration(seconds: 10), vsync: this);

    _animation = Tween(begin: 0.0, end: 100.0).animate(controller)
      ..addListener(() {
        setState(() {
          _completed = _animation.value;
        });
      });

    controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return CustomPaint(painter: CirclePainter(_completed));
  }
}