Circle animation

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: CircleAnimation(),
    );
  }
}

class CircleAnimation extends StatefulWidget {
  @override
  _CircleAnimationState createState() => _CircleAnimationState();
}

class _CircleAnimationState extends State<CircleAnimation>
    with SingleTickerProviderStateMixin {
  Animation<double> animation;
  AnimationController controller;
  bool isLeft = true;
  bool runOnce = false;

  @override
  void initState() {
    super.initState();
    controller =
        AnimationController(duration: const Duration(seconds: 2), vsync: this);
    animation = Tween<double>(begin: 0, end: 100).animate(controller)
      ..addListener(() {
        setState(() {});
      });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
          title: RaisedButton(
        child: Text("Press me"),
        onPressed: (() {
          if (!controller.isAnimating) {
            if (runOnce) {
              isLeft = !isLeft;
            } else {
              runOnce = true;
            }
            controller.reset();
            controller.forward();
          }
        }),
      )),
      body: CustomPaint(
        painter:
            CirclePainter(isLeft ? animation.value : 100 - animation.value),
        child: Container(
          width: double.infinity,
        ),
      ),
    );
  }
}

class CirclePainter extends CustomPainter {
  double x;
  CirclePainter(this.x);

  @override
  void paint(Canvas canvas, Size size) {
    Offset center = Offset(size.width * x / 100, 20);
    canvas.drawCircle(center, 30, Paint()..color = Colors.green);
  }

  @override
  bool shouldRepaint(CirclePainter oldDelegate) => this.x != oldDelegate.x;
}