loading_animation.dart

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


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

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

class RotatingLoadingAnimation extends StatefulWidget{
  @override
  _RotatingLoadingAnimationState createState() => _RotatingLoadingAnimationState();
}

class _RotatingLoadingAnimationState extends State<RotatingLoadingAnimation> with SingleTickerProviderStateMixin{
  
  AnimationController _controller;
  
  bool _completed = false;
  
  @override
  void initState(){
    _controller = AnimationController(vsync: this, duration: Duration(seconds: 3),);
    
    _controller.forward().whenComplete(()=>setState((){_completed = true;}),);
   
    _controller.addListener((){
      setState((){});
    });
    super.initState();
  }
  
  @override
  Widget build(BuildContext context) {
    
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        mainAxisSize: MainAxisSize.max,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: [
          Center(
            child: !_completed?Stack(
              children: [
                Transform.rotate(
                  angle: 20 * _controller.value,
                  child: Container(
                    height: 50,
                    width: 50,
                    child: CustomPaint(
                      painter: RollingThingy(color: Colors.green),
                    ),
                  ),
                ),
                Positioned(
                  top: 5,
                  left: 5,
                  bottom: 5,
                  right: 5,
                  child: Padding(
                    padding: const EdgeInsets.all(3.0),
                    child: Transform.rotate(
                      angle: 20 - (20 * _controller.value),
                      child: Container(
                        height: 40,
                        width: 40,
                        child: CustomPaint(
                          painter: RollingThingy(color: Colors.greenAccent),
                        ),
                      ),
                    ),
                  ),
                ),
              ],
            ): Container(
                  height: 50,
                  width: 50,
                  child: CustomPaint(
                    painter: CustomCheckIcon(),
                  ),
                ),
             
          ),
          SizedBox(height: 20),
          Text(
            !_completed?'${(_controller.value * 100).round()}%':'Completed', 
               style: TextStyle(
                 fontSize: 18,
                 fontWeight: FontWeight.w700,
               ),
              ),
        ],
      ),
    );
  }
}

class RollingThingy extends CustomPainter{
  
  final Color color;
  RollingThingy({this.color});
  
  @override
  void paint(Canvas canvas, Size size) {
    
    Paint paint = Paint()
      ..color = color??Colors.orange
      ..strokeWidth = 6.3
      ..style = PaintingStyle.stroke
      ..strokeCap = StrokeCap.round;
    
    canvas.drawArc(Offset(0, 0) & size, 0, 2, false, paint);
    canvas.drawArc(Offset(0, 0) & size, 3, 2, false, paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
   
    return false;
  }
  
}

class CustomCheckIcon extends CustomPainter{
  @override
  
  
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..color = Colors.green
      ..strokeWidth = 6.3
      ..style = PaintingStyle.stroke
      ..strokeCap = StrokeCap.round;
    
    double centerX = size.width/2;
    double centerY = size.height/2;
    
    Path path = Path()
      ..moveTo(centerX-15, centerY)
      ..lineTo(centerX -8, centerY + 8)
      ..lineTo(centerX + 15, centerY -8);
    
    double radius = min(centerX, centerY);
    
    canvas.drawCircle(Offset(centerX, centerY), radius, paint);
    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return false;
  }
  
  
}