https://youtu.be/gtaEpsQpTSo

paintAnimation.dart

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

void main() => runApp(App());

class App extends StatelessWidget {
  const App({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: Main(),);
  }
}

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

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

class _MainState extends State<Main> {

  double size = 400.0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: Text("Paint Animation"),
        centerTitle: true,
      ),
      body: Container(
        width: MediaQuery.of(context).size.width,
        alignment: Alignment.center,
        child: Stack(
          children: [
            Container(
              width: this.size,
              height: this.size,
              decoration: BoxDecoration(
                image: DecorationImage(
                  fit: BoxFit.cover,
                  image: NetworkImage("https://cdn.pixabay.com/photo/2017/01/14/12/59/iceland-1979445__480.jpg")
                )
              ),
            ),
            Positioned(
              top: 0,
              bottom: 0,
              left: 0,
              right: 0,
              child: AnimContainer(size: this.size,),
            )
          ],
        ),
      ),
    );
  }
}

class AnimContainer extends StatefulWidget {
  final double size;
  const AnimContainer({Key? key, required this.size}) : super(key: key);

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

class _AnimContainerState extends State<AnimContainer> with SingleTickerProviderStateMixin{

  AnimationController? ct;
  Animation? anim;

  @override
  void initState() {
    this.ct = AnimationController(vsync: this, duration: Duration(seconds: 3),)
      ..addListener(() {
        if(!this.mounted) return;
        setState(() {});
      });
    this.anim = Tween<double>(begin: widget.size, end: 0).animate(this.ct!);
    setState(() {});
    Future.delayed(Duration(seconds: 3), () async => await this.ct!.forward());
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return (this.anim == null || this.ct == null) ? Container() : CustomPaint(
      painter: Mc(size: Size(widget.size, this.anim!.value)),
    );
  }
}

class Mc extends CustomPainter{
  final Size size;
  const Mc({required this.size});
  @override
  void paint(Canvas canvas, Size size) {
    final pathPaint = Paint()
      ..color = Colors.white
      ..style = PaintingStyle.fill
      ..strokeWidth = 4;
    Path path = Path();
    path.lineTo(0.0,this.size.width);
    path.lineTo(this.size.height*0.90,this.size.width);
    path.lineTo(this.size.height*0.80,this.size.width/2);
    path.lineTo(this.size.height*0.90,0.0);
    path.close();
    canvas.drawPath(path, pathPaint);
  }

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

}