make half circle from top for Container by (path.quadraticBezierTo) Flutter

The solution is when I delete margin it worked well.

The reason I used margin because I want my Container to be in the bottom of the screen, I replied margin by Positioned : bottom: 0 inside Stack

Stack(
 children: <Widget>[
   Positioned(
     bottom: 0,
     child: ClipPath(
     clipper: MyClipper(),
     child: Container(
//       margin: EdgeInsets.only(top: size.height * 0.5),
         height: size.height * 0.45,
         width: size.width,
         color: Colors.white,
         ),
      ),
   ),
 ]
),

You can copy paste run full code below
You can reference https://blog.usejournal.com/how-to-draw-custom-shapes-in-flutter-aa197bda94bf

Do a Quadratic Bezier to C, having B as a control.

code snippet

Path getClip(Size size) {
    var path = Path();

    path.moveTo(0.0, height);
    path.quadraticBezierTo(
        size.width / 2, size.height * 0.2, size.width, size.height - height);
    path.lineTo(size.width, 0.0);
    path.lineTo(0.0, 0.0);

    path.close();

    return path;
  }

full code

import 'package:flutter/material.dart';

class ClipPainter extends CustomPainter {
  final CustomClipper<Path> clipper;

  ClipPainter(this.clipper);

  @override
  void paint(Canvas canvas, Size size) {
    var paint = Paint();
    paint.color = Colors.blue[800];
    paint.style = PaintingStyle.fill;
    canvas.drawPath(clipper.getClip(size), paint);
  }

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

class ArcClipper extends CustomClipper<Path> {
  ArcClipper(this.height);

  ///The height of the arc
  final double height;

  @override
  Path getClip(Size size) {
    var path = Path();

    path.moveTo(0.0, height);
    path.quadraticBezierTo(
        size.width / 2, size.height * 0.2, size.width, size.height - height);
    path.lineTo(size.width, 0.0);
    path.lineTo(0.0, 0.0);

    path.close();

    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) {
    ArcClipper oldie = oldClipper as ArcClipper;
    return height != oldie.height;
  }
}

class Arc extends StatelessWidget {
  const Arc({
    Key key,
    @required this.height,
    @required this.child,
  }) : super(key: key);

  /// The widget which one of [edge]s is going to be clippddddded as arc
  final Widget child;

  ///The height of the arc
  final double height;

  @override
  Widget build(BuildContext context) {
    var clipper = ArcClipper(height);
    return CustomPaint(
      painter: ClipPainter(clipper),
      child: ClipPath(
        clipper: clipper,
        child: child,
      ),
    );
  }
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Home(),
    );
  }
}

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('Test'),
        ),
        body: Center(
            child: Container(
          width: 100,
          height: 100,
          decoration: BoxDecoration(
            border: Border.all(),
          ),
          child: Arc(
              height: 50,
              child: Container(
                  decoration: BoxDecoration(
                    border: Border.all(),
                  ),
                  width: 100,
                  height: 100)),
        )));
  }
}