MaskLayer

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    theme: ThemeData(primarySwatch: Colors.blueGrey),
    home: PracticeApp(),
  ));
}

class PracticeApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.grey,
      appBar: AppBar(
        title: Text('PracticeApp'),
        centerTitle: true,
      ),
      body: Stack(
        children: [
          Container(
            decoration: BoxDecoration(
              image: DecorationImage(
                image: NetworkImage(
                    'https://flutter.github.io/assets-for-api-docs/assets/widgets/owl-2.jpg'),
                fit: BoxFit.cover,
              ),
            ),
          ),
          CustomPaint(
            painter: HolePainter(),
            child: Container(),
          ),
          Row(
            mainAxisAlignment: MainAxisAlignment.spaceBetween,
            children: [
              Container(
                decoration: BoxDecoration(
                  borderRadius: BorderRadius.all(
                    Radius.circular(50),
                  ),
                  color: Colors.grey.shade600.withOpacity(0.8),
                ),
                padding: EdgeInsets.all(10.0),
                margin: EdgeInsets.all(20),
                child: IconButton(
                  icon: Icon(Icons.flash_on),
                  onPressed: null,
                ),
              ),
              Container(
                decoration: BoxDecoration(
                  borderRadius: BorderRadius.all(
                    Radius.circular(50),
                  ),
                  color: Colors.grey.shade600.withOpacity(0.8),
                ),
                padding: EdgeInsets.all(10.0),
                margin: EdgeInsets.all(20),
                child: IconButton(
                  icon: Icon(Icons.photo_camera),
                  onPressed: null,
                ),
              ),
            ],
          )
        ],
      ),
    );
  }
}

class HolePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    //print('${size.height},${size.width}');
    final paint = Paint();
    paint.color = Colors.grey.withOpacity(0.8);
    canvas.drawPath(
      Path.combine(
        PathOperation.difference,
        Path()
          ..addRRect(RRect.fromLTRBR(
              0, 0, size.width, size.height, Radius.circular(0))),
        Path()
          ..addOval(Rect.fromCircle(
              center: Offset(size.width / 2, size.height / 2), radius: 180))
          ..close(),
      ),
      paint,
    );
  }

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