How to draw a custom rectangle with border radius in Flutter?

Question

My UI draw like this picture:

So I write some code by using CodePainter in Flutter. Here's my code:

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

class BackgroundShape extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final Paint paint = Paint();
    paint.color = Colors.black;
    var smallRect = Alignment.bottomCenter.inscribe(Size(100, 50), Rect.fromLTWH(size.width/2 -35, size.height-40, 40, 30));
    var path = Path();
    path.fillType = PathFillType.evenOdd;
    path.addRRect(RRect.fromRectAndCorners(Rect.fromLTWH(0, 0, size.width, size.height), bottomLeft: Radius.circular(10), bottomRight: Radius.circular(10)));
    path.addRRect(RRect.fromRectAndCorners(smallRect, topLeft: Radius.circular(10), topRight: Radius.circular(10)));
    path.lineTo(0, size.height);
    path.lineTo(size.width/2 - 35, size.height);
    path.lineTo(size.width/2 - 35, size.height-40);
    path.lineTo(size.width/2 + 35, size.height-40);
    path.lineTo(size.width/2 + 35, size.height);
    path.lineTo(size.width, size.height);
    path.lineTo(size.width, 0);
    path.close();

    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => false;
}

Here is my result:

How to add border radius? please help me:D thanks!

Answer 1

For some reason, I'm not able to run completely the code you've provided, but taking consideration your current requirements and from the comments. I've created the following samples for your references:

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter Demo'),
      ),
      body: Center(
        child: Container(
          width: 200,
          height: 200,
          child: CustomPaint(
            painter: PainterTwo(),
          ),
        ),
      ),
    );
  }
}

class PainterOne extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    double w = size.width;
    double h = size.height;
    double r = 15;

    var paint1 = Paint()
      ..color = Color(0xff888888)
      ..style = PaintingStyle.fill;

    RRect fullRect = RRect.fromRectAndRadius(
      Rect.fromCenter(center: Offset(w / 2, h / 2), width: w, height: h),
      Radius.circular(r),
    );
    canvas.drawRRect(fullRect, paint1);
  }

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

class PainterTwo extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..color = Colors.blue
      ..style = PaintingStyle.stroke
      ..strokeWidth = 8.0;

    Path path = Path();
    path.addRRect(RRect.fromRectAndRadius(
        Rect.fromLTWH(
            size.width / 2, size.height / 2, size.width / 4, size.height / 4),
        Radius.circular(15)));
    canvas.drawPath(path, paint);
  }

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

Output of PainterOne():

Output of PainterTwo():