Canvas 类的clipPath 方法不裁剪给定区域

Canvas 类的clipPath 方法不裁剪指定区域。一个正方形被绘制成蓝色。clipPath 方法应该将它剪辑成一个三角形。它没有发生。告诉。下面是完整的代码和截图。

主要.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
 @override
 Widget build(BuildContext context) {
  return MaterialApp(
   debugShowCheckedModeBanner: false,
   title: 'Name App',
   theme: ThemeData(
    primarySwatch: Colors.blue,
   ),
   home: Scaffold(
    appBar: AppBar(
     title: Text('Name Page'),
    ),
    body: MyHomePage(),
   ),
  );
 }
}

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

class _MyHomePageState extends State<MyHomePage> {

 @override
 void initState() {
  super.initState();

 }

 @override
 void dispose() {
  super.dispose();

 }

 void _f1() {
  setState(() {

  });
 }

 @override
 Widget build(BuildContext context) {
  return Stack(
   children: [
    Align(
     alignment: Alignment.center,
     child: CustomPaint(
      //верхний слой
      foregroundPainter: A(),
      //средний слой
      child: Container(
       width: 300.0,
       height: 300.0,
       decoration: BoxDecoration(
        border: Border.all(
         width: 1.0,
         color: Colors.pink
        ),
       ),
      ),
     ),
    ),
   ],
  );
 }

}

class A extends CustomPainter {

 @override
 void paint(Canvas canvas, Size size) {

  //синий цвет, заполнение
  final paint = Paint()
   ..style = PaintingStyle.fill
   ..color = Colors.blue;

  //квадрат
  canvas.drawRRect(
   RRect.fromRectAndRadius(
    Rect.fromLTWH(0, 0, 250, 250),
    Radius.circular(0)
   ),
   paint,
  );

  //область треугольника, которая должна обрезаться
  canvas.clipPath(
   Path()
    ..lineTo(0.0, 0.0)
    ..lineTo(0, size.height - 50)
    ..lineTo(size.width - 50, size.height - 50)
    ..close()
  );

 }

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

}