Flutter:如何将前景添加到可消除的小部件中

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Foreground Example'),
        ),
        body: Stack(
          children: [
            Container(
              color: Colors.blue,
            ),
            Center(
              child: Text(
                'Background',
                style: TextStyle(
                  fontSize: 30,
                  color: Colors.white,
                ),
              ),
            ),
            Positioned(
              top: 100,
              left: 100,
              child: Container(
                width: 200,
                height: 200,
                color: Colors.red,
              ),
            ),
            Positioned(
              top: 150,
              left: 150,
              child: Text(
                'Foreground',
                style: TextStyle(
                  fontSize: 20,
                  color: Colors.white,
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}