如何在TextField中“预选”文本，以便输入新内容立即替换以前的文本？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  final TextEditingController _controller = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('TextField预选文本示例'),
        ),
        body: Center(
          child: TextField(
            controller: _controller,
            decoration: InputDecoration(
              hintText: '请输入文本',
            ),
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            _controller.text = '新内容';
            _controller.selection = TextSelection(
              baseOffset: 0,
              extentOffset: _controller.text.length,
            );
          },
          child: Icon(Icons.edit),
        ),
      ),
    );
  }
}