如何在Flutter中使用TextField小部件的输入来设置计时器的持续时间？

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

class TimerPage extends StatefulWidget {
  @override
  _TimerPageState createState() => _TimerPageState();
}

class _TimerPageState extends State<TimerPage> {
  final TextEditingController _controller = TextEditingController();
  Timer _timer;
  int _duration = 0;
  
  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Timer'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            TextField(
              controller: _controller,
              keyboardType: TextInputType.number,
              decoration: InputDecoration(
                labelText: '请输入持续时间（秒）',
              ),
            ),
            SizedBox(height: 20),
            RaisedButton(
              child: Text('开始计时'),
              onPressed: () {
                setState(() {
                  _duration = int.parse(_controller.text);
                  _startTimer();
                });
              },
            ),
            SizedBox(height: 20),
            Text('剩余时间：$_duration 秒'),
          ],
        ),
      ),
    );
  }
  
  void _startTimer() {
    if (_timer != null) {
      _timer.cancel();
    }
    _timer = Timer.periodic(Duration(seconds: 1), (timer) {
      setState(() {
        if (_duration > 0) {
          _duration--;
        } else {
          _timer.cancel();
        }
      });
    });
  }
}

void main() {
  runApp(MaterialApp(
    home: TimerPage(),
  ));
}