组合我的秒表代码的开始和停止按钮- Flutter

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Stopwatch',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: StopwatchPage(),
    );
  }
}

class StopwatchPage extends StatefulWidget {
  @override
  _StopwatchPageState createState() => _StopwatchPageState();
}

class _StopwatchPageState extends State<StopwatchPage> {
  bool isRunning = false;
  Stopwatch stopwatch = Stopwatch();

  void startStopwatch() {
    setState(() {
      isRunning = true;
      stopwatch.start();
    });
  }

  void stopStopwatch() {
    setState(() {
      isRunning = false;
      stopwatch.stop();
    });
  }

  void resetStopwatch() {
    setState(() {
      isRunning = false;
      stopwatch.reset();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Stopwatch'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              stopwatch.elapsed.toString(),
              style: TextStyle(fontSize: 48),
            ),
            SizedBox(height: 20),
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                RaisedButton(
                  onPressed: isRunning ? stopStopwatch : startStopwatch,
                  child: Text(isRunning ? 'Stop' : 'Start'),
                ),
                SizedBox(width: 20),
                RaisedButton(
                  onPressed: resetStopwatch,
                  child: Text('Reset'),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}