在构建期间调用setState()或markNeedsBuild()。尝试在flutter中创建一个简单的计时器

import 'package:flutter/material.dart';

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

class TimerApp extends StatefulWidget {
  @override
  _TimerAppState createState() => _TimerAppState();
}

class _TimerAppState extends State<TimerApp> {
  int _seconds = 0;
  bool _isRunning = false;

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

  void startTimer() {
    _isRunning = true;
    Future.delayed(Duration(seconds: 1), () {
      if (_isRunning) {
        setState(() {
          _seconds++;
        });
        startTimer();
      }
    });
  }

  void stopTimer() {
    _isRunning = false;
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Timer App'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text(
                'Seconds: $_seconds',
                style: TextStyle(fontSize: 24),
              ),
              SizedBox(height: 20),
              RaisedButton(
                child: Text('Stop Timer'),
                onPressed: stopTimer,
              ),
            ],
          ),
        ),
      ),
    );
  }
}