计时器 |字段 '_timer@559426790' 尚未初始化

Question

我正在尝试按照教程制作番茄钟应用程序计时器。教书的小哥用这段代码成功制作了app

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:intl/intl.dart';
// Checking
void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _seconds = 00;
  int _minutes = 25;
  Timer _timer;
  var f = NumberFormat("00");

  void _stopTimer(){
      if (_timer!=null){
        _timer.cancel();
        _seconds = 0;
        _minutes = 25;
      }
  }

  void _startTimer(){
    if (_timer != null){
      _stopTimer();
    }
  if (_minutes > 0){
  _seconds = _minutes * 60;
  }
  if (_seconds >60){
  _minutes = (_seconds/60).floor();
  _seconds -= (_minutes * 60);
  }
  _timer = Timer.periodic(Duration(seconds: 1), (timer) {
    setState(() {
      if (_seconds > 0){
        _seconds--;
      }
      else {
        if (_minutes >0){
          _seconds = 59;
          _minutes--;
        }
        else {
          _timer.cancel();
          print("Timer Complete");
        }
      }
    });
  });
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.black,
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Row(
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.start,
            children: [
              Text(
                "${f.format(_minutes)} : ${f.format(_seconds)}",
                style: TextStyle(
                  color: Colors.white,
                  fontSize: 48,
                ),
              ),
            ],
          ),
          SizedBox(
            height: 300,
          ),
          Row(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: [
              RaisedButton(
                onPressed: (){
                  setState(() {
                    _stopTimer();
                  });
                },
                color: Colors.black,
                shape: CircleBorder(
                  side: BorderSide(color: Colors.orange[300])
                ),
                child: Padding(
                  padding: const EdgeInsets.all(40.0),
                  child: Text(
                      "Stop",
                    style: TextStyle(
                      color: Colors.white,
                      fontSize: 24,
                    ),
                  ),
                ),
              ),
              RaisedButton(
                onPressed: (){
                  _startTimer();
                },
                color: Colors.orange[300],
                shape: CircleBorder(
                    side: BorderSide(color: Colors.orange[300])
                ),
                child: Padding(
                  padding: const EdgeInsets.all(40.0),
                  child: Text(
                    "Start",
                    style: TextStyle(
                      color: Colors.white,
                      fontSize: 24,
                    ),
                  ),
                ),
              ),
            ],
          ),
        ],
      ),
    );
  }
}

不幸的是，如果我复制代码，它会触发空安全。即使添加“late Timer _timer”，我也会收到以下错误“Field '_timer@559426790' has not been initialized”。有人可以向我解释原因并为我提供解决方案吗？我是新手，很想了解它。提前谢谢你

Answer 1

你没有初始化Timer，你需要在initState中添加late并初始化Timer或者设置类似Timer _timer = Timer(Duration(milliseconds: 1), () {});[=16的东西=]

例子

  int _start = 1;
  Timer _timer = Timer(Duration(milliseconds: 1), () {});

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

  void startTimer() {
    const oneSec = Duration(milliseconds: 100);
    _timer = Timer.periodic(
      oneSec,
          (timer) {
        if (_start == 0) {
          setState(() {
            timer.cancel();
          });
        } else {
          setState(() {
            _start--;
          });
        }
      },
    );
  }

计时器 |字段 '_timer@559426790' 尚未初始化

Timer | Field '_timer@559426790' has not been initialized

android

timer

dart

flutter