React 中的倒数计时器答案

【问题标题】：Countdown timer in ReactReact 中的倒数计时器
【发布时间】：2017-04-14 15:10:15
【问题描述】：

我在 JavaScript 中看到了很多倒数计时器，并希望在 React 中使用一个。

我借用了我在网上找到的这个功能：

secondsToTime(secs){
    let hours = Math.floor(secs / (60 * 60));

    let divisor_for_minutes = secs % (60 * 60);
    let minutes = Math.floor(divisor_for_minutes / 60);

    let divisor_for_seconds = divisor_for_minutes % 60;
    let seconds = Math.ceil(divisor_for_seconds);

    let obj = {
        "h": hours,
        "m": minutes,
        "s": seconds
    };
    return obj;
  };

然后我自己写了这段代码

  initiateTimer = () => {
    let timeLeftVar = this.secondsToTime(60);
    this.setState({ timeLeft: timeLeftVar })
  };

  startTimer = () => {
    let interval = setInterval(this.timer, 1000);
    this.setState({ interval: interval });
  };

  timer = () => {
    if (this.state.timeLeft >0){
      this.setState({ timeLeft: this.state.timeLeft -1 });
    }
    else {
      clearInterval(this.state.interval);
      //this.postToSlack();
    }
  };

目前点击它会将屏幕上的时间设置为：Time Remaining: 1 m : 0 s 但它不会将其减少到 Time Remaining: 0 m : 59 s 然后 Time Remaining: 0 m : 58 s 等等等等

我想我需要使用不同的参数再次调用该函数。我该怎么做呢？

编辑：我忘了说，我想要这个功能，以便我可以使用秒到分和秒

【问题讨论】：

the React documentation examples 之一是一个可以自我更新的时钟，看起来它会相当有用...
@T.J.Crowder 这是半有用的。他们只是得到一个时间，虽然可以通过 componentDidMount 返回它，而我只想从起始位置提取秒和分钟..
也许您可以使用 Stack Snippets 在问题中添加一个可运行的 minimal reproducible example support React and JSX，这样我们就可以看到问题的实际效果。
@T.J.Crowder 发现在 JSfiddle 中创建一个非常困难，因为我在许多文件中使用了许多带有许多道具的组件
@T.J.Crowder 来自问题，你觉得什么有意义？（看看我是否可以为解释不太清楚的事情添加更多知识）

标签： javascript reactjs countdown

【解决方案1】：

在本机反应中：

用法：

时间戳属性必须以秒为单位

const refTimer = useRef();
  
  const timerCallbackFunc = timerFlag => {
    // Setting timer flag to finished
    console.warn(
      'You can alert the user by letting him know that Timer is out.',
    );
  };
    
    
 <Timer
 ref={refTimer}
 timestamp={moment(item?.time_left).diff(moment(), 'seconds')}
 timerCallback={timerCallbackFunc}
 textStyle={styles.timerTextAHL}
 />

Timer.js

import React, {
  useState,
  useEffect,
  useRef,
  forwardRef,
  useImperativeHandle,
} from 'react';
import { Text, View } from 'react-native';

const Timer = forwardRef((props, ref) => {
  // For Total seconds
  const [timeStamp, setTimeStamp] = useState(
    props.timestamp ? props.timestamp : 0,
  );
  // Delay Required
  const [delay, setDelay] = useState(props.delay ? props.delay : 1000);

  // Flag for informing parent component when timer is over
  const [sendOnce, setSendOnce] = useState(true);

  // Flag for final display time format
  const [finalDisplayTime, setFinalDisplayTime] = useState('');

  useInterval(() => {
    if (timeStamp > 0) {
      setTimeStamp(timeStamp - 1);
    } else if (sendOnce) {
      if (props.timerCallback) {
        props.timerCallback(true);
      } else {
        console.log('Please pass a callback function...');
      }
      setSendOnce(false);
    }
    setFinalDisplayTime(secondsToDhms(timeStamp));
  }, delay);

  function secondsToDhms(seconds) {
    seconds = Number(seconds);
    var d = Math.floor(seconds / (3600 * 24));
    var h = Math.floor((seconds % (3600 * 24)) / 3600);
    var m = Math.floor((seconds % 3600) / 60);
    var s = Math.floor(seconds % 60);

    var dDisplay = d > 0 ? d + 'd ' : '';
    var hDisplay = h > 0 ? h + 'h ' : '';
    var mDisplay = m > 0 ? m + 'm ' : '';
    var sDisplay = s > 0 ? s + 's ' : '';
    return dDisplay + hDisplay + mDisplay + sDisplay;
  }

  const refTimer = useRef();
  useImperativeHandle(ref, () => ({
    resetTimer: () => {
      // Clearing days, hours, minutes and seconds
      // Clearing Timestamp
      setTimeStamp(props.timestamp);
      setSendOnce(true);
    },
  }));

  return (
    <View ref={refTimer} style={props.containerStyle}>
      <Text style={props.textStyle}>{sendOnce ? finalDisplayTime : '0'}</Text>
    </View>
  );
});

function useInterval(callback, delay) {
  const savedCallback = useRef();

  // Remember the latest function.
  useEffect(() => {
    savedCallback.current = callback;
  }, [callback]);

  // Set up the interval.
  useEffect(() => {
    function tick() {
      savedCallback.current();
    }
    if (delay !== null) {
      const id = setInterval(tick, delay);
      return () => {
        clearInterval(id);
      };
    }
  }, [delay]);
}

export default Timer;

【讨论】：

【解决方案2】：

这是一个 TypeScript 版本的 React 倒数计时器。我使用了 Masood 和 M.Georgiev 兄弟的代码

import React, {useState, useEffect, useCallback} from "react";

const Minute_to_Seconds = 60;
const Seconds_to_milliseconds = 1000;

export interface CounterProps {
    minutes:number,
    statusAlert: (status: string)=>void,
}


export interface TimerProps {

    initialMinute: number,
    initialSeconds: number,
}

const Counter: React.FC<CounterProps> = (props) => {

    const convert_Minutes_To_MiliSeconds = (minute:number) => {

        return  minute * Minute_to_Seconds * Seconds_to_milliseconds;
    }

    const convert_Mili_Seconds_To_Hour = (miliseconds:number) => {

        return new Date(miliseconds).toISOString().slice(11, -5);
    }

    const convert_Mili_Seconds_To_Minute = (miliseconds:number) => {

        return new Date(miliseconds).toISOString().slice(11, -5);
    }

    const [timer_State, setTimer_State]=useState(0);

    const [timerCount, setTimerCount] = useState(convert_Minutes_To_MiliSeconds(props.minutes));

    useEffect(() => {


        if (timerCount > 0) {

            const interval = setInterval(() => {

                    if (timer_State === 0) {

                        props.statusAlert("start");
                        setTimer_State(1);
                    }


                    let tempTimerCount = timerCount;
                    tempTimerCount -= Seconds_to_milliseconds;
                    setTimerCount(tempTimerCount);
                },
                (timer_State === 0)
                    ? 0

                    : Seconds_to_milliseconds

            );
            return () => {


                clearInterval(interval);
            }


        }
        else{

            props.statusAlert("end");
        }



    }, [

        timer_State,
        timerCount,
        props,
    ]);

    return (
        <p>
            Time left: {convert_Mili_Seconds_To_Hour(timerCount)}
        </p>
    );
}



const Timer: React.FC<TimerProps> = (props) => {

    const [ minutes, setMinutes ] = useState(props.initialMinute);
    const [seconds, setSeconds ] =  useState(props.initialSeconds);

    useEffect(()=>{
        const myInterval = setInterval(() => {
            if (seconds > 0) {
                setSeconds(seconds - 1);
            }
            if (seconds === 0) {
                if (minutes === 0) {
                    clearInterval(myInterval)
                } else {
                    setMinutes(minutes - 1);
                    setSeconds(59);
                }
            }
        }, 1000)
        return ()=> {
            clearInterval(myInterval);
        };
    });

    return (
        <div>
            { ((minutes === 0) && (seconds === 0))
                ? "Press F5 to Refresh"
                : <h1> {minutes}:{seconds < 10 ?  `0${seconds}` : seconds}</h1>
            }
        </div>
    )
}


const RCTAPP=()=> {

    const status_Alert2=(status: string)=> {

        console.log("__________________________==================== status: ", status);
        if (status==="start"){
            alert("Timer started");
        }
        else{
            alert("Time's up");
        }
    }

    return (
        <div style={{textAlign: "center"}}>

            <Counter
                minutes={1}
                // minutes={0.1}
                statusAlert={status_Alert2}
            />

            <Timer
                initialMinute={0}
                initialSeconds={30}
            />

        </div>

    );
}


export default RCTAPP;

【讨论】：

【解决方案3】：

我遇到了同样的问题，我找到了这个 npm 包进行倒计时。

安装包

npm install react-countdown --save 或

yarn add react-countdown
将包导入到您的文件中

import Countdown from 'react-countdown';
在渲染方法中调用导入的“倒计时”并传递日期

<Countdown date={new Date('2021-09-26T10:05:29.896Z').getTime()}> 或

<Countdown date={new Date("Sat Sep 26 2021")}>

这里有一个例子。

import React from "react";
import ReactDOM from "react-dom";
import Countdown from "react-countdown";

// Random component
const Completionist = () => <span>You are good to go!</span>;

ReactDOM.render(
  <Countdown date={new Date('2021-09-26T10:05:29.896Z').getTime()}>
    <Completionist />
  </Countdown>,
  document.getElementById("root")
);

详细文档可以看这里https://www.npmjs.com/package/react-countdown

【讨论】：

【解决方案4】：

当您使用功能组件时，上面的代码是一个不错的选择：

import React, { useState, useEffect } from "react";
import { MessageStrip } from "@ui5/webcomponents-react";
import "./Timer.scss";

const nMinuteSeconds = 60;
const nSecondInMiliseconds = 1000;

const convertMinutesToMiliseconds = (minute) =>
  minute * nMinuteSeconds * nSecondInMiliseconds;

const convertMilisecondsToHour = (miliseconds) => new Date(miliseconds).toISOString().slice(11, -5);

export default function Counter({ minutes, onTimeOut }) {
  let [timerCount, setTimerCount] = useState(
    convertMinutesToMiliseconds(minutes)
  );
  let interval;

  useEffect(() => {
    if (interval) {
      clearInterval(interval);
    }

    interval = setInterval(() => {
      if (timerCount === 0 && interval) {
        onTimeOut();
        clearInterval(interval);
      }

      setTimerCount((timerCount -= nSecondInMiliseconds));
    }, nSecondInMiliseconds);
  }, []);

  return (
    <>
      <MessageStrip design="Information" hide-close-button>
        Time left: {convertMilisecondsToHour(timerCount)}
      </MessageStrip>
    </>
  );
}

【讨论】：

【解决方案5】：

功能： 1)开始 2)重置

功能组件

import {useState, useCallback} from 'react';
const defaultCount = 10;
const intervalGap = 300;

const Counter = () => {
    const [timerCount, setTimerCount] = useState(defaultCount);
    
    const startTimerWrapper = useCallback((func)=>{
        let timeInterval: NodeJS.Timer;
        return () => {
            if(timeInterval) {
                clearInterval(timeInterval)
            }
            setTimerCount(defaultCount)
            timeInterval = setInterval(() => {
                func(timeInterval)
            }, intervalGap)
        }
    }, [])

    const timer = useCallback(startTimerWrapper((intervalfn: NodeJS.Timeout) => {
         setTimerCount((val) => {
            if(val === 0 ) {
                clearInterval(intervalfn);
                return val
            } 
            return val - 1
        })
    }), [])

    return <>
        <div> Counter App</div>
        <div> <button onClick={timer}>Start/Reset</button></div>
        <div> {timerCount}</div>
    </>
}
export default Counter;

【讨论】：

如何引用 NodeJS.Timer ？这给了我例外。

【解决方案6】：

这是一个简单的实现，使用钩子和@dan-abramov 的 useInterval 实现

import React, {useState, useEffect, useRef} from 'react'
import './styles.css'

const STATUS = {
  STARTED: 'Started',
  STOPPED: 'Stopped',
}

const INITIAL_COUNT = 120

export default function CountdownApp() {
  const [secondsRemaining, setSecondsRemaining] = useState(INITIAL_COUNT)
  const [status, setStatus] = useState(STATUS.STOPPED)

  const secondsToDisplay = secondsRemaining % 60
  const minutesRemaining = (secondsRemaining - secondsToDisplay) / 60
  const minutesToDisplay = minutesRemaining % 60
  const hoursToDisplay = (minutesRemaining - minutesToDisplay) / 60

  const handleStart = () => {
    setStatus(STATUS.STARTED)
  }
  const handleStop = () => {
    setStatus(STATUS.STOPPED)
  }
  const handleReset = () => {
    setStatus(STATUS.STOPPED)
    setSecondsRemaining(INITIAL_COUNT)
  }
  useInterval(
    () => {
      if (secondsRemaining > 0) {
        setSecondsRemaining(secondsRemaining - 1)
      } else {
        setStatus(STATUS.STOPPED)
      }
    },
    status === STATUS.STARTED ? 1000 : null,
    // passing null stops the interval
  )
  return (
    <div className="App">
      <h1>React Countdown Demo</h1>
      <button onClick={handleStart} type="button">
        Start
      </button>
      <button onClick={handleStop} type="button">
        Stop
      </button>
      <button onClick={handleReset} type="button">
        Reset
      </button>
      <div style={{padding: 20}}>
        {twoDigits(hoursToDisplay)}:{twoDigits(minutesToDisplay)}:
        {twoDigits(secondsToDisplay)}
      </div>
      <div>Status: {status}</div>
    </div>
  )
}

// source: https://overreacted.io/making-setinterval-declarative-with-react-hooks/
function useInterval(callback, delay) {
  const savedCallback = useRef()

  // Remember the latest callback.
  useEffect(() => {
    savedCallback.current = callback
  }, [callback])

  // Set up the interval.
  useEffect(() => {
    function tick() {
      savedCallback.current()
    }
    if (delay !== null) {
      let id = setInterval(tick, delay)
      return () => clearInterval(id)
    }
  }, [delay])
}

// https://stackoverflow.com/a/2998874/1673761
const twoDigits = (num) => String(num).padStart(2, '0')

这里是codeandbox的实现：https://codesandbox.io/s/react-countdown-demo-gtr4u?file=/src/App.js

【讨论】：

谢谢，帮了我很多忙！只是：不要在ms 参数中使用null 到setInterval！

【解决方案7】：

用户输入倒计时

界面截图

import React, { Component } from 'react';
import './App.css';

class App extends Component {
  constructor() {
    super();
    this.state = {
      hours: 0,
      minutes: 0,
      seconds:0
    }
    this.hoursInput = React.createRef();
    this.minutesInput= React.createRef();
    this.secondsInput = React.createRef();
  }

  inputHandler = (e) => {
    this.setState({[e.target.name]: e.target.value});
  }

  convertToSeconds = ( hours, minutes,seconds) => {
    return seconds + minutes * 60 + hours * 60 * 60;
  }

  startTimer = () => {
    this.timer = setInterval(this.countDown, 1000);
  }

  countDown = () => {
    const  { hours, minutes, seconds } = this.state;
    let c_seconds = this.convertToSeconds(hours, minutes, seconds);

    if(c_seconds) {

      // seconds change
      seconds ? this.setState({seconds: seconds-1}) : this.setState({seconds: 59});

      // minutes change
      if(c_seconds % 60 === 0 && minutes) {
        this.setState({minutes: minutes -1});
      }

      // when only hours entered
      if(!minutes && hours) {
        this.setState({minutes: 59});
      }

      // hours change
      if(c_seconds % 3600 === 0 && hours) {
        this.setState({hours: hours-1});
      }

    } else {
      clearInterval(this.timer);
    }
  }


  stopTimer = () => {
    clearInterval(this.timer);
  }

  resetTimer = () => {
    this.setState({
      hours: 0,
      minutes: 0,
      seconds: 0
    });
    this.hoursInput.current.value = 0;
    this.minutesInput.current.value = 0;
    this.secondsInput.current.value = 0;
  }


  render() {
    const { hours, minutes, seconds } = this.state;

    return (
      <div className="App">
         <h1 className="title"> (( React Countdown )) </h1>
         <div className="inputGroup">
            <h3>Hrs</h3>
            <input ref={this.hoursInput} type="number" placeholder={0}  name="hours"  onChange={this.inputHandler} />
            <h3>Min</h3>
            <input  ref={this.minutesInput} type="number"  placeholder={0}   name="minutes"  onChange={this.inputHandler} />
            <h3>Sec</h3>
            <input   ref={this.secondsInput} type="number"  placeholder={0}  name="seconds"  onChange={this.inputHandler} />
         </div>
         <div>
            <button onClick={this.startTimer} className="start">start</button>
            <button onClick={this.stopTimer}  className="stop">stop</button>
            <button onClick={this.resetTimer}  className="reset">reset</button>
         </div>
         <h1> Timer {hours}: {minutes} : {seconds} </h1>
      </div>

    );
  }
}

export default App;

【讨论】：

你好，如何在时间间隔为零后调用函数

【解决方案8】：

显示使用 Date.now() 倒计时的基本概念，而不是减去会随时间漂移的倒计时。

class Example extends React.Component {
  constructor() {
    super();
    this.state = {
      time: {
        hours: 0,
        minutes: 0,
        seconds: 0,
        milliseconds: 0,
      },
      duration: 2 * 60 * 1000,
      timer: null
    };
    this.startTimer = this.start.bind(this);
  }

  msToTime(duration) {
    let milliseconds = parseInt((duration % 1000));
    let seconds = Math.floor((duration / 1000) % 60);
    let minutes = Math.floor((duration / (1000 * 60)) % 60);
    let hours = Math.floor((duration / (1000 * 60 * 60)) % 24);

    hours = hours.toString().padStart(2, '0');
    minutes = minutes.toString().padStart(2, '0');
    seconds = seconds.toString().padStart(2, '0');
    milliseconds = milliseconds.toString().padStart(3, '0');

    return {
      hours,
      minutes,
      seconds,
      milliseconds
    };
  }

  componentDidMount() {}

  start() {
    if (!this.state.timer) {
      this.state.startTime = Date.now();
      this.timer = window.setInterval(() => this.run(), 10);
    }
  }

  run() {
    const diff = Date.now() - this.state.startTime;
    
    // If you want to count up
    // this.setState(() => ({
    //  time: this.msToTime(diff)
    // }));
    
    // count down
    let remaining = this.state.duration - diff;
    if (remaining < 0) {
      remaining = 0;
    }
    this.setState(() => ({
      time: this.msToTime(remaining)
    }));
    if (remaining === 0) {
      window.clearTimeout(this.timer);
      this.timer = null;
    }
  }

  render() {
    return ( <
      div >
      <
      button onClick = {
        this.startTimer
      } > Start < /button> {
        this.state.time.hours
      }: {
        this.state.time.minutes
      }: {
        this.state.time.seconds
      }. {
        this.state.time.milliseconds
      }:
      <
      /div>
    );
  }
}

ReactDOM.render( < Example / > , document.getElementById('View'));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="View"></div>

【讨论】：

这是一种更稳定的方法。然而问题是，暂停和继续更具挑战性。
@Tosh 不是真的......你知道它被暂停时经过了多少时间，你存储它。继续您找出差异并设置新的开始时间。

【解决方案9】：

这是一个使用钩子的解决方案，Timer 组件，我正在用钩子复制上面相同的逻辑

import React from 'react'
import { useState, useEffect } from 'react';

const Timer = (props:any) => {
    const {initialMinute = 0,initialSeconds = 0} = props;
    const [ minutes, setMinutes ] = useState(initialMinute);
    const [seconds, setSeconds ] =  useState(initialSeconds);
    useEffect(()=>{
    let myInterval = setInterval(() => {
            if (seconds > 0) {
                setSeconds(seconds - 1);
            }
            if (seconds === 0) {
                if (minutes === 0) {
                    clearInterval(myInterval)
                } else {
                    setMinutes(minutes - 1);
                    setSeconds(59);
                }
            } 
        }, 1000)
        return ()=> {
            clearInterval(myInterval);
          };
    });

    return (
        <div>
        { minutes === 0 && seconds === 0
            ? null
            : <h1> {minutes}:{seconds < 10 ?  `0${seconds}` : seconds}</h1> 
        }
        </div>
    )
}

export default Timer;

【讨论】：

你每隔一秒初始化并清除间隔，我认为最好把空数组作为useEffect的依赖

【解决方案10】：

class Example extends React.Component {
  constructor() {
    super();
    this.state = { time: {}, seconds: 5 };
    this.timer = 0;
    this.startTimer = this.startTimer.bind(this);
    this.countDown = this.countDown.bind(this);
  }

  secondsToTime(secs){
    let hours = Math.floor(secs / (60 * 60));

    let divisor_for_minutes = secs % (60 * 60);
    let minutes = Math.floor(divisor_for_minutes / 60);

    let divisor_for_seconds = divisor_for_minutes % 60;
    let seconds = Math.ceil(divisor_for_seconds);

    let obj = {
      "h": hours,
      "m": minutes,
      "s": seconds
    };
    return obj;
  }

  componentDidMount() {
    let timeLeftVar = this.secondsToTime(this.state.seconds);
    this.setState({ time: timeLeftVar });
  }

  startTimer() {
    if (this.timer == 0 && this.state.seconds > 0) {
      this.timer = setInterval(this.countDown, 1000);
    }
  }

  countDown() {
    // Remove one second, set state so a re-render happens.
    let seconds = this.state.seconds - 1;
    this.setState({
      time: this.secondsToTime(seconds),
      seconds: seconds,
    });
    
    // Check if we're at zero.
    if (seconds == 0) { 
      clearInterval(this.timer);
    }
  }

  render() {
    return(
      <div>
        <button onClick={this.startTimer}>Start</button>
        m: {this.state.time.m} s: {this.state.time.s}
      </div>
    );
  }
}

ReactDOM.render(<Example/>, document.getElementById('View'));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="View"></div>

【讨论】：

【解决方案11】：

您必须在剩余的秒数内每秒setState（每次调用间隔）。这是一个例子：

class Example extends React.Component {
  constructor() {
    super();
    this.state = { time: {}, seconds: 5 };
    this.timer = 0;
    this.startTimer = this.startTimer.bind(this);
    this.countDown = this.countDown.bind(this);
  }

  secondsToTime(secs){
    let hours = Math.floor(secs / (60 * 60));

    let divisor_for_minutes = secs % (60 * 60);
    let minutes = Math.floor(divisor_for_minutes / 60);

    let divisor_for_seconds = divisor_for_minutes % 60;
    let seconds = Math.ceil(divisor_for_seconds);

    let obj = {
      "h": hours,
      "m": minutes,
      "s": seconds
    };
    return obj;
  }

  componentDidMount() {
    let timeLeftVar = this.secondsToTime(this.state.seconds);
    this.setState({ time: timeLeftVar });
  }

  startTimer() {
    if (this.timer == 0 && this.state.seconds > 0) {
      this.timer = setInterval(this.countDown, 1000);
    }
  }

  countDown() {
    // Remove one second, set state so a re-render happens.
    let seconds = this.state.seconds - 1;
    this.setState({
      time: this.secondsToTime(seconds),
      seconds: seconds,
    });
    
    // Check if we're at zero.
    if (seconds == 0) { 
      clearInterval(this.timer);
    }
  }

  render() {
    return(
      <div>
        <button onClick={this.startTimer}>Start</button>
        m: {this.state.time.m} s: {this.state.time.s}
      </div>
    );
  }
}

ReactDOM.render(<Example/>, document.getElementById('View'));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="View"></div>

【讨论】：

这看起来不错。一个问题是它不会停在 0 并且会变为负数？解决这个问题，我会接受的；）
嗯，它类似于您在初始代码中的内容。检查是否还有几秒钟，然后执行clearInterval。更新了我的答案。
您还可以进行更多优化，例如重置计时器、暂停等，但问题的目标是如何倒计时并将其反映在渲染中。
干杯，由于某些奇怪的原因，我的仍然是负数。我什至 console.logged(seconds) 显示它为 0，所以必须进一步调试
@FabianSchultz 您的解决方案很棒。构建倒数计时器组件并开始使用对我很有帮助。代码非常干净。继续努力！

【解决方案12】：

setInterval 的一个缺点是它会减慢主线程的速度。您可以使用requestAnimationFrame 来设置倒数计时器来防止这种情况发生。例如，这是我的通用倒数计时器组件：

class Timer extends Component {
  constructor(props) {
    super(props)
    // here, getTimeRemaining is a helper function that returns an 
    // object with { total, seconds, minutes, hours, days }
    this.state = { timeLeft: getTimeRemaining(props.expiresAt) }
  }

  // Wait until the component has mounted to start the animation frame
  componentDidMount() {
    this.start()
  }

  // Clean up by cancelling any animation frame previously scheduled
  componentWillUnmount() {
    this.stop()
  }

  start = () => {
    this.frameId = requestAnimationFrame(this.tick)
  }

  tick = () => {
    const timeLeft = getTimeRemaining(this.props.expiresAt)
    if (timeLeft.total <= 0) {
      this.stop()
      // ...any other actions to do on expiration
    } else {
      this.setState(
        { timeLeft },
        () => this.frameId = requestAnimationFrame(this.tick)
      )
    }
  }

  stop = () => {
    cancelAnimationFrame(this.frameId)
  }

  render() {...}
}

【讨论】：

不错！但我认为您可以通过防止太多渲染来进行优化。您不必每帧都 setState （并重新渲染）（每秒约 30 帧）。只有当timeLeft（以秒为单位）发生变化时，您才能setState。也许使用 shouldComponentUpdate ？我说的对吗？

【解决方案13】：

问题在于您的“this”值。计时器函数无法访问“状态”道具，因为在不同的上下文中运行。我建议你这样做：

...
startTimer = () => {
  let interval = setInterval(this.timer.bind(this), 1000);
  this.setState({ interval });
};

如您所见，我为您的计时器功能添加了“绑定”方法。这允许计时器在调用时访问您的反应组件的相同“this”（这是一般使用 javascript 时的主要问题/改进）。

另一种选择是使用另一个箭头函数：

startTimer = () => {
  let interval = setInterval(() => this.timer(), 1000);
  this.setState({ interval });
};

【讨论】：

啊，是的，我确实忘记绑定了。这并不能解决我没有倒计时的主要问题？