React Native实现验证码倒计时功能
实现倒计时功能使用的是核心方法 setInterval，setInterval() 方法会不停地调用函数，直到 clearInterval() 被调用或窗口被关闭。

startCountDown() {
    this.interval = setInterval(() => { this.setState({ countdown: this.getCountdown() - 1 }); },1000); }

当 App 切换至后台之后，setInterval 将不会执行，而当 App 从后台切换至前台时，setInterval 将继续执行，所以对于切换至后台的情况需要监听 App 状态，记录变化的时间。React Native 提供了 AppState 监听 App 状态改变，AppState 一共有三种状态，包括以下三种。

• active - 应用正在前台运行
• background - 应用正在后台运行。用户既可能在别的应用中，也可能在桌面
• inactive - 这是一个过渡状态，不会在正常的 React Native 应用中出现

componentDidMount() {
    AppState.addEventListener('change',this._handleAppStateChange);
}

componentWillUnmount() {
    AppState.removeEventListener('change',this._handleAppStateChange);
    this.interval && clearInterval(this.interval);
  }

 _handleAppStateChange = (nextAppState) => {
    if (this.state.appState === 'active' && nextAppState.match(/inactive|background/)) {
          this.backgroundTime = new Date().getTime() / 1000;
    }
    if (this.state.appState.match(/inactive|background/) && nextAppState === 'active') {
          this.backgroundTime = utils.fomatFloat(new Date().getTime() / 1000 - this.backgroundTime,0);
     }
     this.setState({appState: nextAppState});
 }

最后完整代码

import React,{
    PureComponent,} from 'react';
import {
    TouchableOpacity,Text,AppState
} from 'react-native';

import * as utils from '../utils';

class CountDown extends PureComponent {
    constructor(props) {
        super(props);
        this.state = {
            appState: AppState.currentState,countdown: -1,disabled: false
        };
        this.backgroundTime = 0;
    }

    componentDidMount() {
        AppState.addEventListener('change',this._handleAppStateChange);
    }

    componentWillUnmount() {
        AppState.removeEventListener('change',this._handleAppStateChange);
        this.interval && clearInterval(this.interval);
    }

    _handleAppStateChange = (nextAppState) => {
        if (this.state.appState === 'active' && nextAppState.match(/inactive|background/)) {
            this.backgroundTime = new Date().getTime() / 1000;
        }
        if (this.state.appState.match(/inactive|background/) && nextAppState === 'active') {
            this.backgroundTime = utils.fomatFloat(new Date().getTime() / 1000 - this.backgroundTime,0);
        }
        this.setState({appState: nextAppState});
    }

    setCountdown(countdown) {
        this.setState({
           countdown: countdown
        });
    }

    getCountdown() {
        return this.state.countdown;
    }

    startCountDown() {
        this.interval = setInterval(() => { if (this.backgroundTime < this.getCountdown()) { this.setState({ countdown: this.getCountdown() - this.backgroundTime - 1 },()=>{ this.backgroundTime = 0; if (this.getCountdown() < 0) { this.interval && clearInterval(this.interval); } if (this.getCountdown() >= 0) { this.setState({ disabled: true }); } else { this.setState({ disabled: false }); } }); } else { this.setCountdown(-1); this.setState({ disabled: false }); this.interval && clearInterval(this.interval); } },1000); this.setState({ disabled: true }); } render() { const { onPress,style,vcodeTextStyle } = this.props; return ( <TouchableOpacity disabled={this.state.disabled} onPress={onPress} style={style}> {this.state.countdown >= 0 ? <Text style={vcodeTextStyle}> {`${this.state.countdown}`}秒 </Text> : <Text style={vcodeTextStyle}> 获取验证码 </Text> } </TouchableOpacity> ); } } export default CountDown; 

其中 utils.fomatFloat 是截取小数点后几位的一个方法

//保留小数点后几位
export function fomatFloat(src,pos) {
    return Math.round(src * Math.pow(10,pos)) / Math.pow(10,pos);
}