ReactNative: 使用AppState的API获取App的状态

一、简介

App在运行的整个过程中，它会有不同的运行状态，例如激活失活状态、前后台进入和离开状态等。开发者可以根据App的不同状态选择合适的时机完成需要的工作。ReactNative中提供了AppState这个API来告知App的状态，它还可以通知状态的改变、甚至用于消息通知的推送等。

二、API

AppState提供了一个属性来获取当前App的状态。

//App的当前状态
AppState.currentState

AppState提供了两个静态方法分别用来添加和移除事件监听

//添加事件监听
//type: 事件的类型，如状态改变：change、内存警告：memoryWarning
//handler：监听的回调
AppState.addEventListener( type: string, handler: Function)

//移除事件监听
//type: 事件的类型，如状态状态：change、内存警告：memoryWarning
//handler：监听的回调
AppState.removeEventListener( type: string, handler: Function)

三、使用

现在就来使用提供的API进行一下简单的测试，示例如下：

index.ios.js

/**
 * Sample React Native App
 * https://github.com/facebook/react-native
 * @flow
 */

import React, { Component } from 'react';

import {
    AppRegistry,
    StyleSheet,
    View,
    Text,
    AppState
} from 'react-native';


export default class ReactNativeDemo extends Component {

    state = { appState: AppState.currentState };

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

    componentWillUnmount() {
        AppState.removeEventListener('change', this.handleAppStateChange);
    }

    handleAppStateChange = (nextAppState) => {
        if (this.state.appState.match(/inactive|background/) && nextAppState === 'active') {
           console.log('App has come to the foreground!')
         }
        this.setState({appState: nextAppState});
        console.log('nextAppState-----',nextAppState);
   };

    constructor(){
        super();
        const currentState = AppState.currentState;
        console.log('currentState-----',currentState);
    }

    render() {
        return (
            <View style={[styles.flex,styles.bgColor,styles.center]}>
                <Text>Current state is: {this.state.appState}</Text>
            </View>
        );
    }
}

const styles = StyleSheet.create({
    flex: {
        flex: 1
    },
    bgColor: {
      backgroundColor: '#1FB9FF'
    },
    center: {
        alignItems: 'center',
        justifyContent: 'center'
    }
});

AppRegistry.registerComponent('ReactNativeDemo', () => ReactNativeDemo);

xcode日志如下：【期间将app进入后台和进入前台过】

2019-12-31 11:47:08.472 [info][tid:com.facebook.react.JavaScript] 'currentState-----', 'active'
2019-12-31 11:47:11.310 [info][tid:com.facebook.react.JavaScript] 'nextAppState-----', 'inactive'
2019-12-31 11:47:11.907 [info][tid:com.facebook.react.JavaScript] 'nextAppState-----', 'background'
2019-12-31 11:47:15.231 [info][tid:com.facebook.react.JavaScript] App has come to the foreground!
2019-12-31 11:47:15.244 [info][tid:com.facebook.react.JavaScript] 'nextAppState-----', 'active'