马克社区,马克吐温社区,马克java社区React Native之didFocus和didBlur; React Native this.props.navigation.addListe didFocus didBlur 第一次加载的时候会调用一次

React Native之didFocus和didBlur

1 didFocus和didBlur解释

    didFocus - the screen focused (if there was a transition, the transition completed)

    didBlur - the screen unfocused (if there was a transition, the transition completed)

didFocus是指当前页面第一次加载的时候会调用一次

didBlur是指当前页面离开的时候会调用一次(前提是当前页面没有被销毁既没有执行componentWillUnmount()函数)

2 测试代码

    import React from 'react';
    import { View, Text, Button} from 'react-native';
    import { createStackNavigator } from 'react-navigation';


    class HomeScreen extends React.Component {


       constructor(props) {
       super(props);
       console.log("HomeScreen constructor start");
            this.didFocusListener = this.props.navigation.addListener(
           'didFocus',
           (obj) => {console.log("HomeScreen didFocus start")}
           );
       this.didBlurListener = this.props.navigation.addListener(
           'didBlur',
           (obj) => {console.log('HomeScreen didBlur start')}
           );
        }

        static navigationOptions = {
            title : 'HomeScreen',
        }

        componentDidMount = () => {
                    console.log("HomeScreen componentDidMount start")
        }

        componentWillUnmount() {
           console.log("HomeScreen componentWillUnmount start")
           this.didFocusListener.remove();
           this.didBlurListener.remove();
        }

        render() {
            return (
                <View style={{flex: 1, alignItems: 'center', justifyContent: 'center'}}>
                   <Text>Home Screen</Text>

                   <Button onPress={() => this.props.navigation.navigate('Details', {
                       itemId:100,
                       otherParam:'chenyu',
                   })} title = "go to Details"/>

                   <Button
               title="Go back"
                  onPress={() => this.props.navigation.goBack()}
                  />
            </View>
        );

        }
    }

    class DetailsScreen extends React.Component {


       constructor(props) {
       super(props);
       console.log("DetailsScreen constructor start");
            this.didFocusListener = this.props.navigation.addListener(
           'didFocus',
           (obj) => {console.log("DetailsScreen didFocus start")}
       );
       this.didBlurListener = this.props.navigation.addListener(
           'didBlur',
           (obj) => {console.log('DetailsScreen didBlur start')}
       );
        }


        static navigationOptions = ({navigation}) => {
            return {
                title : navigation.getParam('otherParam', 'no-values'),
            };
        };
        componentDidMount = () => {
            console.log("DetailsScreen componentDidMount start")
        }

        componentWillUnmount() {
       console.log("DetailsScreen componentWillUnmount start")
            this.didFocusListener.remove();
       this.didBlurListener.remove();
        }

        render() {
            const {navigation} = this.props;
            const itemId = navigation.getParam('itemId', 'no-values');
            const otherParam = navigation.getParam('otherParam', 'no-values');

            return (
                <View style={{ flex: 1, alignItems: 'center', justifyContent: 'center' }}>
               <Text>Details Screen</Text>
           <Text>itemId:{JSON.stringify(itemId)}</Text>
           <Text>otherParam:{JSON.stringify(otherParam)}</Text>
           <Button
               title="Go to Details... again"
               onPress={() => this.props.navigation.push('Details', {
                   itemId: Math.floor(Math.random() * 100),
           })}
           />
           <Button
               title="Go to Home"
               onPress={() => this.props.navigation.navigate('Home')}
           />
           <Button
               title="Go back"
               onPress={() => this.props.navigation.goBack()}
           />
           <Button
               title="Go popToTop"
               onPress={() => this.props.navigation.popToTop()}
           />
               </View>
           );
        }
    }


    const RootStack = createStackNavigator(
        {
            Home: HomeScreen,
            Details: DetailsScreen,
        },
        {
            initialRouteName: 'Home',
        }
    );


    export default class App extends React.Component {

        constructor(props) {
       super(props);
        }
        render() {
            return <RootStack/>;
        }


    }

3 运行结果

2个页面分别如下

在控制台过来React Native命令

adb logcat | grep ReactNativeJS

1) 程序起来打印日志如下

    I/ReactNativeJS(21233): HomeScreen constructor start
    I/ReactNativeJS(21233): HomeScreen componentDidMount start
    I/ReactNativeJS(21233): HomeScreen didFocus start

这里的didFocus start是在componentDidMount后面执行

2 ) 然后点击go to DETAILS按钮日志如下

    I/ReactNativeJS(21233): DetailsScreen constructor start
    I/ReactNativeJS(21233): DetailsScreen componentDidMount start
    I/ReactNativeJS(21233): HomeScreen didBlur start
    I/ReactNativeJS(21233): DetailsScreen didFocus start

然后执行了HomeScreen didBlur start,但是并没有执行HomeScreen componentWillUnmount start,因为页面还没有销毁,所以执行了HomeScreen didBlur start.

3 )然后在在第二个页面点击"GO BACK"或者按下返回键,日志打印如下

I/ReactNativeJS(21233): DetailsScreen componentWillUnmount start
I/ReactNativeJS(21233): HomeScreen didFocus start

发现没有,既然执行了componentWillUnmount函数,说明页面已经销毁,既然销毁了,就没有执行DetailsScreen didBlur start,因为前面的页面没有死,所以不会重新加载再次调用首页的constructor和componentDidMount方法.从前面日志打印

可以看出,另外一个页面执行新页面的constructor函数和componentDidMount函数才执行之前页面的didBlur start,所以估计这里是来不及执行页面就销毁了,所以没有打印DetailsScreen didBlur start.

4 )然后再次点击返回物理键日志如下

I/ReactNativeJS(23183): HomeScreen componentWillUnmount start

只调用了componentWillUnmount函数,所以页面销毁了,HomeScreen didBlur start来不及打印.

4 总结

didFocus只会在当前页面的constructor函数和componentDidMount函数后面执行

didBlur只会在当前页面没有调用componentWillUnmount函数,然后离开当前页面才执行,也意味着,这个页面没有死但是去了另外一个页面才会调用,如果自己页面死了,就不会调用到这里.

作者：chen.yu
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