`componentDidMount()` function is not called after

问题:

I am using stackNavigator for navigating between screens. I am calling two API's in componentDidMount() function in my second activity. When i load it first time, it gets loaded successfully. Then i press back button to go back to first activity. Then, if i am again going to second activity, the APIs are not called and I get render error. I am not able to find any solution for this. Any suggestions would be appreciated.

回答1:

If anyone coming here in 2019, try this:

import {NavigationEvents} from 'react-navigation';

Add the component to your render:

<NavigationEvents onDidFocus={() => console.log('I am triggered')} />

Now, this onDidFocus event will be triggered every time when the page comes to focus despite coming from goBack() or navigate.

回答2:

If the upvoted syntax that uses NavigationEvents component is not working, you can try with this:

// no need to import anything more

// define a separate function to get triggered on focus
onFocusFunction = () => {
  // do some stuff on every screen focus
}

// add a focus listener onDidMount
async componentDidMount () {
  this.focusListener = this.props.navigation.addListener('didFocus', () => {
    this.onFocusFunction()
  })
}

// and don't forget to remove the listener
componentWillUnmount () {
  this.focusListener.remove()
}

回答3:

React-navigation keeps the component mounted even if you navigate between screens. You can use the component to react to those events :

<NavigationEvents
  onDidFocus={() => console.log('hello world')}
/>

More info about this component here.

回答4:

The React-navigation documentation explicitly described this case:

Consider a stack navigator with screens A and B. After navigating to A, its componentDidMount is called. When pushing B, its componentDidMount is also called, but A remains mounted on the stack and its componentWillUnmount is therefore not called.

When going back from B to A, componentWillUnmount of B is called, but componentDidMount of A is not because A remained mounted the whole time.

Now there is 3 solutions for that:

Subscribing to lifecycle events

...

React Navigation emits events to screen components that subscribe to them. There are four different events that you can subscribe to: willFocus, willBlur, didFocus and didBlur. Read more about them in the API reference.

Many of your use cases may be covered with the withNavigationFocus higher-order-component, the <NavigationEvents /> component, or the useFocusState hook.

1. the withNavigationFocus higher-order-component
2. the <NavigationEvents /> component
3. the useFocusState hook (deprecated)

withNavigationFocus higher-order-component

import React from 'react';
import { Text } from 'react-native';
import { withNavigationFocus } from 'react-navigation';

class FocusStateLabel extends React.Component {
  render() {
    return <Text>{this.props.isFocused ? 'Focused' : 'Not focused'}</Text>;
  }
}

// withNavigationFocus returns a component that wraps FocusStateLabel and passes
// in the navigation prop
export default withNavigationFocus(FocusStateLabel);

<NavigationEvents /> component

import React from 'react';
import { View } from 'react-native';
import { NavigationEvents } from 'react-navigation';

const MyScreen = () => (
  <View>
    <NavigationEvents
      onWillFocus={payload => console.log('will focus', payload)}
      onDidFocus={payload => console.log('did focus', payload)}
      onWillBlur={payload => console.log('will blur', payload)}
      onDidBlur={payload => console.log('did blur', payload)}
    />
    {/*
      Your view code
    */}
  </View>
);

export default MyScreen;

useFocusState hook

First install library yarn add react-navigation-hooks

import { useNavigation, useNavigationParam, ... } from 'react-navigation-hooks'

function MyScreen() {   const focusState = useFocusState();   return <Text>{focusState.isFocused ? 'Focused' : 'Not Focused'}</Text>; }

Here is my personal solution, using onDidFocus() and onWillFocus() to render only when data has been fetched from your API:

import React, { PureComponent } from "react";
import { View } from "react-native";
import { NavigationEvents } from "react-navigation";

class MyScreen extends PureComponent {
  state = {
    loading: true
  };

  componentDidMount() {
    this._doApiCall();
  }

  _doApiCall = () => {
    myApiCall().then(() => {
      /* Do whatever you need */
    }).finally(() => this.setState({loading: false}));
  };

  render() {
    return (
      <View>
        <NavigationEvents 
              onDidFocus={this._doApiCall} 
              onWillFocus={() => this.setState({loading: true})}
        />
        {!this.state.loading && /*
        Your view code
        */}
      </View>
    );
  }
}

export default MyScreen;

回答5:

You want to perform something after every time navigating to a component using drawernavigator or stacknavigator ; for this purpose NavigationEvents fits better than componentDidMount .

import {NavigationEvents} from "react-navigation";
<NavigationEvents onDidFocus={()=>alert("Hello, I'm focused!")} />

Note : If you want to do the task every time after focusing on a component using drawer navigation or stack navigation then using NavigationEvents is better idea. But if you want to do the task once then you need to use componenetDidMount .

回答6:

In React, componentDidMount is called only when component is mounted.I think what you are trying to do is call your API on going back in StackNavigator. You can pass a callback function as parameter when you call navigate like this on Parent Screen:

  navigate("Screen", {
     onNavigateBack: this.handleOnNavigateBack
  }); 
  handleOnNavigateBack = () => {//do something};

And on Child Screen

this.props.navigation.state.params.onNavigateBack();
this.props.navigation.goBack();