Cancel All Subscriptions and Asyncs in the componentWillUnmount Method, how?

问题

I'm getting an error message due to an async method issue. In my terminal I'm seeing:

Warning: Can't call setState (or forceUpdate) on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.
- node_modules/fbjs/lib/warning.js:33:20 in printWarning
- node_modules/fbjs/lib/warning.js:57:25 in warning
- node_modules/react-native/Libraries/Renderer/ReactNativeRenderer-dev.js:12196:6 in warnAboutUpdateOnUnmounted
- node_modules/react-native/Libraries/Renderer/ReactNativeRenderer-dev.js:13273:41 in scheduleWorkImpl
- node_modules/react-native/Libraries/Renderer/ReactNativeRenderer-dev.js:6224:19 in enqueueSetState
- node_modules/react/cjs/react.development.js:242:31 in setState
* router/_components/item.js:51:16 in getImage$
- node_modules/regenerator-runtime/runtime.js:62:44 in tryCatch
- node_modules/regenerator-runtime/runtime.js:296:30 in invoke
- ... 13 more stack frames from framework internals

I noticed it's specifically pointing out the getImage$

Here's the code I'm using for that section:

export default class extends Component {
    constructor(props) {
        super(props);
        const { item } = props

        const bindThese = { item }
        this.boundActionCreators = bindActionCreators(bindThese)

        this.state = {
            image: require('../../static/logo.png'),
            ready: false,
            showOptions: this.props.showOptions
        }

        this.getImage = this.getImage.bind(this)
        this.renderNotAdmin = this.renderNotAdmin.bind(this)
        this.renderAdmin = this.renderAdmin.bind(this)
        this.handleOutOfStock = this.handleOutOfStock.bind(this)
    }

    async getImage(img) {
        let imgUri = await Amplify.Storage.get(img)
        let uri = await CacheManager.get(imgUri).getPath()

        this.setState({
            image: { uri },
            ready: true
        })
    }

    componentDidMount() {
        this.getImage(this.props.item.image)
    }

I'm trying to figure out how to use a componentWillUnmount with this async method. How do I go about it?

Thanks!

回答1:

You can use isMounted React pattern to avoid memory leaks here.

In your constructor:

constructor(props) {
    super(props);

    this._isMounted = false;
// rest of your code
}

componentDidMount() {
    this._isMounted = true;
    this._isMounted && this.getImage(this.props.item.image);

}

in your componentWillUnmount

componentWillUnmount() {
   this._isMounted = false;
}

While in you getImage()

async getImage(img) {
    let imgUri = await Amplify.Storage.get(img)
    let uri = await CacheManager.get(imgUri).getPath()

    this._isMounted && this.setState({
        image: { uri },
        ready: true
    })
}

A recommend approach to use Axios which is based cancellable promise pattern. So you can cancel any network call while unmounting the component with it's cancelToken subscription. Here is resource for Axios Cancellation

回答2:

From the React blog

Just set a _isMounted property to true in componentDidMount and set it to false in componentWillUnmount, and use this variable to check your component’s status.

It goes on to say that ideally, this would instead be fixed by using cancellable callbacks, although the first solution seems suitable here.

What you definitely shouldn't do is use the isMounted() function, which may be deprecated.

回答3:

You need to set this.mounted = false in componentWillUnmount() method and this.mounted = true in componentDidMount() method.

The setState update is conditional based need to declare in componentDidMount() method.

componentDidMount() {
        this.mounted = true;
        var myVar =  setInterval(() => {
                let nextPercent = this.state.percentage+10;
                if (nextPercent >= 100) {
                    clearInterval(myVar);
                }
                if(this.mounted) {
                    this.setState({ percentage: nextPercent });
            }
        }, 100);
}

componentWillUnmount(){
      this.mounted = false;
}

来源：https://stackoverflow.com/questions/52061476/cancel-all-subscriptions-and-asyncs-in-the-componentwillunmount-method-how