how to manage component mount and unmount to update state values?

Question

I would like to know how to manage state property when the component mounts and unmounts.

I have a lot of different components in my application to maintain the application flow. I know about function componentdidmount and componentWillUnmount. and I also tried the solution about _isMounted=true on componentdidmount function and check _isMounted properties value when I update setState and then update _isMounted=false on componentWillUnmount function. but this won't work when more two components come in the picture.

For example following links:

as per the example, I have made a common class which will update the value of a component in setMounted function and will return value in getMounted function to validate component is mounted or not. These methods work correctly on a single screen when I call another screen from a stack and update some values then comes back on the previous page and refresh page it will ismount=false.

class Mount {
    isMounted=false;
    getMounted=()=>{
        return isMounted;
    }
    setMounted=mounted=>{
        isMounted=mounted;
    }
}

var mount=new Mount();
export default mount;

class example extends component{
    componentDidMount=async()=>{
        mount.setMounted(true);
        await this.loadScreen();
        this.willFocusSubscription = this.props.navigation.addListener(
            'willFocus',
            async() => {
            await this.loadScreen();
            }
        );
    }
    loadScreen=async()=>{
        //some other stuff
        if(mount.getMounted()){//second time value is false
            this.setState({value:'value'});
        }
    }
    componentWillUnmount() {
        mount.setMounted(false);
    }
    //renderview where i call example2 on buttonclick

}
class example2 extends component{
    componentDidMount=async()=>{
        mount.setMounted(true);
        await this.loadScreen();
    }
    loadScreen=async()=>{
        //some other stuff
        if(mount.getMounted()){
            this.setState({value:'value'});
            this.props.navigation.goBack();
        }
    }
    componentWillUnmount() {
        mount.setMounted(false);
        this.willFocusSubscription.remove();
    }
}

It was showing following warning before using mount functions: Can't perform a React state update on an unmounted component

score 0 · Accepted Answer · answered Aug 02 '19 at 07:37

You are creating only a single instance of your Mount class that is exported and shared across every instance of every component. You will need to create a new instance of Mount for each component instance:

class Mount {
   ...
}
// export the Mount class
export default Mount;

class example extends component{
    constructor(props) {
      super(props);
      // create an instance of Mount for each component instance
      this.mount = new Mount();
    }
    componentDidMount=async()=>{
        this.mount.setMounted(true);
        await this.loadScreen();
        this.willFocusSubscription = this.props.navigation.addListener(
            'willFocus',
            async() => {
            await this.loadScreen();
            }
        );
    }
    loadScreen=async()=>{
        //some other stuff
        if(this.mount.getMounted()){//second time value is false
            this.setState({value:'value'});
        }
    }
    componentWillUnmount() {
        this.mount.setMounted(false);
    }
    //renderview where i call example2 on buttonclick

}

Notice the addition of the constructor and the use of this.mount instead of mount throughout.

your solution worked, I guess I have some other problem and it doesn't belong to the mount of a component. I have API call in load function which is async so maybe response comes in the unmounted second screen and thats why it is giving me mounted=false on willfocus subscription when load calls second time on first component. sorry bit confusing — Karishma Kavankar, Aug 06 '19 at 06:10

how to manage component mount and unmount to update state values?

1 Answers1