2

I am quite new in react native and found simple solution from tutorial for fetching&loading case

export const useFetching = (callback) => {
    const [isLoading, setIsLoading] = useState(false);
    const [error, setError] = useState('');

    const fetching = async(data) => {
        try{
            setIsLoading(true);
            await callback(data);
        } catch(e) {
            setError(e.message);
        } finally {
            setIsLoading(false);
        }
    }
    return [fetching, isLoading, error];
}

I use it with signUp func from react-navigation docs as a callback, which is supposed to change the screen to main app:

signIn: async (data) => {
    const token = await AuthManager.login(data);
    if (token)
        dispatch({ type: 'SIGN_IN', token: 'dummy-auth-token' });

But when I call it I get "Can't perform a React state update on an unmounted component". I found out that:

  1. The "finally" block from useFetching causes the error
  2. Empty useEffect useEffect(() => {return () => {}},[]) inserted into login screen removes the problem

I'm wondering, is there any normal solution in this case and why useEffect helps here?

TDjey
  • 23
  • 4

1 Answers1

1

Changing state on unmounted component is illegal measure in react.

This is simple events plan in your component life cycle:

  1. Api request sended
  2. Component was unmounted
  3. Api request returns with data to your app
  4. Ooooops, api caller (in your case it is fetch function) see that component was unmounted and return error, so your app execute this block of code: catch(e) { setError(e.message); }, but there is problem - setState (setError).

finally block code executed too, so you must check is component mounted before setState.

Simple solution

CubeStorm
  • 138
  • 6