State 不会异步更新 function

Question

Could you please help me understand why my state was not updated when I called two async functions in the first useEffect? and what is the best way to handle async data in the case that I don't know which one comes first (API1 or API2)?

Thank you!

const MyClass = () => {
    const [myState, setMyState] = useState([]);

    useEffect(() => {
        callApi1();
        callApi2();
    }, []);

    const callApi1 = () => {
        fetch(...).then(result => {
            // the result of API 1 always comes first and result is not empty
            setMyState(result);
        )};
    }

    const callApi2 = () => {
        fetch(...).then(result => {
            // the result of API 2 always comes 5 - 10 seconds after the API 1
            console.log(myState) => [], WHY?
        });
    }
}

Answer 1

(1.) "... why my state was not updated..."

Your state was updated, but the callback function captures the old state of myState (as a closure ). That means myState inside the callback function will always stay the same as it was when the function was created. (And it is created only when callApi2() is invoked.)

You can not access the current up-to-date state inside an asynchronous callback.

(2.) "...best way to handle async data in the case that I don't know which one comes first"

It depends on your use case.
Generally, you would set some states from your callbacks (eg your setMyState(result) ), and a different part of your program will do something else dependent on these states, eg useEffect(()=>{ /* do something */ }, [ myState ]) .

eg:

const MyClass = () => {
    const [myState, setMyState] = useState([]);
    const [myState2, setMyState2] = useState([]);
    const [allDone, setAllDone] = useState(false);

    useEffect(() => {
        callApi1();
        callApi2();
    }, []);

    useEffect(() => {
        console.log( 'myState/myState2:', myState, myState2);
        if( myState.length && myState2.length ){
            setAllDone(true);
        }
    }, [ myState, myState2 ]);

    const callApi1 = () => {
        fetch(...).then(result => {
            setMyState(result);
        )};
    }

    const callApi2 = () => {
        fetch(...).then(result => {
            setMyState2(result);
        });
    }
}