错误的 React 挂钩事件监听器的行为

Question

I'm playing around with React Hooks and am facing a problem. It shows the wrong state when I'm trying to console log it using a button handled by event listener.

CodeSandbox: https://codesandbox.io/s/lrxw1wr97m

1. Click on 'Add card' button 2 times
2. In first card, click on Button1 and see in console that there are 2 cards in state (correct behaviour)
3. In first card, click on Button2 (handled by event listener) and see in console that there is only 1 card in state (wrong behaviour)

Why does it show the wrong state?
In first card, Button2 should display 2 cards in the console. Any ideas?

 const { useState, useContext, useRef, useEffect } = React; const CardsContext = React.createContext(); const CardsProvider = props => { const [cards, setCards] = useState([]); const addCard = () => { const id = cards.length; setCards([...cards, { id: id, json: {} }]); }; const handleCardClick = id => console.log(cards); const handleButtonClick = id => console.log(cards); return ( <CardsContext.Provider value={{ cards, addCard, handleCardClick, handleButtonClick }} > {props.children} </CardsContext.Provider> ); }; function App() { const { cards, addCard, handleCardClick, handleButtonClick } = useContext( CardsContext ); return ( <div className="App"> <button onClick={addCard}>Add card</button> {cards.map((card, index) => ( <Card key={card.id} id={card.id} handleCardClick={() => handleCardClick(card.id)} handleButtonClick={() => handleButtonClick(card.id)} /> ))} </div> ); } function Card(props) { const ref = useRef(); useEffect(() => { ref.current.addEventListener("click", props.handleCardClick); return () => { ref.current.removeEventListener("click", props.handleCardClick); }; }, []); return ( <div className="card"> Card {props.id} <div> <button onClick={props.handleButtonClick}>Button1</button> <button ref={node => (ref.current = node)}>Button2</button> </div> </div> ); } ReactDOM.render( <CardsProvider> <App /> </CardsProvider>, document.getElementById("root") );

 <script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script> <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script> <div id='root'></div>

I am using React 16.7.0-alpha.0 and Chrome 70.0.3538.110

BTW, if I rewrite the CardsProvider using a сlass, the problem is gone. CodeSandbox using class: https://codesandbox.io/s/w2nn3mq9vl

Answer 1

This is a common problem for functional components that use the useState hook. The same concerns are applicable to any callback functions where useState state is used, eg setTimeout or setInterval timer functions .

Event handlers are treated differently in CardsProvider and Card components.

handleCardClick and handleButtonClick used in the CardsProvider functional component are defined in its scope. There are new functions each time it runs, they refer to cards state that was obtained at the moment when they were defined. Event handlers are re-registered each time the CardsProvider component is rendered.

handleCardClick used in the Card functional component is received as a prop and registered once on component mount with useEffect . It's the same function during the entire component lifespan and refers to stale state that was fresh at the time when the handleCardClick function was defined the first time. handleButtonClick is received as a prop and re-registered on each Card render, it's a new function each time and refers to fresh state.

Mutable state

A common approach that addresses this problem is to use useRef instead of useState . A ref is basically a recipe that provides a mutable object that can be passed by reference:

const ref = useRef(0);

function eventListener() {
  ref.current++;
}

In this case a component should be re-rendered on a state update like it's expected from useState , refs aren't applicable.

It's possible to keep state updates and mutable state separately but forceUpdate is considered an anti-pattern in both class and function components (listed for reference only):

const useForceUpdate = () => {
  const [, setState] = useState();
  return () => setState({});
}

const ref = useRef(0);
const forceUpdate = useForceUpdate();

function eventListener() {
  ref.current++;
  forceUpdate();
}

State updater function

One solution is to use a state updater function that receives fresh state instead of stale state from the enclosing scope:

function eventListener() {
  // doesn't matter how often the listener is registered
  setState(freshState => freshState + 1);
}

In this case a state is needed for synchronous side effects like console.log , a workaround is to return the same state to prevent an update.

function eventListener() {
  setState(freshState => {
    console.log(freshState);
    return freshState;
  });
}

useEffect(() => {
  // register eventListener once

  return () => {
    // unregister eventListener once
  };
}, []);

This doesn't work well with asynchronous side effects, notably async functions.

Manual event listener re-registration

Another solution is to re-register the event listener every time, so a callback always gets fresh state from the enclosing scope:

function eventListener() {
  console.log(state);
}

useEffect(() => {
  // register eventListener on each state update

  return () => {
    // unregister eventListener
  };
}, [state]);

Built-in event handling

Unless the event listener is registered on document , window or other event targets that are outside of the scope of the current component, React's own DOM event handling has to be used where possible, this eliminates the need for useEffect :

<button onClick={eventListener} />

In the last case the event listener can be additionally memoized with useMemo or useCallback to prevent unnecessary re-renders when it's passed as a prop:

const eventListener = useCallback(() => {
  console.log(state);
}, [state]);

• Previous edition of this answer suggested to use mutable state that was applicable to initial useState hook implementation in React 16.7.0-alpha version but isn't workable in final React 16.8 implementation. useState currently supports only immutable state.*

Answer 2

A much cleaner way to work around this is to create a hook I call useStateRef

function useStateRef(initialValue) {
  const [value, setValue] = useState(initialValue);

  const ref = useRef(value);

  useEffect(() => {
    ref.current = value;
  }, [value]);

  return [value, setValue, ref];
}

You can now use the ref as a reference to the state value.

Answer 3

Short answer for me was that useState has a simple solution for this:

function Example() {
  const [state, setState] = useState(initialState);

  function update(updates) {
    // this might be stale
    setState({...state, ...updates});
    // but you can pass setState a function instead
    setState(currentState => ({...currentState, ...updates}));
  }

  //...
}

Answer 4

Short answer for me

this WILL NOT not trigger re-render ever time myvar changes.

const [myvar, setMyvar] = useState('')
  useEffect(() => {    
    setMyvar('foo')
  }, []);

This WILL trigger render -> putting myvar in []

const [myvar, setMyvar] = useState('')
  useEffect(() => {    
    setMyvar('foo')
  }, [myvar]);

Answer 5

Check the console and you'll get the answer:

React Hook useEffect has a missing dependency: 'props.handleCardClick'. Either include it or remove the dependency array. (react-hooks/exhaustive-deps)

Just add props.handleCardClick to the array of dependencies and it will work correctly.

Answer 6

This way your callback will have updated state values always ;)

 // registers an event listener to component parent
 React.useEffect(() => {

    const parentNode = elementRef.current.parentNode

    parentNode.addEventListener('mouseleave', handleAutoClose)

    return () => {
        parentNode.removeEventListener('mouseleave', handleAutoClose)
    }

}, [handleAutoClose])

Answer 7

To build off of Moses Gitau's great answer , if you are developing in Typescript, to resolve type errors make the hook function generic:

function useStateRef<T>(initialValue: T | (() => T)): 
   [T, React.Dispatch<React.SetStateAction<T>>, React.MutableRefObject<T>] {
  const [value, setValue] = React.useState(initialValue);

  const ref = React.useRef(value);

  React.useEffect(() => {
    ref.current = value;
  }, [value]);

  return [value, setValue, ref];
}

Answer 8

Starting from the answer of @Moses Gitau, I'm using a sligthly different one that doesn't give access to a "delayed" version of the value (which is an issue for me) and is a bit more minimalist:

import { useState, useRef } from 'react';

function useStateRef(initialValue) {
    const [, setValueState] = useState(initialValue);

    const ref = useRef(initialValue);

    const setValue = (val) => {
        ref.current = val;
        setValueState(val); // to trigger the refresh
    };

    const getValue = (val) => {
        return ref.current;
    };

    return [getValue , setValue];
}
export default useStateRef;

This is what I'm using

Example of usage:

const [getValue , setValue] = useStateRef(0);

const listener = (event) => {
    setValue(getValue() + 1);
};

useEffect(() => {
    window.addEventListener('keyup', listener);

    return () => {
        window.removeEventListener('keyup', listener);
    };
}, []);

Edit: It now gives getValue and not the reference itself. I find it better to keep things more encapsulated in that case.

Answer 9

after changing the following line in the index.js file the button2 works well:

useEffect(() => {
    ref.current.addEventListener("click", props.handleCardClick);
    return () => {
        ref.current.removeEventListener("click", props.handleCardClick);
    };
- }, []);
+ });

you should not use [] as 2nd argument useEffect unless you want it to run once.

more details: https://reactjs.org/docs/hooks-effect.html

Answer 10

我遇到了类似的问题，我的事件函数从上下文中获取了过时的值，这篇文章帮助了我。