关于react.js:react-useCallback-闭包陷阱

1. 为什么须要useCallback

• 在应用useCallback 和 useMemo钩子之前，如果每次批改parent的count，都会导致child从新渲染

const Child = (props) => {
  console.log('Child render');
  return (
    <div>
      <button onClick={props.clickCallback}>Child click</button>
    </div>
  )
}

const parent = () => {
  const [count, setCount] = useState(1);
  const cb = () => {
    console.log(count);
  };
  
  return (
    <div>
      parent
      <button onClick={() => setCount()}>click</button>
      <Child clickCallback={cb} />
    </div>
  )
}

2. 场景复现

• 函数组件中，个别 useCallback 和 useMemo都是搭配应用， 模仿componentShouldUpdate钩子，防止不必要的渲染。优化之后，咱们就发现，在parent中，一直的setCount批改状态，也不会触发Child组件的更新。彷佛解决了 父函数组件更新状态，导致子组件的不必要更新问题。然而当点击子组件的button时，就会发现，打印的是 count的初始值 1，这也就是 useCallback的闭包陷阱，不能拿到最新的state。

const Child = (props) => {
  console.log('Child render');
  return (
    <div>
      <button onClick={props.clickCallback}>Child click</button>
    </div>
  )
}

const parent = () => {
  const [count, setCount] = useState(1);
  
  const cb = React.useCallback(() => {
    console.log(count);
  }, []);

  const ChildMemo = React.useMemo(() => <Child clickCallback={cb} />, [cb]);

  return (
    <div style={{ 
      width: '100vw', height: '100vh', 
      justifyContent: 'center', alignItems: 'center', 
      display: 'flex', 
      flexDirection: 'column' }}
     >
      parent count : {count}
      <button onClick={() => setCount(count + 1)}>click</button>
      {ChildMemo}
    </div>
  )
}

3. 解决形式有两种： 两种形式都是将callback保留在一个援用对象中，区别就在于，形式2不须要去批改子组件，然而也减少了了解的老本

• 放弃useCallback, 应用 useRef钩子，用ref.current去援用箭头函数。应用useRef还须要对子组件回调的调用批改一下。

const Child = (props) => {
  console.log('Child render');
  return (
    <div>
      <button onClick={() => {
        props.callbackRef.current();
           }}
       >Child click</button>
    </div>
  )
}

const parent = () => {
  const [count, setCount] = useState(1);
  
  const cbRef = React.useRef(null);
  cbRef.current = () => {
    console.log(count);
  };

  const ChildMemo = React.useMemo(
    () => <Child callbackRef={cbRef} />,
    []
  );

  return (
    <div style={{ 
      width: '100vw', height: '100vh', 
      justifyContent: 'center', alignItems: 'center', 
      display: 'flex', 
      flexDirection: 'column' }}
     >
      parent count : {count}
      <button onClick={() => setCount(count + 1)}>click</button>
      {ChildMemo}
    </div>
  )
}

• 自定义一个hook，应用 useCallback搭配useRef

const useImmediateValue = (cb) => {
  const ref = useRef(cb);
  ref.current = cb;
  const val = useCallback(() => {
     ref.current();
  }, []);
  return val;
}

const Child = (props) => {
  console.log('Child render');
  return (
    <div>
      <button onClick={props.clickCallback}>Child click</button>
    </div>
  )
}

const parent = () => {
  const [count, setCount] = useState(1);
  
  const cb = useImmediateValue(() => {
    console.log(count);
  })

  const ChildMemo = React.useMemo(
    () => <Child clickCallback={cb} />,
    []
  );

  return (
    <div style={{ 
      width: '100vw', height: '100vh', 
      justifyContent: 'center', alignItems: 'center', 
      display: 'flex', 
      flexDirection: 'column' }}
     >
      parent count : {count}
      <button onClick={() => setCount(count + 1)}>click</button>
      {ChildMemo}
    </div>
  )
}