React Hooks useCallback导致子级重新渲染

Question

我正在尝试使用新的Hooks从类组件转变为功能组件。但是，感觉useCallback会获得不必要的子级渲染，这与类组件中的类函数不同。

下面有两个相对简单的片段。第一个是我的示例编写为类，第二个是我的示例重写为功能组件。目的是使功能组件与类组件具有相同的行为。

类组件测试用例

class Block extends React.PureComponent {
  render() {
    console.log("Rendering block: ", this.props.color);

    return (
        <div onClick={this.props.onBlockClick}
          style = {
            {
              width: '200px',
              height: '100px',
              marginTop: '12px',
              backgroundColor: this.props.color,
              textAlign: 'center'
            }
          }>
          {this.props.text}
         </div>
    );
  }
};

class Example extends React.Component {
  state = {
    count: 0
  }
  
  
  onClick = () => {
    console.log("I've been clicked when count was: ", this.state.count);
  }
  
  updateCount = () => {
    this.setState({ count: this.state.count + 1});
  };
  
  render() {
    console.log("Rendering Example. Count: ", this.state.count);
    
    return (
      <div style={{ display: 'flex', 'flexDirection': 'row'}}>
        <Block onBlockClick={this.onClick} text={'Click me to log the count!'} color={'orange'}/>
        <Block onBlockClick={this.updateCount} text={'Click me to add to the count'} color={'red'}/>
      </div>
    );
  }
};

ReactDOM.render(<Example/>, document.getElementById('root'));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.3/umd/react-dom.production.min.js"></script>

<div id='root' style='width: 100%; height: 100%'>
</div>

功能组件测试用例

const Block = React.memo((props) => {
  console.log("Rendering block: ", props.color);
  
  return (
      <div onClick={props.onBlockClick}
        style = {
          {
            width: '200px',
            height: '100px',
            marginTop: '12px',
            backgroundColor: props.color,
            textAlign: 'center'
          }
        }>
        {props.text}
       </div>
  );
});

const Example = () => {
  const [ count, setCount ] = React.useState(0);
  console.log("Rendering Example. Count: ", count);
  
  const onClickWithout = React.useCallback(() => {
    console.log("I've been clicked when count was: ", count);
  }, []);
  
  const onClickWith = React.useCallback(() => {
    console.log("I've been clicked when count was: ", count);
  }, [ count ]);
  
  const updateCount = React.useCallback(() => {
    setCount(count + 1);
  }, [ count ]);
  
  return (
    <div style={{ display: 'flex', 'flexDirection': 'row'}}>
      <Block onBlockClick={onClickWithout} text={'Click me to log with empty array as input'} color={'orange'}/>
      <Block onBlockClick={onClickWith} text={'Click me to log with count as input'} color={'cyan'}/>
      <Block onBlockClick={updateCount} text={'Click me to add to the count'} color={'red'}/>
    </div>
  );
};

ReactDOM.render(<Example/>, document.getElementById('root'));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.3/umd/react-dom.production.min.js"></script>

<div id='root' style='width: 100%; height: 100%'>
</div>

在第一个（类组件）中，我可以通过红色块更新计数，而无需重新渲染任何一个块，并且可以通过橙色块自由地控制台记录当前计数。

在第二个（功能组件）中，通过红色块更新计数将触发红色和青色块的重新渲染。这是因为useCallback将创建其功能的新实例，因为计数已更改，从而导致块获得新的onClick道具并因此重新渲染。橙色块不会重新呈现，因为用于橙色useCallback的{​​{1}}不依赖于计数值。这样做很好，但是当您单击橙色块时，它不会显示计数的实际值。

我认为拥有onClick的意义在于，使子代不会获得具有相同功能的新实例，并且不必进行不必要的重新渲染，但这似乎在回调函数使用第二秒时仍会发生一个变量，根据我的经验，该变量会经常发生。

那么我将如何在不重新渲染子级的情况下在功能组件中实现此useCallback函数？有可能吗？

更新（解决方案）： 使用下面的Ryan Cogswell的答案，我精心制作了一个自定义钩子，以轻松地创建类函数。

onClick

然后我可以像这样轻松地在函数组件中使用它，只需将onClick传递给孩子。它将不再重新渲染子级，而是仍然使用更新的var。

const useMemoizedCallback = (callback, inputs = []) => {
    // Instance var to hold the actual callback.
    const callbackRef = React.useRef(callback);

    // The memoized callback that won't change and calls the changed callbackRef.
    const memoizedCallback = React.useCallback((...args) => {
      return callbackRef.current(...args);
    }, []);

    // The callback that is constantly updated according to the inputs.
    const updatedCallback = React.useCallback(callback, inputs);

    // The effect updates the callbackRef depending on the inputs.
    React.useEffect(() => {
        callbackRef.current = updatedCallback;
    }, inputs);

    // Return the memoized callback.
    return memoizedCallback;
};

Answer 1

useCallback将避免由于父项中发生的某些更改而不必要的子项重新渲染，而这是回调的依赖项中的不部分。为了避免在涉及回调的依赖项时重新渲染子项，您需要使用ref。引用是等效于实例变量的钩子。

下面，我使用onClickMemoized指向当前onClickRef（通过onClick进行设置）的useEffect，以便它委派给该函数的一个已知版本状态的当前值。

我还更改了updateCount以使用功能更新语法，因此它不需要依赖count。

const Block = React.memo(props => {
  console.log("Rendering block: ", props.color);

  return (
    <div
      onClick={props.onBlockClick}
      style={{
        width: "200px",
        height: "100px",
        marginTop: "12px",
        backgroundColor: props.color,
        textAlign: "center"
      }}
    >
      {props.text}
    </div>
  );
});

const Example = () => {
  const [count, setCount] = React.useState(0);
  console.log("Rendering Example. Count: ", count);

  const onClick = () => {
    console.log("I've been clicked when count was: ", count);
  };
  const onClickRef = React.useRef(onClick);
  React.useEffect(
    () => {
      onClickRef.current = onClick;
    },
    [count]
  );

  const onClickMemoized = React.useCallback(() => {
    onClickRef.current();
  }, []);

  const updateCount = React.useCallback(() => {
    setCount(prevCount => prevCount + 1);
  }, []);

  return (
    <div style={{ display: "flex", flexDirection: "row" }}>
      <Block
        onBlockClick={onClickMemoized}
        text={"Click me to log with empty array as input"}
        color={"orange"}
      />
      <Block
        onBlockClick={updateCount}
        text={"Click me to add to the count"}
        color={"red"}
      />
    </div>
  );
};

ReactDOM.render(<Example />, document.getElementById("root"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.3/umd/react-dom.production.min.js"></script>

<div id='root' style='width: 100%; height: 100%'>
</div>

当然，钩子的优点在于您可以将这种有状态逻辑分解为自定义钩子：

import React from "react";
import ReactDOM from "react-dom";

const Block = React.memo(props => {
  console.log("Rendering block: ", props.color);

  return (
    <div
      onClick={props.onBlockClick}
      style={{
        width: "200px",
        height: "100px",
        marginTop: "12px",
        backgroundColor: props.color,
        textAlign: "center"
      }}
    >
      {props.text}
    </div>
  );
});

const useCount = () => {
  const [count, setCount] = React.useState(0);

  const logCount = () => {
    console.log("I've been clicked when count was: ", count);
  };
  const logCountRef = React.useRef(logCount);
  React.useEffect(
    () => {
      logCountRef.current = logCount;
    },
    [count]
  );

  const logCountMemoized = React.useCallback(() => {
    logCountRef.current();
  }, []);

  const updateCount = React.useCallback(() => {
    setCount(prevCount => prevCount + 1);
  }, []);
  return { count, logCount: logCountMemoized, updateCount };
};
const Example = () => {
  const { count, logCount, updateCount } = useCount();
  console.log("Rendering Example. Count: ", count);

  return (
    <div style={{ display: "flex", flexDirection: "row" }}>
      <Block
        onBlockClick={logCount}
        text={"Click me to log with empty array as input"}
        color={"orange"}
      />
      <Block
        onBlockClick={updateCount}
        text={"Click me to add to the count"}
        color={"red"}
      />
    </div>
  );
};

const rootElement = document.getElementById("root");
ReactDOM.render(<Example />, rootElement);