javascript - 我们如何知道 React 的 ref.current 值何时发生了变化？

Question

通常，有了 Prop ，我们可以写

componentDidUpdate(oldProps) {
  if (oldProps.foo !== this.props.foo) {
    console.log('foo prop changed')
  }
}

为了检测 Prop 变化。

但是如果我们使用 React.createRef() ，我们如何检测 ref 何时更改为新组件或 DOM 元素？ React 文档并没有真正提及任何内容。

即，

class Foo extends React.Component {
  someRef = React.createRef()

  componentDidUpdate(oldProps) {
    const refChanged = /* What do we put here? */

    if (refChanged) {
      console.log('new ref value:', this.someRef.current)
    }
  }

  render() {
    // ...
  }
}

我们是否应该自己实现某种旧值的东西？

即，

class Foo extends React.Component {
  someRef = React.createRef()
  oldRef = {}

  componentDidMount() {
    this.oldRef.current = this.someRef.current
  }

  componentDidUpdate(oldProps) {
    const refChanged = this.oldRef.current !== this.someRef.current

    if (refChanged) {
      console.log('new ref value:', this.someRef.current)

      this.oldRef.current = this.someRef.current
    }
  }

  render() {
    // ...
  }
}

这是我们应该做的吗？我原以为 React 会为此添加一些简单的功能。

Answer 1

React 文档推荐使用 callback refs检测 ref值变化。
Hook

export function Comp() {
  const onRefChange = useCallback(node => {
    if (node === null) { 
      // DOM node referenced by ref has been unmounted
    } else {
      // DOM node referenced by ref has changed and exists
    }
  }, []); // adjust deps

  return <h1 ref={onRefChange}>Hey</h1>;
}

useCallback习惯于 prevent double calling带有 null 的 ref 回调和元素。
您可以 trigger re-renders通过使用 useState 存储当前 DOM 节点来进行更改:

const [domNode, setDomNode] = useState(null);
const onRefChange = useCallback(node => {
  setDomNode(node); // trigger re-render on changes
  // ...
}, []);

类组件

export class FooClass extends React.Component {
  state = { ref: null, ... };

  onRefChange = node => {
    // same as Hooks example, re-render on changes
    this.setState({ ref: node });
  };

  render() {
    return <h1 ref={this.onRefChange}>Hey</h1>;
  }
}

注: useRef 不通知 ref变化。还有 no luck与 React.createRef()/对象引用。
这是一个测试用例，在触发 onRefChange 时删除并重新添加一个节点打回来 :

const Foo = () => {
  const [ref, setRef] = useState(null);
  const [removed, remove] = useState(false);

  useEffect(() => {
    setTimeout(() => remove(true), 3000); // drop after 3 sec
    setTimeout(() => remove(false), 5000); // ... and mount it again
  }, []);

  const onRefChange = useCallback(node => {
    console.log("ref changed to:", node);
    setRef(node); // or change other state to re-render
  }, []);

  return !removed && <h3 ref={onRefChange}>Hello, world</h3>;
}

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

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.10.1/umd/react.production.min.js" integrity="sha256-vMEjoeSlzpWvres5mDlxmSKxx6jAmDNY4zCt712YCI0=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.10.1/umd/react-dom.production.min.js" integrity="sha256-QQt6MpTdAD0DiPLhqhzVyPs1flIdstR4/R7x4GqCvZ4=" crossorigin="anonymous"></script>

<script> var {useState, useEffect, useCallback} = React</script>

<div id="root"></div>