React Hooks：为什么是.current null for useRef Hook？

Question

I have a simple example of a component:

function App() {
  const observed = useRef(null);
  console.log(observed.current);

  return (
    <div ref={observed} className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some magic happen!</h2>
    </div>
  );
}

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

I would expect that observed.current would be of type element and current would not be empty but the div element with all its properties. My understanding would be:

1. The ref is initialised with a value of null
2. Null is overwritten by the ref

But as it turns out, .current remains empty. This is bad since I want to pass observed to a function that expects an argument of type Element.

https://codesandbox.io/embed/purple-forest-0460k

Answer 1

Ref.current is null because the ref is not set till after the function returns and the content is rendered. The useEffect hook fires every time the value of contents of the array passed to it changes. By passing observed in the array and by passing a callback that logs observed to the console, the useEffect hook can be leveraged to accomplish your task.

  useEffect(() => {
    console.log(observed.current);
  }, [observed]);

Edit: This only works on the first render as changes to a ref do not trigger re-renders. But the general statement about useEffect still holds. If you want to observe changes to a ref of a dom element, you can use a callback as ref.

  <div 
    ref={el => { console.log(el); observed.current = el; }} // or setState(el)
    className="App"
  >

Code Sandbox

Answer 2

At the time the console.log happens, the ref isn't yet set. Try putting your console.log into a useEffect hook and it works as you want it to.

import React, { useRef, useEffect } from "react";
import ReactDOM from "react-dom";

import "./styles.css";

function App() {
  const observed = useRef(null);

  useEffect(() => {
    console.log(observed.current);
  }, [observed]);

  return (
    <div ref={observed} className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some magic happen!</h2>
    </div>
  );
}

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

Answer 3

This is what I ended up doing since calling useEffect on rlvRef did not capture all events.

const rlvRef = useRef()
const [refVisible, setRefVisible] = useState(false)

useEffect(() => {
  if (!refVisible) { 
    return
  }
  // detected rendering
}, refVisible)

return (
  <RecyclerListView
    ref={el => { rlvRef.current = el; setRefVisible(!!el); }}
    ... 
  />
)

Answer 4

I needed to set a value into the useRef()-variable which was retrieved by an async http-request and was saved in a useState()-variable. So i needed to provide it as an dependency as well in the useEffect()-function:

    const [object, setObject] = useState('');

    const fetchObject = useCallback(async () => {
        const res = await fetch(apiUrl + "/object/", {});
        setObject({ ...await res.json() });
    }, [apiUrl]);

    const jsonEditor = React.createRef();

    useEffect(() => {
        fetchObject();
        if (jsonEditor.current && object.json) {
            jsonEditor.current.jsonEditor.set(JSON.parse(object.json));
        }
    }, [fetchObject, object.json]);

return (
    <div>
        <JsonEditor
            ref={jsonEditor}
            onChangeText={onChangeTextHandler}
        />
    </div>);