使用 react useRef() -- 为什么我的 ref 是 null？

Question

I have a simple example

function Node() {
    const [hidden, setHidden] = useState(true);
    const inputRef = useRef(null)
    console.log(inputRef);
    return (
        <div>
            {!hidden && <h2 ref={inputRef}>Hello World</h2>}
            {hidden && <button onClick={() => setHidden(false)}>Show Child</button>}
        </div>
    )
}

Upon clicking the button, I would expect that the h2 DOM element is attached to my ref. However, I found that the ref.current is still null upon logging, but if I expand the object, it contains the DOM node.

How am I supposed to access the DOM element via my ref? At the time I want to reference it, for example inputRef.current.getBoundingClientRect() , it's always shown as null. Your help is much appreciated!

Answer 1

Your ref is not initialize when setHidden set to false if you want to use it you need some interactions like this

        import React, { useRef, useState, useEffect } from 'react';
    
    function App() {
        const [hidden, setHidden] = useState(true);
        const inputRef = useRef(null)
        console.log(inputRef);
        const handleChange=()=>{
        console.log(inputRef);
          
        }

useEffect(()=>{
   console.log(inputRef);
},[hidden])
        return (
            <div>
              <input onChange ={handleChange}></input>
                {!hidden && <h2 ref={inputRef}>Hello World</h2>}
                {hidden && <button onClick={() => setHidden(false)}>Show Child</button>}
            </div>
        )
    }
    
    export default App;

Answer 2

You are trying to use the ref in the render phase, but the ref will be populate once React paint the screen, in the commit phase. So, call it in a useEffect or useLayoutEffect

// run synchronously after all DOM mutations
React.useLayoutEffect(() => {
  // do something with inputRef
}, [])