React refs 在 useCallback 中如何表现？

Question

I wouldn't expect the following React app to work properly, but it does. I'd expect the useCallback hook to capture and preserve the initial value of the ref. I understand that the ref couldn't be listed in the dependency array so maybe this is a special case only intended for refs?

Why isn't the content of newTodoRef.current.value captured by useCallback just once when App first renders?

import React, { useCallback, useReducer, useRef } from 'react';

type Todo = { id: number, text: string }
type ActionType = { type: 'ADD', text: string} | { type: 'REMOVE', id: number}

const todoReducer = (state: Todo[], action: ActionType) => {
    switch(action.type) {
        case 'ADD': return [ ...state, { id: state.length, text: action.text }]
        case 'REMOVE': return state.filter(({ id }) => id !== action.id) // this is buggy, but that's beside the point
        default: throw new Error()
    }
}

function App() {
    const [todos, dispatch] = useReducer(todoReducer, [])

    const newTodoRef = useRef<HTMLInputElement>(null)

    const onAddTodo = useCallback(() => {
        if (newTodoRef.current) {
            dispatch({ type: "ADD", text: newTodoRef.current.value })
            newTodoRef.current.value = ''
        }
    }, [])

    return (
        <div>
            {todos.map(todo => (
                <div key={todo.id}>
                    {todo.text}
                    <button onClick={() => dispatch({ type:"REMOVE", id: todo.id })}>Remove</button>
                </div>
            ))}
            <input type="text" ref={newTodoRef}/>
            <button onClick={onAddTodo}>ADD</button>
        </div>
    )
}

export default App;

Answer 1

Why isn't the content of newTodoRef.current.value captured by useCallback just once when App first renders?

The reference to the top level object, newTodoRef , is captured in this way. The reason this works out fine is that the ref is a mutable object, and the same object on every render. Once react has created the div element on the page, it will mutate newTodoRef , changing its .current property to the element. Then later, you access newTodoRef , which is still the same object, and you get it's .current property. The property has changed in the meantime, but the object has not.