状态是<empty string>在按键事件上调用函数时

Question

In React, I have a number of buttons (imagine a PIN layout with numbers) that update the state on click. I also added an event listener to the document so pressing keys on the keyboard updates the pin too. However, there's a strange problem. When I add a number by clicking a button, the state is working correctly and everything is fine, but when I press a key on a physical keyboard, the state updates, but logs as <empty string> !

Here is the code:

export default function Keypad() {
    const [pin, setPin] = useState("");

    function addNumber(num) {
        console.log(pin); // returns the correct pin with handleKeyClick, returns <empty string> with handleKeyDown
        if (pin.length < 6) { // only works if the pin is not <empty string>
            setPin((pin) => [...pin, num.toString()]); // works correctly with both handleKeyClick and handleKeyDown even if pin logged <empty string>!
        }
    }

    function handleKeyClick(num) {
        addNumber(num);
    }

    function handleKeyDown(e) {
        if (!isNaN(e.key)) {
            addNumber(e.key);
        }
    }

    useEffect(() => {
        document.addEventListener("keydown", handleKeyDown);

        return () => {
            document.removeEventListener("keydown", handleKeyDown);
        };
    }, []);

    return (
        <div>
            {/* just one button for example */}
            <button onClick={() => handleKeyClick(9)}>9</button>
        </div>
    )
}

I guess this is because document can't access the pin state, but if it was the case, the setPin shouldn't work either. Am I right?

Answer 1

Your component does not keep a reference when listening to DOM events, this answer has some neat code for listening to window events using a fairly simple hook. When applied to your code, it works as expected:

 const {useState, useEffect, useRef} = React; // Hook function useEventListener(eventName, handler, element = window){ // Create a ref that stores handler const savedHandler = useRef(); // Update ref.current value if handler changes. // This allows our effect below to always get latest handler ... // ... without us needing to pass it in effect deps array ... // ... and potentially cause effect to re-run every render. useEffect(() => { savedHandler.current = handler; }, [handler]); useEffect( () => { // Make sure element supports addEventListener // On const isSupported = element && element.addEventListener; if (!isSupported) return; // Create event listener that calls handler function stored in ref const eventListener = event => savedHandler.current(event); // Add event listener element.addEventListener(eventName, eventListener); // Remove event listener on cleanup return () => { element.removeEventListener(eventName, eventListener); }; }, [eventName, element] // Re-run if eventName or element changes ); }; const Keypad = (props) => { const [pin, setPin] = useState([]); function addNumber(num) { console.log(pin); // returns the correct pin with handleKeyClick, returns <empty string> with handleKeyDown if (pin.length < 6) { // only works if the pin is not <empty string> setPin((pin) => [...pin, num.toString()]); // works correctly with both handleKeyClick and handleKeyDown even if pin logged <empty string>! } } function handleKeyClick(num) { addNumber(num); } function handleKeyDown(e) { if (!isNaN(e.key)) { addNumber(e.key); } } useEventListener("keydown", handleKeyDown) return ( <div> {/* just one button for example */} <button onClick={() => handleKeyClick(9)}>9</button> </div> ) return "Hello World" } ReactDOM.render(<Keypad />, document.getElementById("root"))

 <div id="root"></div> <script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.development.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.2/umd/react-dom.development.js"></script>