来自在React.js UseEffect挂钩中创建的事件处理程序的访问状态

Question

In my component I'm setting up an event listener within the useEffect hook:

useEffect(() => {
  const target = subtitleEl.current;
  target.addEventListener("click", () => {
    console.log("click");
    onSubtitleClick();
  });

  return () => {
    target.removeEventListener("click", null);
  };
}, []);

.. but when I call onSubtitleClick , my state is stale - it's the original value. Example code here . How can I access state from the event handler that was setup with the useEffect?

Answer 1

Your event listener registers a function (reference) which has count as 0 in the environment it is defined and when a new render happens, your reference is not being updated which is registered with that element and that registered function reference still knows that count is 0 even though count has been changed but that updated function was not registered which knows the updated value in its context. So you need to update event listener with new function reference.

useEffect(() => {
    const target = subtitleEl.current;
    target.addEventListener("click", onSubtitleClick);

    return () => {
      console.log("removeEventListener");
      target.removeEventListener("click", onSubtitleClick);
    };
}, [onSubtitleClick]);

However, you don't need that messy code to achieve what you are doing now or similar stuff. You can simply call that passed function on click and don't attach to element through ref but directly in jsx.

<div
    className="panelText"
    style={{ fontSize: "13px" }}
    onClick={onSubtitleClick}
    ref={subtitleEl}
  >
    Button2
</div>

Answer 2

I think basically the addeventlistener creates a closure with its own version of count separate to the parent component's. A simple fix is to allow the useEffect function to rerun on changes (remove the [] arg):

useEffect(() => {
  const target = subtitleEl.current;
  target.addEventListener("click", () => {
    console.log("click");
    onSubtitleClick();
  });

  return () => {
    target.removeEventListener("click", null);
  };
}); // removed [] arg which prevents useeffect being run twice