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@gragland
Last active June 26, 2022 17:04
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import { useState, useRef, useEffect, useCallback } from 'react';
// Usage
function App(){
// State for storing mouse coordinates
const [coords, setCoords] = useState({ x: 0, y: 0 });
// Event handler utilizing useCallback ...
// ... so that reference never changes.
const handler = useCallback(
({ clientX, clientY }) => {
// Update coordinates
setCoords({ x: clientX, y: clientY });
},
[setCoords]
);
// Add event listener using our hook
useEventListener('mousemove', handler);
return (
<h1>
The mouse position is ({coords.x}, {coords.y})
</h1>
);
}
// 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
);
};
@Dygerydoo

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It's a simple mistake but you forgot to import useState in the example

import { useRef, useEffect, useCallback } from 'react';

// Usage
function App(){
  // State for storing mouse coordinates
  const [coords, setCoords] = useState({ x: 0, y: 0 });

@gragland

gragland commented Jul 2, 2019

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Author

@ikabirov The 3rd parameter is the element to add the event listener to. I've changed the default from global to window to make it a little more clear to people not used to seeing global (which is defined if node is rendering server-side, but irrelevant in this case since useEffect is only run client-side).
@Dygerydoo Thanks, fixed!

@josebrito

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Some ESLint configurations may complain about eslint/consistent-return with this approach. I suggest returning a no-op function instead. Change:

if (!isSupported) return;

To:

if (!isSupported) return () => {};

@statianzo

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Might want to throw or warn when !isSupported. Failing silently is a good way to cause extra debugging time.

@setbap

setbap commented Sep 14, 2019

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hello .that good to attach an event to HTML node
so I add a state to reference HTML node ( i can't speak English well :) )
the code that I forked from code sandbox:https://codesandbox.io/s/useeventlistener-yyxnq
ref, setRef is optional, if we don't use it evet attached to the window
[i think that not be in a nice way but I learn to react and I want to suggest my way to help me learn new thing]

@yossisp

yossisp commented Jan 16, 2020

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there's a bug on line 38 in this code:

useEffect(() => {
    savedHandler.current = handler;
  }, [handler]);

if the handler is changed then removeEventListener must also be called.

@jedwards1211

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@yossisp it adds the real event listener just once on mount and removes it once on dismount, and the real event listener delegates to handler:

      // Create event listener that calls handler function stored in ref
      const eventListener = event => savedHandler.current(event);

@yossisp

yossisp commented Jan 20, 2020

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@jedwards1211 the problem is that event listeners are removed only when the component unmounts. But if the component rerenders the element will change which will add an event listener without removing the old one. This is what happened to me.

@jedwards1211

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@yossisp I don't know what you observed in the debugger but you must be misunderstanding, because the semantics of useEffect guarantees that the cleanup function will run (removing the old listener) before the function runs again (adding the new listener). And the cleanup function's closure will have element bound to the old element.

@larts85

larts85 commented Nov 30, 2021

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You may add the following line element = !element ? window : element before isSupported constant in order to avoid the following error window is not defined in Next.Js projects. Also remove default element value in hook parameters!

@TusharShahi

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I am wondering if the props like eventName,event and listener will not change then why run effects again. If useEventListener is run again why should we care about the new values, since the listener is already attached in the first place. What case am i missing?

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