Avoid duplicate listeners when using useReactiveVar with React.StrictMode #7581
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src/react/hooks/useReactiveVar.ts
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| // equivalent to the following, but shorter.: | ||
| // useEffect(() => { | ||
| // const mute = rv.onNextChange(setValue); | ||
| // return () => mute(); | ||
| // }, [value]) |
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Great idea to make this alternative code even more verbose, to emphasize the shortness of our version. 😂
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Fixes #7509. Because StrictMode renders functional components twice, duplicate listeners were added to
onNextChange. The trickier part of this is that it doesn't cause inconsistent results until the priority of the update is affected (in the reproduction, returning an unresolved Promise from the button'sonClickhandler resulted in a different priority level in React so the associated duplicate listeners were interleaved with other work resulting in a strange result). We now setonNextChangeonly on the initial mount and when the reactive var's value has changed.Checklist: