Race condition issue with <-deref about hx HOT 6 CLOSED

Wow! Great find. Your description makes some sense to me; useEffect definitely fires after rendering is done. I can see how the race condition occurs; that's a tricky one!

I can think of two potential fixes:

1. Use useLayoutEffect instead to fire synchronously after render

However, this might have negative effects; per the React docs:

Updates scheduled inside useLayoutEffect will be flushed synchronously, before the browser has a chance to paint.

This means that when Concurrent React lands, we might be over-scheduling the state changes to re-render the component.

This actually begs the question how re-renders are being scheduled now while we're using useEffect. 🤔

2. Manually manage the subscription via refs

Something like:

(let [[v u] (<-state @a)
     atom-identity (<-ref nil)
     sub (<-ref (gensym "<-"))]
  (when (and (not= a @atom-identity)
    (when (not (nil? @atom-identity))
      (remove-watch a @sub))
    (add-watch a @sub (fn [_ _ _ v'] (u v'))))
  v)

This completely removes the act of subscribing from React's lifecycle. But once again I'm not sure how this interacts with React's scheduler.

I might reach out to the React team to see the best way to handle this case. I imagine this would come up anytime you're subscribing to some external event bus.

from hx.

Looks like there is already an issue in the React repo: facebook/react#14988

gaeron posted this:

As for fixing the original issue, the strategy is to read the mutable value in useEffect. If it changed since the value captured during render, setState.

Which seems like an easy fix.

from hx.

from hx.

Would you be able to verify that 6d3b7cb fixes the issue?

from hx.

I’ll have to test it on Monday, but I applied pretty much the same code (sans comparison) in a local version and that did the trick. The comparison should be ok, since there can’t be a second thread mutating the atom behind the scenes.

Thanks!

from hx.

Cool. Closing this for now. Re-open if you still run into issues.

from hx.