onBecome(Un)Observed intercept hooks can remove each other

[jamiewinder]

The way onBecomeObserved and onBecomeUnobserved are implemented means they can inadvertently cancel each other.

Source of the issue is here:
https://github.com/mobxjs/mobx/blob/master/src/api/become-observed.ts#L44

If you add a hook, the behaviour is 'call the original hook, then the new one'. The disposer function that is returned has the behavior of 'restore the original hook'.

This means that if a hook is disposed (with the return value), any subsequent hooks added since that was added will also be removed. Example here:
https://codesandbox.io/s/k0y70x7vy7

(You'd expect one of the two callbacks be called, but it isn't because the first was removed).

I started a fix for this, but the solution wasn't as immediately straightforward as I thought and since I don't know when I'll have time to give it another go I wanted to record this for now.

[michalwarda]

Hey @mweststrate I'll be happy to try to fix this issue. Is there anything I should take into account before doing that? Any suggestions or we can discuss it after I try to implement it?

[mweststrate]

Hey! Feel free to open a PR, it is probably easier to discuss with some code at hand :-) Op ma 1 okt. 2018 om 11:25 schreef Michał Warda <[email protected]>:

…

Hey @mweststrate <https://github.com/mweststrate> I'll be happy to try to fix this issue. Is there anything I should take into account before doing that? Any suggestions or we can discuss it after I try to implement it? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#1537 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABvGhEYMZI-xvz0rgpsg08usOhC-9eSNks5ugd-egaJpZM4T3ACs> .

[michalwarda]

@mweststrate Do you have any suggestions on how can we continue this? I have some time currently so I can adjust the PR.

[mweststrate]

released in 4.8.0 / 5.8.0

[lock]

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