fix: sync mutations are applied independently

[koba04]

fixes #994.
A codesandbox in #994 now works correctly. https://codesandbox.io/s/swr-basic-forked-qrbpc?file=/src/App.js

This is the same approach in #735, so this causes unnecessary renders. But React batches updates in an event hander and has unstable_batchedUpdates for the API, which means developers can make those updates in a batched way.
On the other hand, the problem described in #994 is hard to avoid for developers and seems to be a more common case, so I think it should be fixed.

[codesandbox-ci]

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Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
eloquent-river-m341r	Issue #994

[shuding]

Thanks for looking into this and creating a minimal reproduction in #994 (comment)!

I'm a little bit worried that with this change, SWR no longer batches synchronous mutations and components will render intermediate result, which is another issue that we don't want to have (we can't ask users to always use unstable_batchedUpdates, and that API isn't officially stable).

However I'm kinda 50/50 on this and it might be a good thing. Since React itself doesn't batch the results by default if you do 2 setStates in a row. So like you said, this should be something that developers should handle.

[shuding]

I just verified that with this fix, intermediate results will not be rendered: https://codesandbox.io/s/swr-basic-forked-d78ie?file=/src/App.js (it won't log any odd numbers). It seems like React deduped them automatically, without unstable_batchedUpdates?

Anyways this is a good fix, thank you very much!

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 998c0d8:

Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
SWR-Basic (forked)	PR
agitated-heyrovsky-iw4hf	Issue #994

[koba04]

@shuding

I'm a little bit worried that with this change, SWR no longer batches synchronous mutations and components will render intermediate result, which is another issue that we don't want to have (we can't ask users to always use unstable_batchedUpdates, and that API isn't officially stable).

I agree with you and I have been hoping unstable_batchedUpdates would be a stable API over five years 😭
Actually, I don't have enough confident for this change yet, so I would need more investigations.

However I'm kinda 50/50 on this and it might be a good thing. Since React itself doesn't batch the results by default if you do 2 setStates in a row. So like you said, this should be something that developers should handle.

Yeah, this is the default behavior of React, so it makes sense for me than optimizing the update with undesirable edge cases.

I just verified that with this fix, intermediate results will not be rendered: https://codesandbox.io/s/swr-basic-forked-d78ie?file=/src/App.js (it won't log any odd numbers). It seems like React deduped them automatically, without unstable_batchedUpdates?

React does batching updates in an event handler by default, so the updates in the CodeSandbox are batched.

[shuding]

Thank you!

[koba04]

Thank you!