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fix(cleanup): remove scheduler code from flush-microtasks #744

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merged 4 commits into from Jul 13, 2020

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kentcdodds
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@kentcdodds kentcdodds commented Jul 13, 2020

What: This removes the scheduler code from flush-microtasks

Why: Because the test that failed is fundamentally flawed

How:

  1. Rewrite the test to illustrate why it's flawed (we can't and shouldn't guarantee that all scheduled tasks will run simply as a result of cleanup)
  2. Rewrite the test to not test for that
  3. Fix the implementation of flush-microtasks to not use the scheduler (basically revert everything we did for Fix build for React next #726 and change the test instead).

Checklist:

- [ ] Documentation added to the docs site N/A

  • Tests
    - [ ] Typescript definitions updated N/A
  • Ready to be merged

Closes #743

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@@ -31,7 +31,7 @@ test('cleanup waits for queued microtasks during unmount sequence', async () =>
const spy = jest.fn()

const Test = () => {
React.useEffect(() => () => setImmediate(spy))
React.useEffect(() => () => setTimeout(spy, 200))
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Users can do all kinds of async things as a result of their cleanup functions and it's not reasonable for them to expect that we'll wait until all those async things are done. They'll have to wait for those things themselves.

So we shouldn't have a setImmediate in here at all.

const spy = jest.fn()

const Test = () => {
React.useEffect(() => () => setImmediate(spy))
React.useEffect(() => spy)
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All we can guarantee is that the cleanup function is called, so that's all we'll test.

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👍 The problem is that we can't know what kind of task is queued from the cleanup function. We only supported setImmediate but any longer running task wasn't caught as far as I can tell.

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codecov bot commented Jul 13, 2020

Codecov Report

Merging #744 into master will not change coverage.
The diff coverage is n/a.

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@@            Coverage Diff            @@
##            master      #744   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          100       100           
  Branches        16        16           
=========================================
  Hits           100       100           

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This one looks good and as @eps1lon said, we were probably only waiting for setImmediate anyways.
Looks like we got ourselves to the wrong adventure 😅 Sorry about that.
I'll close the PR for mocking shceduler (#743).
This PR will resolve https://github.com/testing-library/react-testing-library/issues/738 and #735.

@kentcdodds kentcdodds merged commit 240900c into master Jul 13, 2020
@kentcdodds kentcdodds deleted the pr/remove-scheduler branch July 13, 2020 18:15
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Thanks for all the work you did anyway @MatanBobi!

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Glad I was able to help @kentcdodds

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🎉 This PR is included in version 10.4.6 🎉

The release is available on:

Your semantic-release bot 📦🚀

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pke commented Nov 19, 2020

@kentcdodds I think the react-native package suffers from a similar bug. When I add jest.useFakeTimers("modern") all tests suddenly time out.

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@kentcdodds I think the react-native package suffers from a similar bug. When I add jest.useFakeTimers("modern") all tests suddenly time out.

Hi @pke!
Did you happen to look at our using fake timers page?
Are you setting back to real timers in the afterEach hook?
A code example would be really helpful :)

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4 participants