Understanding `act` behaviour

[mhuebert]

I have been trying to use act for the first time, and having some issues, and so I'm wondering if my expectations are wrong about what it is supposed to do, or if I am "doing it wrong".

What is the current behavior?

The only way I can observe the results of state changes I initiate is by using a timeout.

Paste the link to your JSFiddle or CodeSandbox example below:

https://codesandbox.io/s/k5zmln6w83?expanddevtools=1&fontsize=14&hidenavigation=1

What is the expected behavior?

What I expect is that by wrapping a state change or render operation in act, all of the resulting state changes / side-effects / re-renders will be complete by the time act returns, so that the operation appears (or is coerced to be) synchronous.

I created an example (https://codesandbox.io/s/k5zmln6w83?expanddevtools=1&fontsize=14&hidenavigation=1) wherein I render a view via unstable_ConcurrentMode. In the view, I create a useState hook with a value of 0. After the view is rendered, I use that hook's setter to change its state to 1.

Below is a log of the steps I take, showing three values at each time: seenByRender, the last state-value that appeared within the render body; calculated, the last value returned from my state-update function; and seenByEffect, the last value observed from a useEffect I create in the view.

1. before act/render:   seenByRender: null, calculated: null, seenByEffect: null
2. after act/render:    seenByRender: 0,    calculated: null, seenByEffect: 0
3. ---incrementing with act/setState---
4. after act/increment: seenByRender: 0,    calculated: 1,    seenByEffect: 0
5. after timeout:       seenByRender: 1,    calculated: 1,    seenByEffect: 1

What I am wanting/expecting is for step 4 to look like step 5, ie, I can somehow test the full consequences of my setState call.

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?

16.8.6, using unstable_ConcurrentMode

[threepointone]

act() isn't concurrent mode compatible yet, I'm afraid. we're working on it.

[mhuebert]

Ok, thanks. Do you know if there is any short-term solution (other than using a timeout) to test concurrent behaviour?

[threepointone]

we're still fleshing out what the concurrent mode story will be, so I hesitate to make any recommendation. We're not sure whether we'll even support the unstable_ConcurrentMode component. maybe you could write your tests in regular sync mode?

[mhuebert]

Ok. I am working on some state-management tools and wanted to make sure they would be compatible with concurrent mode; maybe it is just too early for that.

[threepointone]

I have a PR open that sort of unifies act's behaviour in sync and concurrent mode #15591 you'll probably have to mock the scheduler for it (docs incoming), but most tests should just work. I'll leave this issue open till we land/release it.

[threepointone]

We have a clearer idea now of how we want to treat this (but no official documentation yet). tl;dr you should use a mocked scheduler (and you'll see instructions if you try using act() with a concurrent mode tree). I'm going to close this, but we will have more to say in the near future.