Hi there, so this is a question about testing Com...

# compose

[s3rius]

Hi there,

so this is a question about testing Compose Pager. I have a ViewModel that wants to listen to the pager's current index, so I hoisted the PagerState into the ViewModel and observe it via snapshotFlow. Basically this:

class VM {
	val pagerState = PagerState(initialPage = 0)
	private val currentIndex = snapshowFlow { pagerState.currentPage }
	// do something with currentIndex
}

This works just fine.

Now I'm trying to unit test my `ViewModel`:

fun test() = runTest {
	val vm = createVm()
	vm.pagerState.scrollTo(n) // <- blocks indefinitely
	// assert that vm does the correct thing
}

Until now I had no trouble testing Compose State classes, but it seems PagerState has some internal mechanism that waits for a layout to happen before scrollTo finishes. Since this isn't happening in my test, the function blocks indefinitely.

I could handle the current-page logic entirely myself but I would end up having to synchronize with the PagerState and end up with two sources of truth.

Any ideas?

[jossiwolf]

@levima

[mattinger]

It makes sense that it has that internal mechanism. scrollToPage is a suspending function that is supposed to wait for the scroll to actually happen.

[mattinger]

I suspect you’d have to spawn another thread somewhere that’s going to alter the state somehow to indicate that the scroll has happened)

[s3rius]

Since it waits for a layout, I could technically turn it into a fully-fledged Compose UI test. But that's a lot of overhead just to test the page change mechanism.

[levima]

We do use the same mechanism in other states, like in LazyLists. But without a layout pass I'm not sure you'll be able to unblock the scrollTo call. So my best advice for now would be to convert it to a compose ui test and file a ticket so we can take a better look at this 😄