Feature request: Document how to display an empty screen [159054196]

Fixed

Bug

Status Update

No update yet.

Description

ri...@soundcloud.com

created issue #1

Jun 16, 2020 09:13AM

The doc about how to load and display paged data with Paging 3.0 clearly explains how to show progress and error views but it doesn't cover how to handle the case of an empty result. Also the samples seem to not support that case.

I'm new to the Paging library and I find it non trivial to determine what is the right LoadingState to listen to. I assume it's the refresh state, but even there it's not clear how to determine whether the result is empty or not.

Maybe this suggestion should go to a separate issue but I think it would be nice to reuse the same LoadStateAdapter approach as that would allow to build nice reusable loading-empty-error page components.

Comments

le...@gmail.com <le...@gmail.com> #2Jun 16, 2020 01:29PM

Yigit, do you have time to fix it?

reemission of the same liveData is racy

du...@google.com <du...@google.com> #3Jun 16, 2020 03:14PM

yea i'll take it.

cc...@google.com <cc...@google.com> #4Jun 16, 2020 04:06PM

Thanks for the detailed analysis. This may not be an issue anymore since we've started using Main.immediate there but I' not sure; I'll try to create a test case.

du...@google.com <du...@google.com> #5Jun 16, 2020 04:30PM

just emitting same live data reproduces the issue.

@Test
fun raceTest() {
val subLiveData = MutableLiveData(1)
val subject = liveData(testScope.coroutineContext) {
emitSource(subLiveData)
emitSource(subLiveData) //crashes
}
subject.addObserver().apply {
testScope.advanceUntilIdle()
}
}

ap...@google.com <ap...@google.com> #6Jun 16, 2020 04:48PM

With 2.2.0-alpha04 (that use Main.immediate), the issue seems to be still there (I tested it by calling emitSource() twice, like your test case)

cc...@google.com <cc...@google.com> Jun 16, 2020 05:11PM

Reassigned to du...@google.com.

ap...@google.com <ap...@google.com> #7Jun 17, 2020 11:33PM

Marked as fixed.

yea sorry immediate does not fix it.
I actually have a WIP fix for it:

https://android-review.googlesource.com/c/platform/frameworks/support/+/1112186

if your case is the one i found (emitting same LiveData multiple times, as shown in #5) you can work around it by adding a dummy transformation.

val subLiveData = MutableLiveData(1)
val subject = liveData(testScope.coroutineContext) {
emitSource(subLiveData.map {it })
emitSource(subLiveData.map {it} )
}