LiveData.switchMap should allow to return `null` instead LiveData [132923666]

Fixed

Feature Request

Status Update

No update yet.

Description

ma...@gmail.com

created issue #1

May 17, 2019 10:59AM

Component used: androidx.lifecycle:lifecycle-livedata-ktx
Version used: 2.2.0-alpha01

LiveData.switchMap breaks the nullability contracts of the corresponding Java Transformations.switchMap.
Transformations.switchMap allows returning null, while the Kotlin extension function does not.
I can write

Transformations.switchMap(fooLiveData) { it?.barLiveData }

but I can't write

fooLiveData.switchMap { it?.barLiveData }

since I'm expected to return a non-nullable LiveData<Y>.
To make it compile, I would need to write an ugly

fooLiveData.switchMap { it?.barLiveData ?: MutableLiveData<Bar>(null) }

The null return value is safely handled in the Java implementation, so I believe there should be no reason to force a non-null return type.

Please consider changing the switchMap function signature from
inline fun <X, Y> LiveData<X>.switchMap(crossinline transform: (X) -> LiveData<Y>): LiveData<Y>
to
inline fun <X, Y> LiveData<X>.switchMap(crossinline transform: (X) -> LiveData<Y>?): LiveData<Y>

Comments

pe...@gmail.com <pe...@gmail.com> #2Nov 21, 2019 02:39PM

Yigit, do you have time to fix it?

reemission of the same liveData is racy

pe...@gmail.com <pe...@gmail.com> #3Nov 21, 2019 03:24PM

yea i'll take it.

[Deleted User] <[Deleted User]> #4Nov 29, 2019 02:45AM

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.

pe...@gmail.com <pe...@gmail.com> #5May 28, 2020 08:51AM

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()
}
}

la...@gtempaccount.com <la...@gtempaccount.com> #6Oct 19, 2020 08:22PM

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)

il...@google.com <il...@google.com> Nov 18, 2021 10:12PM

Status: New

il...@google.com <il...@google.com> Feb 1, 2022 09:34PM

Assigned to jb...@google.com.

ap...@google.com <ap...@google.com> #7Feb 2, 2022 11:27PM

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} )
}

jb...@google.com <jb...@google.com> #8Feb 2, 2022 11:30PM

Marked as fixed.

Project: platform/frameworks/support
Branch: androidx-master-dev

commit af12e75e6b4110f48e44ca121466943909de8f06
Author: Yigit Boyar <yboyar@google.com>
Date: Tue Sep 03 12:58:11 2019

Fix coroutine livedata race condition

This CL fixes a bug in liveData builder where emitting same
LiveData source twice would make it crash because the second
emission registry could possibly happen before first one is
removed as source.

We fix it by using a suspending dispose function. It does feel
a bit hacky but we cannot make DisposableHandle.dispose async
and we do not want to block there. This does not mean that there
is a problem if developer disposes it manually since our emit
functions take care of making sure it disposes (and there is
no other way to add source to the underlying MediatorLiveData)

Bug: 140249349
Test: BuildLiveDataTest#raceTest_*

Change-Id: I0b464c242a583da4669af195cf2504e2adc4de40

M lifecycle/lifecycle-livedata-ktx/api/2.2.0-alpha05.txt
M lifecycle/lifecycle-livedata-ktx/api/current.txt
M lifecycle/lifecycle-livedata-ktx/api/public_plus_experimental_2.2.0-alpha05.txt
M lifecycle/lifecycle-livedata-ktx/api/public_plus_experimental_current.txt
M lifecycle/lifecycle-livedata-ktx/api/restricted_2.2.0-alpha05.txt
M lifecycle/lifecycle-livedata-ktx/api/restricted_current.txt
M lifecycle/lifecycle-livedata-ktx/src/main/java/androidx/lifecycle/CoroutineLiveData.kt
M lifecycle/lifecycle-livedata-ktx/src/test/java/androidx/lifecycle/BuildLiveDataTest.kt

https://android-review.googlesource.com/1112186

https://goto.google.com/android-sha1/af12e75e6b4110f48e44ca121466943909de8f06