Implement APIs to pass results between Fragments [149787344]

Fixed

Feature Request

Status Update

No update yet.

Description

jb...@google.com

created issue #1

Feb 19, 2020 12:07AM

We should add a clean way of passing results between two Fragments.

It should be able to properly handle process death and with it, we can deprecate target fragment.

Target fragment requires directly accessing the instance of another Fragment and not only is that discouraged, but the state of the Fragment being accessed is unpredictable so it is also dangerous. Target fragment is also not supported in Navigation.

Long term, the Fragment Lifecycle methods will be driven with a LifecycleObserver and target fragment cannot exist in that world.

Comments

ap...@google.com <ap...@google.com> #2Apr 2, 2020 08:43PM

Yigit, do you have time to fix it?

reemission of the same liveData is racy

ap...@google.com <ap...@google.com> #3Apr 3, 2020 05:22PM

yea i'll take it.

ap...@google.com <ap...@google.com> #4Apr 3, 2020 06:12PM

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.

jb...@google.com <jb...@google.com> #5Apr 3, 2020 06:32PM

Marked as fixed.

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()
}
}

de...@gmail.com <de...@gmail.com> #6May 15, 2020 01:28PM

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)

de...@gmail.com <de...@gmail.com> #7May 15, 2020 02:10PM

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> #8May 15, 2020 07:37PM

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

b9...@gmail.com <b9...@gmail.com> #9May 18, 2020 10:34AM

        LifecycleEventObserver observer = new LifecycleEventObserver() {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    lifecycle.removeObserver(this);    // only remove observer when onDestroy
                    mResultListeners.remove(requestKey);
                }
            }
        };
        lifecycle.addObserver(observer);

I think it would be better remove observer right after the callback is fired, sometimes we don't want the observer keep listening. In that way we could wrap the whole task in one block (like coroutine style), more flexible.

il...@google.com <il...@google.com> #10May 18, 2020 08:20PM

Re #9 - you can certainly clear out your FragmentResultListener after you receive a result if that's the behavior you want for your business logic. The clean up mentioned in your comment is only to prevent memory leaks and not for business logic purposes.