IssueTracker

Yigit, do you have time to fix it?

reemission of the same liveData is racy

yea i'll take it.

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.

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()
        }
    }

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)

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} )
        }

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

This is really weird but I believe this issue resurfaced in 2.7.1. I was using 2.5.3 and it was working just fine but today i updated the dependency and this started happening.

Same for me. I was using 2.7.1. When I selected the arrow to navigate back twice quickly, the app would go back one screen, then the screen would go completely blank. When I reverted to 2.5.3, that issue went away.

Same for me. I was using 2.7.2 and, oh man, when I encountered this issue I didn't even know where to start debugging. Thank god it worked just fine when I downgraded to 2.5.3.

same here, just updated from 2.5.3 to 2.7.3 and screen is blank when navigating for second time on same screen

To reproduce this issue, I attached a basic navigation project that consists of screens with buttons that go from one Screen to another.

Additionally, I attached two videos. One using Navigation 2.6.0 shows it is working fine, while the other video using Navigation 2.7.1 shows the strange behavior of a blank flash.

I tested all 2.7.x versions and the 2.8.0 alpha, and it's still happening.

Additionally, in some cases, the transition of the screens is that it enters from the upper right corner to the lower left. This behavior is bizarre and erratic since there is no defined animation, and it happens randomly on the screen. Only when many buttons on the screens are pressed quickly.

Feel free to tag me if there is any advance :(. (I am new here, IDK how this works)

#15 you should report this in a new ticket

Here is a solution that works for me. The blank screen seems to happen if there's no screen in the previous history to navigate to when a user double-clicks the back arrow too quickly.

if (navController.previousBackStackEntry != null) {
                        navController.popBackStack()
                    }

And here is my solution for bottom navigation, always check if the current route is not equal to the route being navigated to:

onClick = {
                    if (selectedItemIndex != index) {
                        localNavController.navigate(
                            route = item.route,
                            builder = {
                                launchSingleTop = true
                                restoreState = true
                                popUpTo(localNavController.graph.startDestinationId) {
                                    saveState = true
                                }
                            }
                        )
                    }
                }