Nested NavHostFragments cannot intercept [111345778]

Fixed

Bug

Status Update

No update yet.

Description

an...@backx.org

created issue #1

Jul 12, 2018 08:44AM

Component used: navigation component
Version used: 1.0.0-alpha02
Devices/Android versions reproduced on: Nexus 5, OnePlus One

We want to implement nested NavHostFragments because we want to navigate between 2 screens that each have 2 different Bottom Navigation Bars. We also thought about nesting NavHostFragments so we can bind ViewModels to a navigation part of our app. Parts that need to clean up the data they were using after being navigated through. The problem we're facing is that the outer most NavHostFragment intercepts the back button navigation before the child NavHostFragments do. The result is that when pressing the back button, it completely goes back to the destination before the child NavHostFragment. Are there any plans to support nesting NavHostFragments or is there a different solution to nested navigation that is seemingly not supported by nesting navigation graphs because I don't see how you can for example navigate between fragments with different Bottom Navigation Bars and navigate the fragments that belong to the Bottom Navigation Bar?

Comments

an...@backx.org <an...@backx.org> #2Jul 12, 2018 09:21AM

Yigit, do you have time to fix it?

reemission of the same liveData is racy

il...@google.com <il...@google.com> #3Jul 12, 2018 07:45PM

Status: Won't Fix (Intended Behavior)

yea i'll take it.

an...@backx.org <an...@backx.org> #4Jul 12, 2018 07:50PM

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.

il...@google.com <il...@google.com> #5Jul 12, 2018 07:59PM

Assigned to il...@google.com.

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()
}
}

an...@backx.org <an...@backx.org> #6Jul 12, 2018 09:47PM

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)

be...@google.com <be...@google.com> #7Jul 13, 2018 01:01AM

Reassigned to jb...@google.com.

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} )
}

il...@google.com <il...@google.com> #8Jul 13, 2018 09:17PM

Marked as fixed, reassigned to il...@google.com.

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

il...@google.com <il...@google.com> #9Jul 13, 2018 09:18PM

And by alpha05, I mean alpha04. Getting ahead of myself :)

an...@backx.org <an...@backx.org> #10Jul 13, 2018 09:21PM

This is great to hear. I'm curious as to what is recommended when for example you need to navigate between screens with different BottomNavigationViews. Is it recommended to have separate activities or nested NavHostFragments? If it's NavHostFragments, how should this best be achieved? Would love some added documentation on this in time for alpha04.

Also thank you very much for the quick responses on the issue!

il...@google.com <il...@google.com> #11Jul 23, 2018 03:59PM

Per the material design guidelines (

https://material.io/design/components/bottom-navigation.html), bottom navigation bars should be consistent and appear on the bottom on every screen so changing the BottomNavigationView isn't really a recommended pattern.

ra...@gmail.com <ra...@gmail.com> #12Jul 30, 2018 11:57PM

1.0.0-alpha04 introduces a new bug where using a BottomNavigationBar 'NavigationUI.setupWithNavController(bottom_bar, navController)' results in the BottomNavBar not being synchronized.

The selected tab icon is not set when the fragment changes, it stays on the last icon that was tapped.