Develop Reference

Android - NavGraph without a startDestination

How can I have a navGraph without a startDestination?
I have a BottomSheet fragment container. When some events are triggered I want to expand this BottomSheet and load a fragment to its fragment container. Until one of those events are triggered I don't want the fragment container to have any fragment loaded. I want it to be empty.
But if don't supply a valid startDestination to the navGraph the app crashes with the exception:
IllegalStateException: no start destination defined
Is this possible? What would be the best way to handle this?
EDIT
I know I can supply a startDestination programmatically. But this is not a graceful approach. It introduces clutter code that needs to be executed when the first fragment load needs to happen and it gets worse when that fragment needs arguments.
The goal is to skip supplying a startDestination altogether.

First solution:
Create empty fragment and set it as startDestination in graph
Second solution:
Wait until event triggered and open BottomSheet with startDestination argument. Read argument of BottomSheet and change start destination
navGraph.setStartDestination(R.id.fragment2)

Set a default startDestination in your navgraph. For me, for example, it is the destination I usually want to start with (R.id.trips_fragment)
Then, in my activity onCreate:
private fun setupMainNavigationGraph() {
val navController = findNavHostFragment().navController
val mainNavigationGraph = navController.navInflater.inflate(R.navigation.main_graph)
mainNavigationGraph.setStartDestination(intent.getIntExtra(START_DESTINATION_INTENT_EXTRA, R.id.trips_fragment))
navController.graph = mainNavigationGraph
}
This way, if I want to use a custom start destination, I pass the start destination ID as an Intent to the activity, and the graph is loaded with the overridden start destination.
Note, the line here
intent.getIntExtra(START_DESTINATION_INTENT_EXTRA, R.id.trips_fragment)
R.id.trips_fragment is the default value.
I use this for integration tests to override the start destination.

Navigation Component - Starting destination is incorrect

I started noticing something in my app the other day and its wildly inconsistent. Sometimes it happens and sometimes it doesn't.
I am using the Navigation Component to handle navigation in the app and I started noticing that sometimes, when popping the backstack via the action bar back button or the device back button, it returns to a fragment that is no longer the starting destination (or at least shouldn't be).
In my case the app starts in MainFragment and once authenticated moves to DashboardFragment. This is a pretty common scenario.
Navigation in the app is pretty flat. most of the time its only 1 level deep so nearly all views are accessible from the dashboard.
The app starts at a login view as many do and then to a dashboard where the session will remain as the "start destination". To accomplish this, its done in the nav_graph using popUpTo and popUpToInclusive.
<fragment
android:id="#+id/mainFragment"
android:name="com.example.view.fragments.MainFragment"
android:label="Welcome">
<action
android:id="#+id/action_mainFragment_to_dashboardFragment"
app:destination="#id/dashboardFragment"
app:popUpTo="#id/mainFragment"
app:popUpToInclusive="true"/>
</fragment>
<fragment
android:id="#+id/dashboardFragment"
android:name="com.example.view.fragments.dashboard.DashboardFragment"
android:label="#string/dashboard_header" >
<action
android:id="#+id/action_dashboardFragment_to_notificationsFragment"
app:destination="#id/notificationsFragment" />
</fragment>
When the user successfully authenticates and its time to go to the dashboard, I use NavController.navigate() to send them there.
findNavController().navigate(
MainFragmentDirections.actionMainFragmentToDashboardFragment()
)
// This should have the same result and it does appear to be affected by the same issue
// findNavController().navigate(R.id.action_mainFragment_to_dashboardFragment)
I have an action bar with a back arrow and a navigation drawer. In the main activity I need to define the AppBarConfiguration and override onSupportNavigateUp()
lateinit var appBarConfiguration: AppBarConfiguration
...
override fun onCreate(savedInstanceState: Bundle?) {
Timber.d("onCreate()")
super.onCreate(savedInstanceState)
_binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
// There is 2 different drawer menu's respectfully.
appBarConfiguration = AppBarConfiguration(
setOf(
R.id.mainFragment,
R.id.dashboardFragment
), binding.drawerLayout
)
setSupportActionBar(binding.toolbar)
setupActionBarWithNavController(navController, appBarConfiguration)
}
...
override fun onSupportNavigateUp(): Boolean {
Timber.d("-> onSupportNavigateUp()")
val breadcrumb = navController
.backStack
.map { it.destination }
.filterNot { it is NavGraph }
.joinToString(" > ") { it.displayName.split('/')[1] }
Timber.d("Backstack: $breadcrumb")
Timber.d("Previous backstack entry: ${navController.previousBackStackEntry?.destination?.displayName}")
return navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp()
}
...
The logs look like this when we step back and it is working correctly
D/MainActivity: -> onSupportNavigateUp()
D/MainActivity: Backstack: dashboardFragment > testingFragment
D/MainActivity: Previous backstack entry: com.example:id/dashboardFragment
D/DashboardFragment: -> onCreateView()
D/BaseFragment: -> onCreateView()
D/DashboardFragment: -> onViewCreated()
I also noticed when using the hamburger in the action bar that it also calls onSupportNavigateUp()
D/MainActivity: -> onSupportNavigateUp()
D/MainActivity: pendingAction: false
D/MainActivity: Backstack: dashboardFragment
D/MainActivity: Previous backstack entry: null
When I use the drawer to navigate to a destination I do see this in the logs and im not sure where/why this is returned or if it has any importance
I/NavController: Ignoring popBackStack to destination com.example:id/mainFragment as it was not found on the current back stack
Now, when its NOT working correctly, this is what the logs look like
D/MainActivity: -> onSupportNavigateUp()
D/MainActivity: Backstack: mainFragment > testingFragment
D/MainActivity: Previous backstack entry: com.example:id/mainFragment
D/MainFragment: -> onCreateView()
D/BaseFragment: -> onCreateView()
D/MainFragment: -> onViewCreated()
This really feels like the popUpTo and popUpToInclusive properties are not being applied (sometimes) when performing the navigation from main fragment to dashboard fragment. It's also suspicious that even though the dashboard fragment is not set as the new starting destination but also its missing from the backstack. Assuming the properties were NOT applied I would expected to see the breadcrumb Backstack: mainFragment > dashboardFragment > testingFragment
Any help would be greatly appreciated!

While there may certainly be a better procedure to follow (as outlined in the Principles of Navigation in the previous comments) the overhead of the change introduces a plethora of new errors and the scope is too large at this time.
Its still unknown why popUpTo and popUpToInclusive are not reliable via XML when navigating (even using NavDirections) however, thus far, passing the NavOptions while navigating seems to resolve the issue.
findNavController().navigate(
MainFragmentDirections.actionMainFragmentToDashboardFragment(),
NavOptions.Builder().setPopUpTo(R.id.mainFragment, true).build()
)
Thus far the issue has yet to arise again.

As it says in Ian's first link:
The back stack always has the start destination of the app at the bottom of the stack.
Your start destination is the "home" one in a navigation graph, the one identified with the little house icon. Set with the startDestination attribute in the XML. When the Navigation library creates a backstack, it always has that destination at the bottom. And it will always be there, even if you try to avoid it with the poUpTo attributes.
That's why if there's a fragment you consider your "home" one, like your dashboard, that needs to explicitly be your startDestination. It's the last thing a user will see if they back out of your app. If you have something like a login or welcome screen as the start destination, they'll back out to that.
Which is why you need to set the "home" fragment as the start destination, and then handle any extra navigation to a login or welcome screen from there. It's just how the Navigation stuff is designed to work. If you try and work around it (I did!) you'll run into other problems, and a lot of the nice features like automatic backstack recreation might not work properly

FragmentManager instantiate correctly fragments but content fragments is invisible

I've been trying to resolve this issue for some time now. My app uses a slidingPaneLayout to show fragments inside it. My issue is in lower API than 28, after some time loading fragments on the slidingPaneLayout, when instantiate new fragments, their content become partially or fully invisible but the fragment itself works correctly (sounds and timer works even with content invisible), if I go background with the app and comes back the content of the fragment re-appears without any issue.
I don't know exactly what is the causing the issue, if someone had this problem before and have some indications it would be a great help for me.
For the slidingPaneLayout I use this library: https://github.com/umano/AndroidSlidingUpPanel
For changing my fragments I use this:
fun showFragment(fragment: Fragment, containerId: Int, tag: String) {
val fragmentTransaction = supportFragmentManager.beginTransaction()
fragmentTransaction.replace(containerId, fragment, tag)
fragmentTransaction.addToBackStack(null)
fragmentTransaction.commit()
}
What it should look like:
What happens when this issue first appears
After first issue all fragments load with content invisible (the orange arrow is from the activity view)
[3
PS: I corrected nearly all memory leaks with canaryleak on the app
PS: I'm using the last version of Fragments, androidx.fragment:fragment:1.2.4
Edit: When i force onStop in my fragment like below:
var firstTime = true
override fun onResume() {
super.onResume()
if(firstTime){
onStop()
firstTime = false
}
}
The fragment view appears correctly I don't know why

How to kill all fragments created by a NavHostFragment?

I have 2 navigation files, and in my Activity, 2 fragments. One of the navigations is always shown inside one of the fragments, but I show the other one only when I need it.
The way they're drawn is the always showing fragment is inside a relativeLayout, and the other fragment is inside the same relativeLayout with it's visibility set as gone. When I need the second navigation, I set the visibility to visible and when I don't need it, I set it to gone again.Visually this works well, but what I want to accomplish is that when I don't want the second navigation, I want to completely kill it and redraw it the next time I need it.
What I've done so far was to get a hold of the NavHostFragment used to start the navigation, and when I dont need it anymore, call popBackStack() on it's navController, but it doesn't work:
val navHost: NavHostFragment? = null
fun createSecondNav() {
navHostLogin = NavHostFragment.create(R.navigation.navigation_second)
theFragment.visibility = View.VISIBLE
supportFragmentManager.beginTransaction()
.replace(R.id.theFragment, navHostLogin!!)
.commit()
}
fun killSecondNav() {
theFragment.visibility = View.GONE
navHostLogin?.navController?.popBackStack() // returns false
navHostLogin = null
}
So how can I completely kill the fragments created by the second navHost?

NavController maintains it's own back-stack, independent form the FragmentManager back-stack.
And popBackStack() without arguments only pops that back-stack once:
Attempts to pop the controller's back stack. Analogous to when the user presses the system Back button when the associated navigation host has focus.
While popBackStack(int destinationId, boolean inclusive) reads:
Attempts to pop the controller's back stack back to a specific destination.
destinationId int: The topmost destination to retain
inclusive boolean: Whether the given destination should also be popped.
So this should be:
navController.popBackStack(R.id.startDestination, true)
I'd wonder why even using two NavController, because one can set the graph at run-time with setGraph(NavGraph graph, Bundle startDestinationArgs):
Sets the navigation graph to the specified graph.
Any current navigation graph data (including back stack) will be replaced.

Why Android Navigation Component screen not go back to previous Fragment,but a method in onViewCreated of previos Fragment being called?

I have 2 fragment call CreateRoomFragment and DisplayPhotoFragment,the navigation graph is look like this:
<navigation>
<fragment
android:id="#+id/createRoomFragment"
android:name="package.room.CreateRoomFragment"
android:label="Create a room"
tools:layout="#layout/fragment_create_room">
<action
android:id="#+id/action_createRoomFragment_to_roomFragment"
app:destination="#id/roomFragment" />
<action
android:id="#+id/action_createRoomFragment_to_displayPhotoFragment"
app:destination="#id/displayPhotoFragment" />
</fragment>
<fragment
android:id="#+id/displayPhotoFragment"
android:name="package.fragment.DisplayPhotoFragment"
android:label="fragment_display_photo"
tools:layout="#layout/fragment_display_photo" >
<argument android:name="bitmap"
app:argType="android.graphics.Bitmap"/>
</fragment>
So when I wanna to move from CreateRoomFragment to DisplayPhotoFragment,I use the do as below:
NavDirections action = CreateRoomFragmentDirections.actionCreateRoomFragmentToDisplayPhotoFragment(selectedPhoto);
Navigation.findNavController(view).navigate(action);
Doing this,I can navigate to DisplayPhotoFragment.
But when I press back button of the device and also the Back arrow from the toolbar,it cant go back to CreateRoomFragment.
I tried this,but still unable to back to previous fragment:
requireActivity().getOnBackPressedDispatcher().addCallback(getViewLifecycleOwner(),
new OnBackPressedCallback(true) {
#Override
public void handleOnBackPressed() {
navController.navigateUp(); //I tried this
navController.popBackStack(R.id.createRoomFragment,false); //and also this
}
});
Main Problem now:
By using the code above,the screen didnt go back to previous Fragment(CreateRoomFragment).It still stuck in DisplayPhotoFragment,but at the same time,an API method in CreateRoomFragment onViewCreated section is being called.
What causing this? and how can I solve this problem?

I had the same problem. For me the issue was that I was using a LiveData boolean to decide when to go to the next fragment. When I then navigated back/up the boolean was still true so it would automatically navigate forward again.

Android maintains a back stack that contains the destinations you've visited. The first destination of your app is placed on the stack when the user opens the app. Each call to the navigate() method puts another destination on top of the stack. Tapping Up or Back calls the NavController.navigateUp() and NavController.popBackStack() methods, respectively, to remove (or pop) the top destination off of the stack.
NavController.popBackStack() returns a boolean indicating whether it successfully popped back to another destination. The most common case when this returns false is when you manually pop the start destination of your graph.
When the method returns false, NavController.getCurrentDestination() returns null. You are responsible for either navigating to a new destination or handling the pop by calling finish() on your Activity.
When navigating using an action, you can optionally pop additional destinations off of the back stack by using popUpTo and popUpToInclusive parameter of the action.
class MyFragment : Fragment() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val onBackPressedCallback = object : OnBackPressedCallback(true) {
override fun handleOnBackPressed() {
if (true == conditionForCustomAction) {
CustomActionHere()
} else NavHostFragment.findNavController(this#MyFragment).navigateUp();
}
}
requireActivity().onBackPressedDispatcher.addCallback(
this, onBackPressedCallback
)
...
}

The best solution for handling navigation using live data is to use the SingleLiveEvent.
You can always use this class which is an extension of MutableLiveData.
https://gist.githubusercontent.com/cdmunoz/ebe5c4104dadc2a461f512ea1ca71495/raw/a17f76754f86a4c0b1a6b43f5c6e6d179535e627/SingleLiveEvent.kt
For a detail run down of this check:
https://proandroiddev.com/singleliveevent-to-help-you-work-with-livedata-and-events-5ac519989c70

Had a similar issue. We still have multiple activities with nav component.
So imagine activity A -> activity B, activity B has its own nav and fragments. When the initial fragment tries to pop the back stack there is nowhere to pop back to and the nav controller does not know to finish the activity. So one solution I found was to do
if (!findNavController().popBackStack()) activity?.finish()
If nav controller can not pop back it will finish activity.

You can use MutableSharedFlow instead on MutableLiveData if you want to observe the Event only once.
in your viewModel:
private val _events = MutableSharedFlow<Event>()
val events = _events.asSharedFlow() // read-only public view
suspend fun postEvent() {
_events.emit(event) // suspends until subscribers receive it
}
In your Activity/Fragment class:
lifecycleScope.launchWhenStarted {
viewModel.events.collect {
}
}
viewModel.postEvent()
This will prevent observing data continuously when going back to fragment.