Develop Reference

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.

Passing data back to previous Fragment with Android Navigation Components

Imagine that FragmentA starts FragmentB on a button click. Passing some arguments to FragmenB is pretty straightforward:
val action = FragmentADirections.actionToFragmentB("some text")
findNavController().navigate(action)
But is there a way to pass data from FragmentB to FragmentA when the user navigates away from FragmentB? With other words, when FragmentB is destroyed as the result of back press.

In your Navigation UI xml:
<fragment
android:id="#+id/FragmentB"
android:name="com.example.FragmentB"
android:label="FragmentB"
tools:layout="#layout/FragmentA">
<action
android:id="#+id/action_FragmentB_to_FragmentA"
app:destination="#id/FragmentA" />
</fragment>
Like you want to navigate with some arguments back to the FragmentA from FragmentB, use this:
findNavController().navigate(
R.id.action_FragmentB_to_FragmentA,
bundleof("key" to <your_value>) // example: 100 (Int value)
)
Then you can just receive the argument in your FragmentA using arguments?.getInt("key") or if you pass String can get it with arguments?.getString("str_key") etc.

Kill fragment in navigation controller

My flow of fragment is like this
Main -> A -> B -> C ->A
In fragment c, it has a submit button which will return to A.
When I press back button in A, I want it back to Main. But it return to fragment c instead.
In fragment C, I use this
findNavController().navigate(R.id.action_c_to_a)
nav_graph.xml
<fragment
android:id="#+id/fragmentC"
android:name="xxx"
android:label="xxx">
<action
app:launchSingleTop="true"
app:popUpTo="#+id/fragmentA"
app:popUpToInclusive="true"
android:id="#+id/action_c_to_a"
app:destination="#id/fragmentA" />
</fragment>

Why not pop up to fragment A? You could just call findNavController().popBackStack(R.id.fragmentA, false) instead of navigating with an action.

try to look at my complete solution with removing/killing fragment from backstack.
Navigation Component set transition animation programmatically

You can override the back button behavior to do this.
Provide custom back navigation

The idea is to set a app:popUpTo without setting app:destination. Indeed, setting a app:destination will create a fragment and add it to the backstack which is not what you want.
To you can just remove :
app:destination="#id/fragmentA"
If you have this :
app:popUpTo="#+id/fragmentA"
And it will really pop the backstack until fragmentA

Navigation popUpTo and PopUpToInclusive aren't clearing the backstack

I'm new to the Android Jetpack Navigation architecture. I'm trying it out on a new app. There's one activity and a few fragments, two of them are login screen and email login screen. I defined those fragments in my navigations XML. The flow of the app is as follows:
Login screen → Email Login screen
What I want is, after navigating to the email login screen, when I press back, the app exits. Meaning the back-stack for login screen is removed. I know login screens aren't supposed to work that way, but I'm still just figuring things out.
I followed the documentation from Google's Get started with the Navigation component. It said, using app:popUpTo and app:popUpToInclusive="true" is supposed to clear the backstack, yet when I press back on email login screen, it still goes back to login instead of exiting.
So, here's what I've tried.
nav_main.xml
<fragment android:id="#+id/loginFragment"
android:name="com.example.myapp.ui.main.LoginFragment"
android:label="#string/login"
tools:layout="#layout/fragment_login" >
<action
android:id="#+id/action_login_to_emailLoginFragment"
app:destination="#id/emailLoginFragment"
app:popEnterAnim="#anim/slide_in_right"
app:popExitAnim="#anim/slide_out_right"
app:popUpTo="#+id/emailLoginFragment"
app:popUpToInclusive="true"/>
</fragment>
<fragment android:id="#+id/emailLoginFragment"
android:name="com.example.myapp.ui.main.EmailLoginFragment"
android:label="EmailLoginFragment"
tools:layout="#layout/fragment_login_email" />
LoginFragment.kt
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View {
binding.emailLoginButton.setOnClickListener {
findNavController().navigate(R.id.action_login_to_emailLoginFragment)
}
return binding.root
}
I gave a click event to a button. In it, I used the Navigation Controller to navigate to the email login screen by giving it the action's ID. In the <action>, there are app:popUpTo and app:popUpToInclusive="true".
After reading the documentation over and over, as well as reading plenty of StackOverflow questions, I found those properties are supposed to remove my login screen off the back-stack. But they don't. The button does navigate to the email login screen, but when I press back, it still goes back to login screen instead of exiting the app. What am I missing?

<action
android:id="#+id/action_login_to_emailLoginFragment"
app:destination="#id/emailLoginFragment"
app:popEnterAnim="#anim/slide_in_right"
app:popExitAnim="#anim/slide_out_right"
app:popUpTo="#+id/loginFragment"
app:popUpToInclusive="true"/>
Your popUpTo is going back to the email login, and then popping it because of the inclusive.
If you will change the popUpTo to your login fragment, it will be navigated back to, and popped as well because of the inclusive flag, which will result in your desired behaviour.

I write this answer for people who have not completely understood the
way popUpTo works and I hope its example helps someone because most
examples for navigation are repetitive in most sites and do not show
the whole picture.
In any <action> if we write a value for app:popUpTo, it means we want to delete some of the fragments from the back stack just after completing the action, but which fragments are going to be removed from the back stack when action is completed?
Its order is Last In First Out so:
All fragments between the last fragment and the fragment defined in popUpTo will be removed.
And if we add app:popUpToInclusive="true", then the fragment defined
in popUpTo will also be removed.
Example:
Consider fragments from A to G in a navigation graph like this:
A->B->C->D->E->F->G
We can go from A to B and then from B to C and so on. Consider the following two actions:
An action E->F we write:
<action
...
app:destination="#+id/F"
app:popUpTo="#+id/C"
app:popUpToInclusive="false"/>
And for F->G we write:
<action
...
app:destination="#+id/G"
app:popUpTo="#+id/B"
app:popUpToInclusive="true"/>
Then after going from E to F using the action E->F, the fragments between the last fragment (F) and C (which is defined in popUpTo of E->F) will be removed. The fragment C will not be removed this time because of app:popUpToInclusive="false" so our back stack becomes:
A->B->C->F (F is currently on Top)
Now if we go to fragment G using action F->G :
all fragments between the last fragment(G) and B (which is defined in popUpTo of F->G ) will be removed but this time the fragment B will also be removed because in F->G action we wrote app:popUpToInclusive="true" . so back stack becomes:
A->G (G is on top now)

These 2 lines make the trick works:
If you want to go from A to B and expect to finish A:
You need to call B with this action:
<fragment
android:id="#+id/fragmentA"
tools:layout="#layout/fragment_a">
<action
android:id="#+id/action_call_B"
app:destination="#+id/fragmentB"
app:popUpTo="#id/fragmentA"
app:popUpToInclusive="true" />
</fragment>
<fragment
android:id="#+id/fragmentB"
tools:layout="#layout/fragment_b">
</fragment>
If you put log to your fragments you can see that fragmentA is destroyed after calling fragmentB with this action.

You can do it in XML just like this answer does, or you can also do it programmatically:
NavOptions navOptions = new NavOptions.Builder().setPopUpTo(R.id.loginRegister, true).build();
Navigation.findNavController(mBinding.titleLogin).navigate(R.id.login_to_main, null, navOptions);

Let's say that your app has three destinations—A, B, and C—along with actions that lead from A to B, B to C, and C back to A. The corresponding navigation graph is shown in figure
With each navigation action, a destination is added to the back stack. If you were to navigate repeatedly through this flow, your back stack would then contain multiple sets of each destination (A, B, C, A, B, C, A, and so on). To avoid this repetition, you can specify app:popUpTo and app:popUpToInclusive in the action that takes you from destination C to destination A, as shown in the following example:
<fragment
android:id="#+id/c"
android:name="com.example.myapplication.C"
android:label="fragment_c"
tools:layout="#layout/fragment_c">
<action
android:id="#+id/action_c_to_a"
app:destination="#id/a"
app:popUpTo="#+id/a"
app:popUpToInclusive="true"/>
After reaching destination C, the back stack contains one instance of each destination (A, B, C). When navigating back to destination A, we also popUpTo A, which means that we remove B and C from the stack while navigating. With app:popUpToInclusive="true", we also pop that first A off of the stack, effectively clearing it. Notice here that if you don't use app:popUpToInclusive, your back stack would contain two instances of destination A

popUpTo its to define the place that you want to go when you press back. If you set popUpInclusive = true, the navigation skipe that place too ( in popUpTo ).

Sample: A -> B -> A
FragmentB.kt
Attempts to pop the controller's back stack
private fun popBackStackToA() {
if (!findNavController().popBackStack()) {
// Call finish on your Activity
requireActivity().finish()
}
}
Back Stack

I faced a similar problem and my approach was simple. In the navigation graph, you have to designate the starting screen. Mine was:
app:startDestination="#id/webview"
It's called the start destination, it is the first screen users see when opening your app, and it's the last screen users see when exiting your app.
If you do not wish your login activity to be shown as you exit the app, just remove it as the start destination and use the fragment that you wish to show last in your case, It's the Email Login screen.
<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/nav"
app:startDestination="#id/Email Login screen">
Also, make sure you override the onBackPressed() method from the host activity code as:
override fun onBackPressed() {
finish()
super.onBackPressed()
}
Now that you have removed the login fragment as the start destination, it's now not obvious what fragment will be shown first when the app opens.
Add a method to implement that in the host activity and call it from the oncreate(). In my case,i created initContent() to handle that logic. This was the code:
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val navHostFragment = supportFragmentManager
.findFragmentById(R.id.nav_host_fragment) as NavHostFragment
navController = navHostFragment.navController
if (savedInstanceState == null) {
initContent()
}
}
private fun initContent() {
if (isNetworkConnected()) {
navController.navigate(R.id.webView)
} else {
navController.navigate(R.id.noInternetFragment)
}
}
Hope this helps someone.

AndroidX navigation navigateUp twice

I am using the new AndroidX navigation framework.
I have a few fragments all linked in a navigation chain.
FragmentA --> FragmentB --> FragmentC
FragmentC has a Cancel button that should send me up all the way back to FragmentA.
Should I do the following:
on FragmentC call the method:
Navigation.findNavController(view).navigateUp();
then on FragmentB listen to some callback and using some passed parameter or argument trigger another navigateUp() function from FragmentB
or is there some method that will do the equivalent of navigateUpTwice()

What I ended up doing was
Navigation.findNavController(view).popBackStack(R.id.fragmant_a,false)

In your navigation.xml file under the action that you have created to navigate to starting fragment (FragmentA in your case) add the following
<action
....
app:popUpTo="#id/fragmentA"
app:popUpToInclusive="false"/>
This will pop up all the fragments until FragmentA and will exclude FragmentA since popUpToInclusive is set to false.
Edit:
The full form of your action under FragmentC tag of your navigation.xml file will be something similar to this:
<fragment
android:id="#+id/FragmentC"
android:name="com.yourdomain.FragmentC"
android:label="FragmentC">
<action
android:id="#+id/action_fragmentC_to_fragmentA"
app:destination="#id/fragmentA"
app:popUpTo="#id/fragmentA"
app:popUpToInclusive="false"/>
</fragment>

Try with this, it works for me.
findNavController().navigateUp()
findNavController().navigateUp()

You can set pop To FragmentA in your action from FragmentB --> FragmentC and when then you press back it goes to Fragment A instead of Fragment B

So I found myself in a situation where I had to navigateUp() twice, but with a twist: I could reach the view through several paths. Think of it like this:
Fragment A --> Fragment C --> Fragment D
Fragment B --> Fragment C --> Fragment D
In a normal situation, using the popTo xml attribute or the popBackStack() method would work. However it is unusable here. Fortunately, what you can do is:
val navController = NavHostFragment.findNavController(this)
navController.navigateUp()
navController.navigateUp
As other people have pointed out, this is absolutely not optimised performance-wise. In any situation where you can reach Fragment D through a single path, use popTo instead.