Develop Reference

Kotlin: Load/initialize next activity in the background

I have an initialization activity in my app which displays the logo, then I show my next activity using
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState) // Call the parent class function
setContentView(R.layout.activity_launcher)
// This starts a new co-routine
// it is important to do it this way, in order to show the UI _before_
// all the initialization happens, otherwise launcher is pointless
GlobalScope.launch {
...
[initialization]
...
startActivity(ActivityTwo)
}
}
The transition takes about three seconds because of all the code that is running inside onCreate belonging to ActivityTwo. Is there a way to "create" the second activity behind the scenes, and then show it. I don't mind if the app stays on the initialization screen for those 3 seconds, but the white transition looks really ugly.

onCreate method is actually creating your activity. You must be doing some heavy computation if your activity is janking while rendering. If you are not satisfied with the transition between the two activities, then apply an animation between them.

FragmentController vs FragmentScenario

Roblectric's FragmentController allowed us to drive the lifecycle of the Fragment to a desired state. It seems that FragmentScenario always takes the Fragment under test to its resumed state. One would assume that calling moveToState(CREATED) would take the Fragment back to through its onCreate(), but in reality, as per the docs, moveToState() simulates an external app interacting with the Fragment under test. This means that moveToState(CREATED) moves the Fragment under test through onPause() and onStop() as it would happen when a new Activity is launched.
To drive the fragment to a different lifecycle state, call moveToState(). This methods supports the following states as arguments: CREATED, STARTED, RESUMED, and DESTROYED. This action simulates a situation where the activity containing your fragment changes its state because it's interrupted by another app or a system action.
Is it possible to somehow direct FragmentScenario to drive the Fragment to a desired state instead of always going through onResume()? How do we test that something happened inside onCreate() and not inside onResume() using FragmentScenario? Am I missing something here?

Not sure why you need to test something during fragment's onCreate, but I had the case to supply my fragment with fake viewmodel(mocking happy case, etc.) - you can do it as follows:
#Test fun yetAnotherTest(){
val fakeViewModel = YourViewModel(fakeDependency1, fakeDependency2,...)
val scenario: FragmentScenario<YourFragment> =
launchFragmentInContainer {
YourFragment().apply {
viewModel = fakeViewModel
}
}
}
and some minor modifications
lateinit var viewmodel: YourViewModel
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
//if this is yetAnotherTest() run, line is skipped
if (!::viewModel.isInitialized) {
//normal setup
viewModel = provideYourViewModel()
}
alternatively, you can provide FragmentFactory for your fragment so you can test fragments with FragmentScenario more easily.

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.

Fragment lifecycle overlap on navigate

I have a single Activity application with multiple Fragments that are being switched by using Navigation components. When I switch between two fragments their onCreate() and onDestroy() methods seem to overlap. Thus making it difficult for me to write initialization and clean up code for fragments when they access the same global objects.
Navigating from Framgent_A to Fragment_B has the following order of methods:
Fragment_B.onCreate()
Fragment_A.onDestroy()
In Fragment_A.onDestroy() I reverse the operations I do in Fragment_A.onCreate(). And in Fragment_B I expect things to be in a neutral state when onCreate() is called. However that is not the case since Fragment_A.onDestroy() has not yet been called.
Is the overlap normal on Android or did I configure something wrong in my Navigation components? Is there another way I could achieve what I am trying to do? I know I could couple both Fragments and make it work, but I don't want either Fragment to know about each other. To me it seems weird that Framgnet_A is still alive when Fragment_B is created, when Fragment_B is supposed to replace Fragment_A.
Any help is greatly appreciated!
Edit:
After groing through the source code while debugging I have found out that in FragmentNavigator.navigate() FragmentTransaction.setReorderingAllowed() is called, which allows reordering of operations, even allowing onCreate() of a new fragment to be called before onDestroy() of the previous. The question still remains, how can I solve my problem of correctly cleaning up global state in one Fragment before initializing the same global state in the next Fragment.

The Android Fragment life-cycle is not really an appropriate callback host for your needs. The navigation controller will replace the two fragments with animation, so both are somehow visible the same time and eventually even onPause() of the exiting fragment is called after onResume() of the entering one.
Solution 1: Use OnDestinationChangedListener
The onDestinationChanged() callback is called before any of the life-cycle events. As a very simplified approach (look out for leaks) you could do the following:
findNavController().addOnDestinationChangedListener { _, destination, _ ->
if(shouldCleanupFor(destination)) cleanup()
}
Solution 2: Abstract the global changes away
Instead of having single navigation points change the global state, have a single point of truth for it. This could be another fragment independent of the navigation hierarchy. This then observes the navigation as before:
findNavController(R.id.nav_graph).addOnDestinationChangedListener { _, destination, _ ->
resetAll()
when(distination.id) {
R.id.fragment_a -> prepareForA()
R.id.fragment_b -> prepareForB()
else -> prepareDefault()
}
}
As an additional advantage you could implement the state changes idempotently as well.

Since you have an activity that controls the inflation of your Fragments you can manually control the lifecycles of the fragment that are being inflated. By calling into below methods you can control which fragment is ready to use global data. You will at this point have to, some how pass data back to Mainactivity to establish which fragment is active since your asking about how to inflate 2 fragment simultaneously which will share an object. Better approach would be to have the MainActivity implement FragmentA and FragmentB-detail with specific classes to do Stuff this way you have to treat your app like Tablet and determine 2 pane mode and which point you can use appropriate classes out of those fragments controlled by your Activity. The included link matches what your trying to accomplish
private void addCenterFragments(Fragment fragment) {
try {
removeActiveCenterFragments();
fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.add(R.id.content_fragment, fragment);
fragmentTransaction.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
activeCenterFragments.add(fragment);
fragmentTransaction.commit();
}catch (Exception e){
Crashlytics.logException(e);
}
}
private void removeActiveCenterFragments() {
if (activeCenterFragments.size() > 0) {
fragmentTransaction = fragmentManager.beginTransaction();
for (Fragment activeFragment : activeCenterFragments) {
fragmentTransaction.remove(activeFragment);
}
activeCenterFragments.clear();
fragmentTransaction.commit();
}
}

Perhaps you could move some the code related to initialization where you assume a neutral state to that fragments onStart() or onCreateView() method. According to the developer documentation this is where initialization should take place.
Another option available is using an Observer /Observable pattern, where you could notify your Activity once onDestroy() in Fragment A is completed. The Activity would then notify Fragment B that it is safe to assume a cleaned up state and begin initialization.

My case was a little bit different, and I would like to share it in case anyone faced the same issue.
I wanted to do an action in onPause() of the current fragment, but not execute that code when one navigates from a fragment to another. What I had to do was to call isRemoving() method to check if the current fragment is being removed or not. It is set to true when NavController.navigate(...) method is called.
override fun onPause() {
super.onPause()
if (!isRemoving()) {
// Write your code here
}
}
Per Google's Fragment.isRemoving() documentation:
Return true if this fragment is currently being removed from its activity. This is not whether its activity is finishing, but rather whether it is in the process of being removed from its activity.

Force LiveData Observer inside a Fragment to receive a change after coming visible again?

I am implementing Android Architecture Components. Imagine a case like the following where your Fragment is observing a LiveData to change its UI. User minimizes the app and the state is changed (in my case from the repository). So the Observer from the Fragment is not triggered with a change because the Fragment is not visible. But then, when the user comes back to the app it doesn't trigger the new state. If the state is changed again (while the Fragment is visible), the Observer receives the change. Do you know any way to force an update when fragment is visible again?
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
vm = ViewModelProviders.of(this, viewModelFactory).get(MyViewModel::class.java)
vm.getStatus()?.observe(this, Observer<MyRepository.Status> { status ->
if (status != null) {
when (status) {
NONE -> setNoneUI()
LOADING -> setLoadingUI()
CONTENT -> setContentUI()
ERROR -> setErrorUI()
}
}
})
}

Actually your activity should receive the latest state from LiveData when it is visible again, as explained in this video and described here:
Always up to date data: If a lifecycle becomes inactive, it receives the latest data upon becoming active again. For example, an activity that was in the
background receives the latest data right after it returns to the
foreground.
I just tested it in my own application, works as described. So there must be another error in your application: Do you set the LiveData correctly from your repository class? Maybe a different thread / postValue problem?