Develop Reference

how can i press back just to close the app?

I implemented this function in the mainActivity of my app to ask to press back twice to exit the application, everything works but it asks me to double back press even when I want to close a fragment, how can I limit it only to the main screen?
override fun onBackPressed() {
if (backPressedTime + 2000 > System.currentTimeMillis()) {
super.onBackPressed()
}else{
Toast.makeText(this, "Premere ancora per tornare indietro", Toast.LENGTH_SHORT).show()
}
backPressedTime = System.currentTimeMillis()
}

You have to use the OnBackPressedCallBack Interface which will provide a function named handleOnBackPressed() that is called when you click navigate back button.
If your app is a Single Activity App, then you have to call finish() function that will finish your activity and this will lead to exit from the app.
You can reach finish() by using requireActivity() in the specific fragment.
If you are using Navigation Component, try to put this function in your First Destination Fragment:
private fun setOnBackButtonClickListener() {
val onBackPressedCallback = object : OnBackPressedCallback(true) {
override fun handleOnBackPressed() {
if (backPressedTime + 2000 > System.currentTimeMillis()) {
requireActivity.finish()
} else {
Toast.makeText(this, "Premere ancora per tornare indietro",
Toast.LENGTH_SHORT).show()
}
}
}
requireActivity().onBackPressedDispatcher.addCallback(this, onBackPressedCallback)
}
and if you are not using Navigation Component, try to use the same function in the fragment you want to end the app from.

Application Crash/Navigation issue with observer when clicking "back" from fragment

I have a fragment A which sends a search query to the network, and if the result is positive uses Android navigation component to navigate to fragment B, and its done using observers.
After navigation to fragment B, i click on "<-" arrow on the top of the screen, but instead of navigating back to fragment A it reloads fragment B again. And if using the native "back" button on the device, the app crashes with "illegalArgumentException navigation destination unknown" error.
I check the internet for clues on this issue, but all i learned is that this happens because i am using .observe in onViewCreated() and when i go back, it gets called again, and because livedata has something in it already, it just navigates me back to B.
I have tried observing in onActivityCreated(), and using getViewLifeCycleOwner, but no success... the only thing that helped is checking if livedata has observers and returning if true, before using .observe, but it seems incorrect.
This is the viewModel:
private val getAssetResult = MutableLiveData<GeneralResponse<Asset>>()
private val updateAssetResult = MutableLiveData<GeneralResponse<Int>>()
private val deleteAssetResult = MutableLiveData<GeneralResponse<Int>>()
init {
state.value = ViewState(false)
Log.d(TAG, "State in init: $state")
}
fun getAssetResult(): LiveData<GeneralResponse<Asset>>{
return getAssetResult
}
fun findAsset(req: GetAssetRequest) {
scope.launch {
setProgressIndicator(true)
val result = repository.getAsset(req)
getAssetResult.postValue(result)
setProgressIndicator(false)
}
}
This is the fragment:
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewModel = ViewModelProvider(this).get(EditAssetViewModel::class.java)
setupViewModel()
initFields()
}
private fun setupViewModel() {
if (viewModel.getAssetResult().hasObservers()) // <- This is the part that prevents the app from crashing.
return
viewModel.getAssetResult().observe(this, Observer {
if (it == null) return#Observer
handleSearchResult(it)
})
if (viewModel.getState().hasObservers())
return
viewModel.getState().observe(this, Observer { handleState(it) })
}
private fun handleSearchResult(response: GeneralResponse<Asset>) {
if (response.singleValue == null) {
Toast.makeText(context!!, response.errorMessage, Toast.LENGTH_SHORT).show()
return
}
targetFragment?.let { it ->
val bundle = bundleOf("asset" to response.singleValue)
when(it) {
"UpdateLocation" ->
Navigation.findNavController(view!!).navigate(R.id.updateLocation, bundle)
"EditAsset" -> {
Navigation.findNavController(view!!).navigate(R.id.editAsset, bundle)
}
}
}
}
if i remove this part from the setupViewModel function:
if (viewModel.getAssetResult().hasObservers())
return
the app will either crash when clicked "back" using the device button or go back to fragment A, just to be navigated back to fragment B because of the .observe function.

Override the method onBackPressed() to handle the "<-" arrow

Seems like the LiveData that you use to signal to fragment A that it should navigate to fragment B is actually an event. An event happens only once and once it is consumed (navigation event is done), it is gone. Therefore, after navigating you need to send a message to the viewmodel that the navigation took place and that the corresponding data holder should be (e.g.) null again. In Fragment A you check that the new value is unequal to null, and only if this is the case, you issue the navigation event. This would prevent fragment A to immediatelly switch to B again in the back scenario.
If you want to learn more about ways to use live data for events, please refer to this article.

OnBackPressedCallback is being called, but app is not going back

I recently updated my dependencies to include the OnBackPressedCallback change from an interface into an abstract class.
I have set things up according to the new documentation here but I feel like things are not working as they should.
My fragment's OnCreate looks a lot like the documentation:
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
requireActivity().onBackPressedDispatcher.addCallback(this) {
backPressed()
}
}
When I press the back button, the code in backPressed() is run, but nothing more happens.
I have tried calling handleBackPressed() and requireActivity().onBackPressedDispatcher.onBackPressed() and requireActivity().onBackPressed() from inside the callback, but those all cause a StackOverflowError because it seems to run that callback recursively.
There has got to be something really obvious I am missing...

There has got to be something really obvious I am missing...
You forget to disable your custom callback in you fragment before asking Activity to handle back pressed.
My solutiuon suitable for me:
#Override
public void onCreate(#Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final OnBackPressedCallback callback = new OnBackPressedCallback(true) {
#Override
public void handleOnBackPressed() {
if (/*situation to handle back pressing*/){
//here handle your backPress in your fragment
} else {
setEnabled(false); //this is important line
requireActivity().onBackPressed();
}
}
};
requireActivity().getOnBackPressedDispatcher().addCallback(this, callback);
}

When you register an OnBackPressedCallback, you are taking on the responsibility for handling the back button. That means that no other on back pressed behavior is going to occur when you get a callback.
If you're using Navigation, you can use your NavController to pop the back stack:
requireActivity().onBackPressedDispatcher.addCallback(this) {
backPressed()
// Now actually go back
findNavController().popBackStack()
}

This works for me in androidx.appcompat:appcompat:1.1.0
requireActivity().onBackPressedDispatcher.addCallback(
this,
object : OnBackPressedCallback(true) {
override fun handleOnBackPressed() {
Log.d(TAG, "Fragment back pressed invoked")
// Do custom work here
// if you want onBackPressed() to be called as normal afterwards
if (isEnabled) {
isEnabled = false
requireActivity().onBackPressed()
}
}
}
)

You can also remove callback instead of setting enabled if it's no longer needed. I use it with nested graph like this because when you touch back in a nested nav graph with it's NavHostFragment, it removes it from main fragment back stack instead of opening last fragment in nested nav graph.
// Get NavHostFragment
val navHostFragment =
childFragmentManager.findFragmentById(R.id.nested_nav_host_fragment)
// ChildFragmentManager of the current NavHostFragment
val navHostChildFragmentManager = navHostFragment?.childFragmentManager
val callback = object : OnBackPressedCallback(true) {
override fun handleOnBackPressed() {
val backStackEntryCount = navHostChildFragmentManager!!.backStackEntryCount
if (backStackEntryCount == 1) {
// We are at the root of nested navigation, remove this callback
remove()
requireActivity().onBackPressed()
} else {
navController?.navigateUp()
}
}
}
requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, callback)

shared element transition is not working when using toolbar home

I'm currently running into a strange problem, similar to here Shared element transition when using ActionBar Back button but with a fragment to fragment shared element transition. It works fine when using the back button. As soon as the toolbar home is triggered it's just blinking. So calling finishAfterTransition() is no option here.
I call the same method both times. The base for back navigation
override fun onBackPressed() {
if (coordinator != null) {
coordinator!!.back()
} else {
finish()
}
}
and the intercepted toolbar home click.
override fun onOptionsItemSelected(item: MenuItem?): Boolean {
return when (item?.itemId) {
android.R.id.home -> {
onBackPressed()
true
}
else -> return super.onOptionsItemSelected(item)
}
}
Update: Now I noted like one out of ten tries it works correctly.
I hope anyone of you guys has a clue why this happens.
Regards coffeelord

unsaved warning on back pressed in fragment

I want to show dialog when user press back or quit from fragment if there are some data unsaved. I am trying to override onbackpressed but unfortunately I got error lateinit property barcodeList has not been initialized. how to solve it?
here is my script on activity:
override fun onBackPressed() {
val theFragment = supportFragmentManager.fragments
for(i in 0 until theFragment.size)
{
if(theFragment[i].tag == "stocker_fragment")
{
StockerFragment().onBackPressed()
}
}
}
and this is in fragment:
fun onBackPressed() {
var check = false
// this barcodeList variable error.
for(i in 0 until barcodeList.size)
{
if(barcodeList[i].barcode.trim()=="")
{
check = true
break
}
}
if (check)
{
AlertHelper(context).onBackPressedAlert()
}
}
FYI: I have initialized barcodeList on onCreateView and everything is fine. only error in onBackPressed.
And my last question is, how do i know if user quit from fragment without pressing back button?

I think the problem is in your onBackPressed() implementation in the Activity. With the line StockerFragment().onBackPressed() you are creating a new instance of the StockerFragment and calling onBackPressed() on it, rather than calling it on the instance that is actively being used.
You should be able to adjust your Activity onBackPressed() like so:
override fun onBackPressed() {
val theFragment = supportFragmentManager.fragments
for(i in 0 until theFragment.size)
{
if(theFragment[i].tag == "stocker_fragment")
{
(theFragment[i] as StockerFragment).onBackPressed()
}
}
}
You can also make this a bit more kotliny like so:
override fun onBackPressed() {
supportFragmentManager.fragments.forEach { fragment ->
if (fragment is StockerFragment) {
fragment.onBackPressed()
}
}
}
You'll probably also want to figure out a way to decide whether the fragment's onBackPressed has determined that the Activity should stick around or not. Then, if the fragment is happy, you should call super.onBackPressed() in the Activity so that the expected back behavior (leave the Activity) happens.