Handling back button in Android Navigation Component

I'd like to know how properly handle system back button action using Navigation Controller. In my app I have two fragments (for ex. fragment1 and fragment2) and I have an action in fragment1 with destination to fragment2. Everything works well except one thing - when user presses system back button in fragment2 I want to show a dialog (using DialogFragment for example) to confirm exit. What is the best way to implement this behavior? If I use app:defaultNavHost="true" in my host fragment then it automatically goes back ignoring my rules. And, additionally, what is this component for?

Should I use "pop to" may be?

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Newest Update - April 25th, 2019

New release androidx.activity ver. 1.0.0-alpha07 brings some changes

More explanations in android official guide: Provide custom back navigation

Example:

public class MyFragment extends Fragment { @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); // This callback will only be called when MyFragment is at least Started. OnBackPressedCallback callback = new OnBackPressedCallback(true /* enabled by default */) { @Override public void handleOnBackPressed() { // Handle the back button event } }; requireActivity().getOnBackPressedDispatcher().addCallback(this, callback); // The callback can be enabled or disabled here or in handleOnBackPressed() } ...
}

Old Updates

UPD: April 3rd, 2019

Now its simplified. More info here

Example:

requireActivity().getOnBackPressedDispatcher().addCallback(getViewLifecycleOwner(), this);
@Override
public boolean handleOnBackPressed() { //Do your job here //use next line if you just need navigate up //NavHostFragment.findNavController(this).navigateUp(); //Log.e(getClass().getSimpleName(), "handleOnBackPressed"); return true; }

Deprecated (since Version 1.0.0-alpha06 April 3rd, 2019) :

Since this, it can be implemented just using JetPack implementation OnBackPressedCallback in your fragment and add it to activity:getActivity().addOnBackPressedCallback(getViewLifecycleOwner(),this);

Your fragment should looks like this:

public MyFragment extends Fragment implements OnBackPressedCallback { @Override public void onActivityCreated(@Nullable Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); getActivity().addOnBackPressedCallback(getViewLifecycleOwner(),this);
} @Override public boolean handleOnBackPressed() { //Do your job here //use next line if you just need navigate up //NavHostFragment.findNavController(this).navigateUp(); //Log.e(getClass().getSimpleName(), "handleOnBackPressed"); return true; } @Override public void onDestroyView() { super.onDestroyView(); getActivity().removeOnBackPressedCallback(this); }
}

UPD:Your activity should extends AppCompatActivityor FragmentActivity and in Gradle file:

 implementation 'androidx.appcompat:appcompat:{lastVersion}'

For anyone looking for a Kotlin implementation see below.

Note that the OnBackPressedCallback only seems to work for providing custom back behavior to the built-in software/hardware back button and not the back arrow button/home as up button within the actionbar/toolbar. To also override the behavior for the actionbar/toolbar back button I'm providing the solution that's working for me. If this is a bug or you are aware of a better solution for that case please comment.

build.gradle

...
implementation "androidx.appcompat:appcompat:1.1.0-rc01"
implementation "androidx.navigation:navigation-fragment-ktx:2.0.0"
implementation "androidx.navigation:navigation-ui-ktx:2.0.0"
...

MainActivity.kt

...
import androidx.appcompat.app.AppCompatActivity
...
class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { setContentView(R.layout.activity_main) ... val navController = findNavController(R.id.nav_host_fragment) val appBarConfiguration = AppBarConfiguration(navController.graph) // This line is only necessary if using the default action bar. setupActionBarWithNavController(navController, appBarConfiguration) // This remaining block is only necessary if using a Toolbar from your layout. val toolbar = findViewById<Toolbar>(R.id.toolbar) toolbar.setupWithNavController(navController, appBarConfiguration) // This will handle back actions initiated by the the back arrow // at the start of the toolbar. toolbar.setNavigationOnClickListener { // Handle the back button event and return to override // the default behavior the same way as the OnBackPressedCallback. // TODO(reason: handle custom back behavior here if desired.) // If no custom behavior was handled perform the default action. navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp() } } /** * If using the default action bar this must be overridden. * This will handle back actions initiated by the the back arrow * at the start of the action bar. */ override fun onSupportNavigateUp(): Boolean { // Handle the back button event and return true to override // the default behavior the same way as the OnBackPressedCallback. // TODO(reason: handle custom back behavior here if desired.) // If no custom behavior was handled perform the default action. val navController = findNavController(R.id.nav_host_fragment) return navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp() }
}

MyFragment.kt

...
import androidx.activity.OnBackPressedCallback
import androidx.fragment.app.Fragment
...
class MyFragment : Fragment() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val onBackPressedCallback = object : OnBackPressedCallback(true) { override fun handleOnBackPressed() { // Handle the back button event } } requireActivity().getOnBackPressedDispatcher().addCallback(this, onBackPressedCallback) }
}

The official documentation can be viewed at

So, I created an interface

public interface OnBackPressedListener { void onBackPressed();
}

And implemented it by all fragments that need to handle back button. In main activity I overrided onBackPressed() method:

@Override
public void onBackPressed() { final Fragment currentFragment = mNavHostFragment.getChildFragmentManager().getFragments().get(0); final NavController controller = Navigation.findNavController(this, R.id.nav_host_fragment); if (currentFragment instanceof OnBackPressedListener) ((OnBackPressedListener) currentFragment).onBackPressed(); else if (!controller.popBackStack()) finish();
}

So, If the top fragment of my Navigation host implements OnBackPressedListener interface, I call its onBackPressed() method, elsewhere I simply pop back stack and close application if the back stack is empty.

The recommended approach is to add an OnBackPressedCallback to the activity's OnBackPressedDispatcher.

requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner) { // handle back event
}

Here is solution that should do what you want, but i think it is a bad solution, because it is going against Android Navigation component idea(letting the android handle the navigation).

Override "onBackPressed" inside your activity

override fun onBackPressed() { when(NavHostFragment.findNavController(nav_host_fragment).currentDestination.id) { R.id.fragment2-> { val dialog=AlertDialog.Builder(this).setMessage("Hello").setPositiveButton("Ok", DialogInterface.OnClickListener { dialogInterface, i -> finish() }).show() } else -> { super.onBackPressed() } }
}

Just add these lines

 override fun onBackPressed() { if(navController.popBackStack().not()) { //Last fragment: Do your operation here finish() }

navController.popBackStack() will just pop your fragment if this is not your last fragment

I written in main activity like this,

override fun onSupportNavigateUp(): Boolean { return findNavController(R.id.my_nav_host_fragment).navigateUp(appBarConfiguration) }

UpdateApr 22, '21

I'm updating my answer to showcase a sample of the recommended approach which is also the accepted answer above.

class MyFragment : Fragment() { ... private val backPressedDispatcher = object : OnBackPressedCallback(true) { override fun handleOnBackPressed() { // Redirect to our own function [email protected]() } } override fun onViewCreated(view: View, savedInstanceState: Bundle?) { ... setHasOptionsMenu(true) //Set this to true in order to trigger callbacks to Fragment#onOptionsItemSelected (requireActivity() as AppCompatActivity).apply { // Redirect system "Back" press to our dispatcher onBackPressedDispatcher.addCallback(viewLifecycleOwner, backPressedDispatcher) // Set toolbar if it is in Fragment's layout. If you have a global toolbar that lives in Activity layout, then you don't need this line. setSupportActionBar(view.findViewById(R.id.toolbar)) // Setup action bar to work with NavController setupActionBarWithNavController(findNavController()) } } override fun onOptionsItemSelected(item: MenuItem): Boolean { return if (item.itemId == android.R.id.home) { // Redirect "Up/Home" button clicks to our own function [email protected]() true } else { super.onOptionsItemSelected(item) } } private fun onBackPressed() { // Work your magic! Show dialog etc. } override fun onDestroyView() { // It is optional to remove since our dispatcher is lifecycle-aware. But it wouldn't hurt to just remove it to be on the safe side. backPressedDispatcher.remove() super.onDestroyView() } }

Original answerJan 3 '19

A little late to the party, but with the latest release of Navigation Component 1.0.0-alpha09, now we have an AppBarConfiguration.OnNavigateUpListener.

Refer to these links for more information:

In 2.1.0-alpha06

If you want to handle backpress only in current fragment

requireActivity().onBackPressedDispatcher.addCallback(this@LoginFragment) { // handle back event
}

For whole Activity

requireActivity().onBackPressedDispatcher.addCallback() { // handle back event
}

FragmentExtenstions.kt

fun Fragment.onBackPressedCustomAction(action: () -> Unit) { requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, object : OnBackPressedCallback(true) { override fun handleOnBackPressed() { action() } })
}

YourPrettyFragment.kt

onBackPressedCustomAction { // Your custom action here
}

This is 2 lines of code can listen for Navigation back press, from fragments, [TESTED and WORKING]

requireActivity().getOnBackPressedDispatcher().addCallback(getViewLifecycleOwner(), new OnBackPressedCallback(true) { @Override public void handleOnBackPressed() { //setEnabled(false); // call this to disable listener //remove(); // call to remove listener //Toast.makeText(getContext(), "Listing for back press from this fragment", Toast.LENGTH_SHORT).show(); }
});

The recommended method worked for me but after updating my library implementation 'androidx.appcompat:appcompat:1.1.0'

Implement as below

 val onBackPressedCallback = object : OnBackPressedCallback(true) { override fun handleOnBackPressed() { // Handle the back button event } } requireActivity().onBackPressedDispatcher.addCallback(this, onBackPressedCallback)

using Kotlin

you can provide your custom back navigation by using OnBackPressedDispatcher

class MyFragment : Fragment() {
override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) // This callback will only be called when MyFragment is at least Started. val callback = requireActivity().onBackPressedDispatcher.addCallback(this) { // Handle the back button event
// and if you want to need navigate up
//NavHostFragment.findNavController(this).navigateUp() } // The callback can be enabled or disabled here or in the lambda
}
}

More explanations in android official guide:

Use this if you're using fragment or add it in your button click listener. This works for me.

requireActivity().onBackPressed()

Called when the activity has detected the user's press of the back key. The getOnBackPressedDispatcher() OnBackPressedDispatcher} will be given chance to handle the back button before the default behavior of android.app.Activity#onBackPressed()} is invoked.

Using navigation components This was good for me:

Navigation.findNavController(requireView()).popBackStack()

android documentation

Kotlin Answer

Use popBackStack() example:

override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) mButton.setOnClickListener { Navigation.findNavController(view).popBackStack() // You need this line. }
}

If you are using BaseFragment for your app then you can add onBackPressedDispatcher to your base fragment.

//Make a BaseFragment for all your fragments
abstract class BaseFragment : Fragment() {
private lateinit var callback: OnBackPressedCallback
/** * SetBackButtonDispatcher in OnCreate */
override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setBackButtonDispatcher()
}
/** * Adding BackButtonDispatcher callback to activity */
private fun setBackButtonDispatcher() { callback = object : OnBackPressedCallback(true) { override fun handleOnBackPressed() { onBackPressed() } } requireActivity().onBackPressedDispatcher.addCallback(this, callback)
}
/** * Override this method into your fragment to handleBackButton */ open fun onBackPressed() { }
}

Override onBackPressed() in your fragment by extending basefragment

//How to use this into your fragment
class MyFragment() : BaseFragment(){
private lateinit var mView: View
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? { mView = inflater.inflate(R.layout.fragment_my, container, false) return mView.rootView
}
override fun onBackPressed() { //Write your code here on back pressed.
}

}

If you use Navigation Component follow the codes below in your onCreateView() method (in this example I want just to close my app by this fragment)

 OnBackPressedCallback backPressedCallback = new OnBackPressedCallback(true) { @Override public void handleOnBackPressed() { new AlertDialog.Builder(Objects.requireNonNull(getActivity())) .setIcon(R.drawable.icon_01) .setTitle(getResources().getString(R.string.close_app_title)) .setMessage(getResources().getString(R.string.close_app_message)) .setPositiveButton(R.string.yes, new DialogInterface.OnClickListener() { @Override public void onClick(DialogInterface dialog, int which) { getActivity().finish(); } }) .setNegativeButton(R.string.no, null) .show(); } }; requireActivity().getOnBackPressedDispatcher().addCallback(this, backPressedCallback);

And if you want the same behavior also for the toolbar back button just add this in your activity:

@Override
public boolean onOptionsItemSelected(MenuItem item) { if (item.getItemId() == android.R.id.home) { getOnBackPressedDispatcher().onBackPressed(); return true; } return super.onOptionsItemSelected(item);
}

just create an extension function to the fragment

fun Fragment.onBackPressedAction(action: () -> Boolean) { requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, object : OnBackPressedCallback(true) { override fun handleOnBackPressed() { this.isEnabled = action() if (!this.isEnabled) { requireActivity().onBackPressed() } } })
}

and after in the fragment put the code into onCreateView (the action must return false to call the activity onBackPressed)

onBackPressedAction { //do something }

This is an answer if you are using the NavController:

Navigation.findNavController(view).navigateUp();

Don't try to hack other methods, like replacing the fragment using FragmentManager and replace() in a transaction. The NavController takes care of this.

Problem

The official method to provide custom back navigation only works for the native button. This method may (and typically must) be declared in other or even multiple fragments with different viewModels to decide what should happen to the navigation (simple example: if x then show a save dialog else navigate back normally).

Solution

onBackPressed simulates a native back press in any fragment.
setNavigationOnClickListener can be set to listen to events from the non-native back button.

Implementation

After setupWithNavController in MainActivity.kt, add:

myToolbar.setNavigationOnClickListener{ onBackPressed()
}

See also Aptin Grabian's answer and poby's comment.

Try this. I think this will help you.

override fun onBackPressed() { when (mNavController.getCurrentDestination()!!.getId()) { R.id.loginFragment -> { onWarningAlertDialog(this, "Alert", "Do you want to close this application ?") } R.id.registerFragment -> { super.onBackPressed() } }
}
private fun onWarningAlertDialog(mainActivity: MainActivity, s: String, s1: String) { val dialogBuilder = AlertDialog.Builder(this) dialogBuilder.setMessage(/*""*/s1) .setCancelable(false) .setPositiveButton("Proceed", DialogInterface.OnClickListener { dialog, id -> finish() }) .setNegativeButton("Cancel", DialogInterface.OnClickListener { dialog, id -> dialog.cancel() }) // create dialog box val alert = dialogBuilder.create() // set title for alert dialog box alert.setTitle("AlertDialogExample") // show alert dialog alert.show() }

if you are actually trying to handle back button specifically then you could use @Jurij Pitulja answer.

But, if you want pop SecondFragment (start fragment FirstFragment) and not return to FirstFragment, then you could use :

Navigation.findNavController(view).popBackStack()

from the SecondFragment. This way you would pop the SecondFragmetn of the back stack, and not return to SecondFragment when you press back button from FirstFragment.

Here is my solution

Use androidx.appcompat.app.AppCompatActivity for the activity that contains the NavHostFragment fragment.

Define the following interface and implement it in all navigation destination fragments

interface InterceptionInterface { fun onNavigationUp(): Boolean fun onBackPressed(): Boolean
}

In your activity override onSupportNavigateUp and onBackPressed:

override fun onSupportNavigateUp(): Boolean { return getCurrentNavDest().onNavigationUp() || navigation_host_fragment.findNavController().navigateUp()
}
override fun onBackPressed() { if (!getCurrentNavDest().onBackPressed()){ super.onBackPressed() }
}
private fun getCurrentNavDest(): InterceptionInterface { val currentFragment = navigation_host_fragment.childFragmentManager.primaryNavigationFragment as InterceptionInterface return currentFragment
}

This solution has the advantage, that the navigation destination fragments don't need to worry about the unregistering of their listeners as soon as they are detached.

I tried Jurij Pitulja solution but I just wasn't able to find getOnBackPressedDispatcher or addOnBackPressedCallback also using Kiryl Tkach's solution wasn't able to find the current fragment, so here's mine:

interface OnBackPressedListener { fun onBackPressed(): Boolean
}
override fun onBackPressed() { val navHostFragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment) val currentFragment = navHostFragment?.childFragmentManager!!.fragments[0] if (currentFragment !is OnBackPressedListener || !(currentFragment as OnBackPressedListener).onBackPressed()) super.onBackPressed()

this way you can decide in fragment whether the activity should take control of back pressed or not.

Alternatively, you have BaseActivity for all your activities, you can implement like this

override fun onBackPressed() { val navHostFragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment) if (navHostFragment != null){ val currentFragment = navHostFragment.childFragmentManager.fragments[0] if (currentFragment !is AuthContract.OnBackPressedListener || !(currentFragment as AuthContract.OnBackPressedListener).onBackPressed()) super.onBackPressed() } else { super.onBackPressed() }
}

Depending on your logic, if you want to close only the current fragment you have to pass viewLifecycleOwner, code is shown below:

 requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, object : OnBackPressedCallback(true) { override fun handleOnBackPressed() { requireActivity().finish() } })

However, if you want to close application on backPressed no matter from what fragment(probably you wouldn't want that!), don't pass the viewLifecycleOwner. Also if you want to disable the back button, do not do anything inside the handleOnBackPressed(), see below:

 requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, object : OnBackPressedCallback(true) { override fun handleOnBackPressed() { // do nothing it will disable the back button } })

My Opinion requireActivity().onBackPressed()

requireActivity().onBackPressed()

I have searched through many threads and none of them work. Finally I found one:

MainActivity.java

@Override
protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); Toolbar mToolbar = findViewById(R.id.topAppBar); setSupportActionBar(mToolbar);
}
@Override
public boolean onSupportNavigateUp() { navController.navigateUp(); return super.onSupportNavigateUp();
}

MyFragment.java

@Override
public void onViewCreated(@NonNull final View view, @Nullable Bundle savedInstanceState) { Toolbar mToolbar = (MainActivity) getActivity().findViewById(R.id.topAppBar); mToolbar.setNavigationOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { // Do something when uses presses back button (showing modals, messages,...) // Note that this will override behaviour of back button } });
}
@Override
public void onStop() { // Reset back button to default behaviour when we leave this fragment Toolbar mToolbar = (MainActivity) getActivity().findViewById(R.id.topAppBar); mToolbar.setNavigationOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { mainActivity.onBackPressed(); } }); super.onStop();
}

I need to support both real back button and toolbar back button with ability to override "Back" click in both cases (to show dialog or something else). I made an additional method in activity and corresponding boolean checks ('onBackPressed' in my case) in fragments:

// Process hardware Back button
override fun onBackPressed() { if (canCloseActivity()) { super.onBackPressed() }
}
// Process toobar Back and Menu button
override fun onSupportNavigateUp(): Boolean { if (canCloseActivity()) { return navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp() } return false
}
// Do real check if has unfinished tasks, return false to override activity closing
private fun canCloseActivity(): Boolean { val currentFragment = navHostFragment.childFragmentManager.primaryNavigationFragment return when { currentFragment is MyFragment && currentFragment.onBackPressed() -> false drawerLayout.isOpen -> { drawerLayout.close() false } fullScreenPreviewLayout.visibility == View.VISIBLE -> { closeFullscreenPreview() false } else -> true }
}

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