How to clear navigation Stack after navigating to another fragment in Android

First, add attributes app:popUpTo='your_nav_graph_id' and app:popUpToInclusive="true" to the action tag.

<fragment    android:id="@+id/signInFragment"    android:name="com.glee.incog2.android.fragment.SignInFragment"    android:label="fragment_sign_in"    tools:layout="@layout/fragment_sign_in" >    <action        android:id="@+id/action_signInFragment_to_usersFragment"        app:destination="@id/usersFragment"        app:launchSingleTop="true"        app:popUpTo="@+id/main_nav_graph"        app:popUpToInclusive="true" /></fragment>

Second, navigate to the destination, using above action as parameter.

findNavController(fragment).navigate(     SignInFragmentDirections.actionSignInFragmentToUserNameFragment())

See the docs for more information.

NOTE: If you navigate using method navigate(@IdRes int resId), you won't get the desired result. Hence, I used method navigate(@NonNull NavDirections directions).

I think your question specifically pertains on how to use the Pop Behavior / Pop To / app:popUpTo (in xml)

In documentation,
Pop up to a given destination before navigating. This pops all non-matching destinations from the back stack until this destination is found.

Example (Simple Job hunting app)
my start_screen_nav graph is like this:

startScreenFragment (start) -> loginFragment -> EmployerMainFragment                            -> loginFragment -> JobSeekerMainFragment

if I want to navigate to EmployerMainFragment and pop all including startScreenFragment then the code will be:

        <action            android:id="@+id/action_loginFragment_to_employerMainFragment"            app:destination="@id/employerMainFragment"            app:popUpTo="@+id/startScreenFragment"            app:popUpToInclusive="true" />

if I want to navigate to EmployerMainFragment and pop all excluding startScreenFragment then the code will be:

        <action            android:id="@+id/action_loginFragment_to_employerMainFragment"            app:destination="@id/employerMainFragment"            app:popUpTo="@+id/startScreenFragment"/>

if I want to navigate to EmployerMainFragment and pop loginFragment but not startScreenFragment then the code will be:

        <action            android:id="@+id/action_loginFragment_to_employerMainFragment"            app:destination="@id/employerMainFragment"            app:popUpTo="@+id/loginFragment"            app:popUpToInclusive="true"/>

OR

        <action            android:id="@+id/action_loginFragment_to_employerMainFragment"            app:destination="@id/employerMainFragment"            app:popUpTo="@+id/startScreenFragment"/>

In my case i needed to remove everything in the back Stack before i open a new fragment so i used this code

navController.popBackStack(R.id.fragment_apps, true);navController.navigate(R.id.fragment_company);

the first line removes the back Stack till it reaches the fragment specified in my case it's the home fragment so it's removes all the back stack completely , and when the user clicks back in the fragment_company he closes the app.