1-) Android RMOS - fragment de geri tuşuna basınca datalar boş olmaması için BackPressed

public class Genel {

 public static Stack<Integer> fragmentYigin = new Stack<Integer>();

 public static void binoncekiFragmenteGit() {
    try{
        fragmentYigin.pop();
        if(fragmentYigin.size()<1){
            MainActivity.mainActivity.finish();
            return;
        }
        int bironceki = fragmentYigin.pop();
        MainActivity.mainActivity.navController.navigate(bironceki);
    }catch (Exception ex){
        MainActivity.mainActivity.finish();
    }
}

}

Kullanımı :

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    Genel.fragmentYigin.push(MainActivity.mainActivity.navController.getCurrentDestination().getId());


    OnBackPressedCallback callback = new OnBackPressedCallback(true /* enabled by default */) {
        @Override
        public void handleOnBackPressed() {
            Genel.binoncekiFragmenteGit();
        }
    };
    requireActivity().getOnBackPressedDispatcher().addCallback(getActivity(), callback);

  }

Açıklama : Her fragment oluştugunda yıgına ekler. geri tuşuna basınca bir onceki kendisi olur. ondan da once -> bironceki fragment olur -> yani iki kere pop yaparız. eğer tek pop dan sonra size sıfır ise uygulamadan çıkar

2-) İKİNCİ YOL AMA SAĞLIKLI BULAMADIM

OnBackPressedCallback callback = new OnBackPressedCallback(true /* enabled by default */) {
    @Override
    public void handleOnBackPressed() {
        MainActivity.mainActivity.navController.navigate(MainActivity.mainActivity.navController.getPreviousBackStackEntry().getDestination().getId());

    }
};
requireActivity().getOnBackPressedDispatcher().addCallback(getActivity(), callback);

<?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/mobile_navigation"
    app:startDestination="@+id/navigation_dashboard">



    <fragment
        android:id="@+id/navigation_dashboard"
        android:name="com.ramzey.doviz.ui.dashboard.DashboardFragment"
        android:label="Piyasalar"
        tools:layout="@layout/fragment_dashboard">
        <action
            android:id="@+id/a1"
            app:destination="@id/navigation_home"
            app:launchSingleTop="true"
            app:popUpTo="@+id/main_nav_graph"
            app:popUpToInclusive="true" />
        <action
            android:id="@+id/a2"
            app:destination="@id/navigation_notifications"
            app:launchSingleTop="true"
            app:popUpTo="@+id/main_nav_graph"
            app:popUpToInclusive="true" />
    </fragment>

    <fragment
        android:id="@+id/navigation_home"
        android:name="com.ramzey.doviz.ui.home.HomeFragment"
        android:label="RamZey Döviz"
        tools:layout="@layout/fragment_home" >
        <action
            android:id="@+id/a1"
            app:destination="@id/navigation_notifications"
            app:launchSingleTop="true"
            app:popUpTo="@+id/main_nav_graph"
            app:popUpToInclusive="true" />
        <action
            android:id="@+id/a2"
            app:destination="@id/navigation_dashboard"
            app:launchSingleTop="true"
            app:popUpTo="@+id/main_nav_graph"
            app:popUpToInclusive="true" />
    </fragment>

    <fragment
        android:id="@+id/navigation_notifications"
        android:name="com.ramzey.doviz.ui.notifications.NotificationsFragment"
        android:label="Alarm Kur"
        tools:layout="@layout/fragment_notifications" >
        <action
            android:id="@+id/a1"
            app:destination="@id/navigation_home"
            app:launchSingleTop="true"
            app:popUpTo="@+id/main_nav_graph"
            app:popUpToInclusive="true" />
        <action
            android:id="@+id/a2"
            app:destination="@id/navigation_dashboard"
            app:launchSingleTop="true"
            app:popUpTo="@+id/main_nav_graph"
            app:popUpToInclusive="true" />
    </fragment>
</navigation>