Prevent android from recreating new instance of a specific Fragment

Question

I have this structure:

MainActivity -> ProfileFragment -> ListFragment

where "->" is a FragmentTransaction.add(....)

My question is, do i need android for reload the old instances of the child fragments(for example when rotating the device) if i do it by "myself" with the implementation?

Because when android recreate the first Activity and reach the onStart() method, the activity itself recreate the ProfileFragment and so on...

I had also tried with some test Log and the results are below

First Creation:

2023-11-02 18:54:59.141 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ProfileFragment instance created from MainActivity
2023-11-02 18:54:59.161 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ProfileFragment: ProfileFragment{4eef489} (7ad3c3ad-8de4-4205-bc33-5e803c9c0f36 id=0x7f090101) onCreateView()
2023-11-02 18:54:59.170 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ListFragment instance created from ProfileFragment
2023-11-02 18:54:59.171 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ListFragment: ListFragment{74e1ea1} (03980350-9ab1-4857-a7f3-d71560f949cf id=0x7f0901bb) onCreateView()

After Rotation:

2023-11-02 18:51:46.127 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ProfileFragment instance created from android
2023-11-02 18:51:46.128 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ListFragment instance created from android
2023-11-02 18:51:46.164 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ProfileFragment instance created from MainActivity
2023-11-02 18:51:46.167 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ProfileFragment: ProfileFragment{9373e8c} (a5b6a134-359d-46bc-a487-270f4e970b41 id=0x7f090101) onCreateView()
2023-11-02 18:51:46.410 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ListFragment: ListFragment{fb9c6ab} (a78556b1-fe09-4b21-b40b-c3fc21fc9728 id=0x7f0901bb) onCreateView()
2023-11-02 18:51:46.416 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ProfileFragment: ProfileFragment{5d59c38} (063fb353-0129-4a4a-9db3-c72578aea443 id=0x7f090101) onCreateView()
2023-11-02 18:51:46.427 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ListFragment instance created from ProfileFragment
2023-11-02 18:51:46.428 16908-16908 Rotation Test           it.uniba.dib.sms222334               D  ListFragment: ListFragment{aad4157} (23af9cd1-15e2-4ddc-aa59-c138b79a5d8e id=0x7f0901bb) onCreateView()

For completeness i put also the constructor for this case

ListFragment:

public ListFragment() {
        Log.d("Rotation Test","ListFragment instance created from android");
        currentUser=SessionManager.getInstance().getCurrentUser();

        currentUserRole=currentUser.getRole();
    }

    public ListFragment(String overrideParams){

    }


    public static ListFragment newInstanceProfile(ProfileFragment.Tab tabPosition, ProfileFragment.Type profileType, User profile) {
        ListFragment myFragment = new ListFragment("justForOverrideTheConstructor");
        Bundle args = new Bundle();
        args.putInt("tab_position", tabPosition.tabPosition.ordinal());
        args.putInt("profile_type", profileType.ordinal());
        args.putParcelable("profileData", profile);
        myFragment.setArguments(args);

        Log.d("Rotation Test","ListFragment instance created from ProfileFragment");

        return myFragment;
    }

ProfileFragment:

public ProfileFragment(){
        Log.d("Rotation Test","ProfileFragment instance created from android");
    }

    private ProfileFragment(String overrideParams){

    }

    public static ProfileFragment newInstance(User profile) {
        ProfileFragment myFragment = new ProfileFragment("justForOverrideTheConstructor");

        Bundle args = new Bundle();
        args.putParcelable("profileData", profile);
        myFragment.setArguments(args);

        Log.d("Rotation Test","ProfileFragment instance created from MainActivity");

        return myFragment;
    }

So when i rotate the device as you can see i have two instances of both fragments, and it's not the result i want.

I saw some solution about setRetainInstance(true) but it's a good solution? I knew it was something used with non-UI-Fragments otherwise it can generate memoryLeak.

Answer 1

You can use these two callbacks to handle that,

override fun onSaveInstanceState(bundle: Bundle){
  // save the data you need here by just putting it into bundle
}

override fun onRestoreInstanceState(bundle: Bundle){
  // retrieve the data you need here by just getting it from bundle
}

So you do not make your fragments non destroyable(which is impossible IMHO, as your activity gets recreated) but you can save the UI state that they have and restore it properly