Executing lifecycle method of backstack fragment problem in android

Question

I have an activity with two fragment and want to be executed first fragment when its back from second fragment using back button. And i am using the add() when navigating first fragment to second fragment. Here is my scenario and code snippet:

First fragment:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {

        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.one_fragment, container, false);    

        final Button button = (Button) view.findViewById(R.id.buttonChange);
        button.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                buttonClicked(v);
            }
        });

        return view;
    }

    public void buttonClicked(View view) {
        FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.fragment_Container, new TwoFragment());
        fragmentTransaction.addToBackStack("sdfsf");
        fragmentTransaction.commit();

}

Moving to Second fragment and here is the code:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.two_fragment, container, false);
        return view;
}

The problem is that, When I am navigating from first to second fragment and then back again in the first fragment using back button first fragment lifecycle method is not executing. Instead of using add() if I use replace() then lifecycle method are executing properly. I know its the difference between add() and replace() but I want to use add() and also want to have navigation callback to handle some logic when I back in the first fragment using back button.

Also tried below code:

fragmentManager.addOnBackStackChangedListener(
        new FragmentManager.OnBackStackChangedListener() {
                public void onBackStackChanged() {
                        Log.e(TAG, "onBackStackChanged: ");
                        // Update your UI here.
                }
});

But its also calling multiple times and creating anomalies.

How can I handle this? Specially handle some logic in first fragment when I back from second fragment.

Answer 1

The easiest way I can think of is to set result when you're done with the second fragment that essentially tells the first fragment to "resume" via its onActivityResult method.

When you create an instance of Fragment B, call #setTargetFragment() and pass in Fragment A as your target fragment. Then when Fragment B is done and going to return to Fragment A, before it exits, you will set the result of it for Fragment A by calling:

getTargetFragment().onActivityResult(getTargetRequestCode(), RESULT_FRAGMENT_B_FINISHED,null)
///// horizontal scroll padding

Note that RESULT_FRAGMENT_B_FINISHED would be some static integer you define somewhere, like

public static final int RESULT_FRAGMENT_B_FINISHED = 123123;

Now in Fragment A all you need to do is override onActivityResult and check that the request code matches the request code integer from setTargetFragment and the result code also matches RESULT_FRAGMENT_B_FINISHED, if so you can run the code that would have been fired from onResume().

#getTargetFragment()

#onActivityResult()

#getTargetRequestCode()

Answer 2

Instead of passing data between the two fragments I recommend you to use a SharedViewModel.

The idea is that the first fragment observe some data for changes and the second one edit this data.

Example:

Shared ViewModel

public class SharedViewModel extends ViewModel {

  private final MutableLiveData<Item> selected = new 
  MutableLiveData<Item>();

  public void select(Item item) {
      selected.setValue(item);
  }

  public LiveData<Item> getSelected() {
      return selected;
  }
}

First fragment

public class FirstFragment extends Fragment {

  public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      SharedViewModel model = 
      ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
      model.getSelected().observe(this, { item ->
         // Update the UI.
      });
  }
}

Second fragment

public class SecondFragment extends Fragment {

  public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      SharedViewModel model = 
      ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
      model.select(new Item("value 1","value 2");
  }
}

You can read about ViewModels, LiveData and Architecture components starting from here: https://developer.android.com/topic/libraries/architecture/viewmodel#java

This answer is assuming that you want to execute some logic based on some data change. If it's not the case, you can explain what kind of logic do you want to execute and I will edit my answer.