onBackStackChanged() called twice when new fragment is added in backstack

Question

So, i have notice interesting problem. When i add new fragment programmatically, method onBackStackChanged from OnBackStackChangedListener is called twice, but it must be call only one time. Here is my code of activity:

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity implements FragmentManager.OnBackStackChangedListener{

    private FragmentManager fragmentManager;
    private Button button1;
    private Button button2;
    private Button button3;

    private Fragment defaultFragment;
    private Fragment previousFragment;
    private Fragment currentFragment;
    private String currentFragmentTag;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button1 = (Button) findViewById(R.id.button);
        button2 = (Button) findViewById(R.id.button2);
        button3 = (Button) findViewById(R.id.button3);

        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showFragment(new FragmentOne(), FragmentOne.TAG);
            }
        });

        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showFragment(new FragmentTwo(), FragmentTwo.TAG);
            }
        });

        button3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showFragment(new FragmentThree(), FragmentThree.TAG);
            }
        });

        setupFragmentManager();

    }

    private void setupFragmentManager() {
        fragmentManager = getSupportFragmentManager();
        fragmentManager.addOnBackStackChangedListener(this);


        FragmentOne fragmentOne = new FragmentOne();
        defaultFragment = fragmentOne;  // fragment for default

        currentFragmentTag = FragmentOne.TAG;
        currentFragment = fragmentOne;

        fragmentManager.beginTransaction()
                .add(R.id.fragments_container, currentFragment, currentFragmentTag)
                .addToBackStack(currentFragmentTag)
                .commit();
    }

    public void showFragment(Fragment fragment, String fragmentTag) {
        previousFragment = currentFragment;
        currentFragment = fragment;
        fragmentManager.beginTransaction()
                .hide(previousFragment)
                .add(R.id.fragments_container, fragment, fragmentTag)
                .addToBackStack(fragmentTag)
                .commit();
    }

    @Override
    public void onBackStackChanged() {
        Log.d("MY_TAG" , "onBackStackChangedListener - " + fragmentManager.getBackStackEntryCount() );

        if (fragmentManager.getBackStackEntryCount() > 0) {    
            String fragmentTag = fragmentManager.getBackStackEntryAt(fragmentManager.getBackStackEntryCount() - 1).getName();
            currentFragment = fragmentManager.findFragmentByTag(fragmentTag);
        } else {
            currentFragment = defaultFragment;
        }

// some code...
    }

    @Override
    public void onBackPressed() {
        if (fragmentManager.getBackStackEntryCount() == 1) { 
            finish();
        } else {
            super.onBackPressed();
        }
    }
}

And fragment class:

public class FragmentOne extends Fragment {

    public static final String TAG = FragmentOne.class.getSimpleName();

    private Context context;

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


        context = getActivity();
        View topLevelView = LayoutInflater.from(context).inflate(R.layout.fragment_layout, container, false);

        TextView textView = (TextView) topLevelView.findViewById(R.id.fragment_Text);

        textView.setText("This is FRAGMENT ONE");

        return topLevelView;
    }

}

FragmentTwo and FragmentThree are identical to FragmentOne.

And when i start my app, in log i see:

onBackStackChangedListener - 1
onBackStackChangedListener - 1

after pressed of button (show next fragment) i see:

onBackStackChangedListener - 2
onBackStackChangedListener - 2

and another one:

onBackStackChangedListener - 3
onBackStackChangedListener - 3

And changing

.add(R.id.fragments_container, currentFragment, currentFragmentTag)

to

.replace(R.id.fragments_container, currentFragment, currentFragmentTag)

doesnt solve the problem.

But it is very strange. Why my OnBackStackChangedListener calls more than one time own method after one addiing the fragment?

Answer 1

Couldn't believe it can happen. Created a similar project and dived into the debugger. Turns out it's a bug in new support library. There's is a bug opened in the tracker.

Switching back to 25.0.0 will work as expected.

compile 'com.android.support:appcompat-v7:25.0.0'
compile 'com.android.support:support-v4:25.0.0'

---

Though not mentioned in release notes, but it is mentioned in the issue tracker, that the issue is fixed in 25.4.0, which is good 😊.