Question

I’m writing a simple Activity which should retrieve and show some data in a Fragment containing a ListView. The problem is that I’m not able to access the ListView because none of the on* methods seems to be called.

The fragment has a LinearLayout which gets populated dynamically by the code in onCreateView. The Activity creates the Fragment in onCreate and adds it to its layout using the FragmentManager returned by getSupportFragmentManager(As a side note: I’m using the support-v4 library).

Adding some Log.d calls in the onAttach/onCreate/onCreateView methods clearly show that none of them is called at all.

What am I doing wrong?

The activity:

public class CharactersActivity extends FragmentActivity {

    private MainCharactersFragment mainFragment = null;

    private String[] charnames = null;


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // R.layout.characters contains only a LinearLayout with id characters_fragment_container
        setContentView(R.layout.characters);
        if (mainFragment == null) {
            mainFragment = new MainCharactersFragment();
            //Add the fragment to the activity's layout.
            //Taken from Google's tutorials.
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.characters_fragment_container, mainFragment).commit();
            //I'd expect onCreateView to be called during this commit, but it is not called.
        }
        //Loads some data to display
        Intent intent = getIntent();
        String[] filenames = intent.getStringArrayExtra(PortableDnDActivity.CHARACTERS_FILENAMES);
        this.charnames = new String[filenames.length];
        for (int i=0; i < filenames.length; i++) {
            this.charnames[i] = getCharacterName(filenames[i]);
        }
    }

    @Override
    public void onStart() {
        // Set the ListAdapter to show the data.
        Log.d("MyLog", "           HERE            ");  // Shown by logs
        ListView view = (ListView) findViewById(R.id.main_characters_view); // This is null
        Log.d("MyLog", "The view is null:" + (view == null));
        mainFragment.setCharsAdapter(this, this.charnames);
        Log.d("MyLog", "                 HERE           2222"); //NOT shown by the logs
    }

The activity layout:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/characters_fragment_container"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >
</LinearLayout>

The fragment:

public class MainCharactersFragment extends Fragment {

    public ListView charsList;
    public Button newCharButton;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // The following is never shown in the logs
        Log.d("MyLog", "                HERE                ");
        View view = inflater.inflate(R.layout.main_characters_fragment, container, false);
        return view;
    }

    public void setCharsAdapter(Context ctx, String[] data) {
        // Raise NullPointerException because at this moment
        // findViewById(R.id.main_characters_view) returns null
        charsList.setAdapter(new ArrayAdapter<String>(ctx, R.id.main_characters_view, data));
    }
    @Override
    public void onAttach(Activity activity) {
        Log.d("MyLog", "            onAttach called           "); //Never shown
    }
}

The fragment layout:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ListView android:id="@+id/main_characters_view"
              android:layout_width="fill_parent"
              android:layout_height="0dp"
              android:layout_weight="0.5"
              />
    <Button android:id="@+id/new_character_button"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text="@string/new_character"
            />

</LinearLayout>

Answer 1

I think the NPE is confusing things. Comment that out of your onStart() so you can address it separately.

Once you do that, you will see you have a few cases of falling to call the super method. Once in MainCharactersFragment.onAttach() and once in CharactersActivity.onStart(). Add those two calls in, clean, and try again.

Given those changes, your code ran fine for me.