Fragment's reference to mActivity becomes null after orientation change. Ineffective fragment state maintenance

Question

My application consists of several fragments. Up until now I've had references to them stored in a custom Application object, but I am beginning to think that I'm doing something wrong.

My problems started when I realized that all my fragment's references to mActivity becomes null after an orientation change. So when I call getActivity() after an orientation change, a NullPointerException is thrown. I have checked that my fragment's onAttach() is called before I make the call to getActivity(), but it still returns null.

The following is a stripped version of my MainActivity, which is the only activity in my application.

public class MainActivity extends BaseActivity implements OnItemClickListener,
        OnBackStackChangedListener, OnSlidingMenuActionListener {

    private ListView mSlidingMenuListView;
    private SlidingMenu mSlidingMenu;

    private boolean mMenuFragmentVisible;
    private boolean mContentFragmentVisible;
    private boolean mQuickAccessFragmentVisible;

    private FragmentManager mManager;

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

        /*
         * Boolean variables indicating which of the 3 fragment slots are visible at a given time
         */
        mMenuFragmentVisible = findViewById(R.id.menuFragment) != null;
        mContentFragmentVisible = findViewById(R.id.contentFragment) != null;
        mQuickAccessFragmentVisible = findViewById(R.id.quickAccessFragment) != null;

        if(!savedInstanceState != null) {
            if(!mMenuFragmentVisible && mContentFragmentVisible) {
                setupSlidingMenu(true);
            } else if(mMenuFragmentVisible && mContentFragmentVisible) {
                setupSlidingMenu(false);
            }

            return;
        }

        mManager = getSupportFragmentManager();
        mManager.addOnBackStackChangedListener(this);

        final FragmentTransaction ft = mManager.beginTransaction();
        ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);

        if (!mMenuFragmentVisible && mContentFragmentVisible) {
            /*
             * Only the content fragment is visible, will enable sliding menu
             */
            setupSlidingMenu(true);
            onToggle();

            ft.replace(R.id.contentFragment, getCustomApplication().getSportsFragment(), SportsFragment.TAG);

        } else if (mMenuFragmentVisible && mContentFragmentVisible) {
            setupSlidingMenu(false);
            /*
             * Both menu and content fragments are visible
             */
            ft.replace(R.id.menuFragment, getCustomApplication().getMenuFragment(), MenuFragment.TAG);
            ft.replace(R.id.contentFragment, getCustomApplication().getSportsFragment(), SportsFragment.TAG);
        }

        if (mQuickAccessFragmentVisible) {
            /*
             * The quick access fragment is visible
             */
            ft.replace(R.id.quickAccessFragment, getCustomApplication().getQuickAccessFragment());
        }

        ft.commit();
    }

    private void setupSlidingMenu(boolean enable) {
        /*
         * if enable is true, enable sliding menu, if false
         * disable it
         */
    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

        // launch the fragment that was clicked from the menu
    }

    @Override
    public void onBackPressed() {
        // Will let the user press the back button when
        // the sliding menu is open to display the content.
        if (mSlidingMenu != null && mSlidingMenu.isMenuShowing()) {
            onShowContent();
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public void onBackStackChanged() {
        /*
         * Change selected position when the back stack changes
         */
        if(mSlidingMenuListView != null) {
            mSlidingMenuListView.setItemChecked(getCustomApplication().getSelectedPosition(), true);    
        }
    }

    @Override
    public void onToggle() {
        if (mSlidingMenu != null) {
            mSlidingMenu.toggle();
        }
    }

    @Override
    public void onShowContent() {
        if (mSlidingMenu != null) {
            mSlidingMenu.showContent();
        }
    }
}

The following is a stripped version of the CustomApplication. My thoughts behind this implementation was to guarantee only one instance of each fragment throughout my application's life cycle.

public class CustomApplication extends Application {

    private Fragment mSsportsFragment;
    private Fragment mCarsFragment;
    private Fragment mMusicFragment;
    private Fragment mMoviesFragment;

    public Fragment getSportsFragment() {
        if(mSsportsFragment == null) {
            mSsportsFragment = new SportsFragment();
        }

        return mSsportsFragment;
    }

    public Fragment getCarsFragment() {
        if(mCarsFragment == null) {
            mCarsFragment = new CarsFragment();
        }

        return mCarsFragment;
    }

    public Fragment getMusicFragment() {
        if(mMusicFragment == null) {
            mMusicFragment = new MusicFragment();
        }

        return mMusicFragment;
    }

    public Fragment getMoviesFragment() {
        if(mMoviesFragment == null) {
            mMoviesFragment = new MoviesFragment();
        }

        return mMoviesFragment;
    }
}

I am very interested in tips on how to best implement multiple fragments and how to maintain their states. For your information, my applicaion consists of 15+ fragments so far. I have done some research and it seems that FragmentManager.findFragmentByTag() is a good bet, but I haven't been able to successfully implement it.

My implementation seems to work good except for the fact that mActivity references become null after orientation changes, which lets me to believe that I may have some memory leak issues as well.

If you need to see more code, please let me know. I purposely avoided including fragment code as I strongly believe issues are related to my Activity and Application implementations, but I may be wrong.

Thanks for your time.

Answer 1

My thoughts behind this implementation was to guarantee only one instance of each fragment throughout my application's life cycle

This is probably part, if not all, of the source of your difficulty.

On a configuration change, Android will re-create your fragments by using the public zero-argument constructor to create a new instance. Hence, your global-scope fragments will not "guarantee only one instance of each fragment".

Please delete this custom Application class. Please allow the fragments to be re-created naturally, or if they need to live for the life of a single activity, use setRetainInstance(true). Do not attempt to reuse fragments across activities.