Question

The documented Android fragment samples (FragmentBasics, NewsReader violate core principles of object oriented design. There are redundant conditionals to establish the currently showing view type, tightly coupling the FragmentActivity with the view types and fragment types. MainActivity is coupled to every class (including the XML):

One might be able to make the excuse for the authors that they are trying to “keep it simple,” or that readers might be confused. If this is the intent–I think readers can handle it; instead, it’s teaching bad programming practices that will result in less maintainable applications.

How can Fragments be implemented such that the views and fragments aren’t tightly coupled to the FragmentActivity?

Answer 1

Starting with the simpler FragmentBasics demo, gut MainActivity as follows:

public class MainActivity extends FragmentActivity
      implements OnHeadlineSelectedListener {
   AbstractNewsView abstractNewsView;

   @Override public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.news_articles);
      abstractNewsView = new AbstractNewsViewProvider(this).get();
      abstractNewsView.onCreate(savedInstanceState);
   }

   @Override public void onArticleSelected(int position) {
      abstractNewsView.onArticleSelected(position);
   }
}

Now its dependency diagram looks like this. You can now add all the news_articles view variants you’d like for different device types and MainActivity doesn’t need to change.

Add a new class AbstractNewsViewProvider whose sole responsibility is to determine which type of view (single or double pane) is being used for the given device. If you’re using Guice or RoboGuice for dependency injection, this would instead be a Provider method in your binding module.

public class AbstractNewsViewProvider {
   private final FragmentActivity fragmentActivity;

   public AbstractNewsViewProvider(FragmentActivity activity) {
      this.fragmentActivity = activity;
   }

   public AbstractNewsView get() {
      if (fragmentActivity.findViewById(R.id.fragment_container) != null) {
           return new SinglePaneNewsView(fragmentActivity);
        } else {
           return new DoublePaneNewsView(fragmentActivity);
        }
   }
}

Add two new classes SinglePaneNewsView and DoublePaneNewsView that implement AbstractNewsView as shown below. These two classes are responsible for the setup of the initial fragment(s) within the respective view type. They are also responsible for handling transitions between fragments, if any.

interface AbstractNewsView extends OnHeadlineSelectedListener {
   public void onCreate(Bundle savedInstanceState);
   @Override public void onArticleSelected(int position);
}


public class SinglePaneNewsView implements AbstractNewsView {

   private final FragmentActivity fragmentActivity;

   public SinglePaneNewsView(FragmentActivity fragmentActivity) {
      this.fragmentActivity = fragmentActivity;
   }

   @Override public void onCreate(Bundle savedInstanceState) {
      // However, if we're being restored from a previous state,
      // then we don't need to do anything and should return or else
      // we could end up with overlapping fragments.
      if (savedInstanceState != null) {
         return;
      }

      // Create an instance of ExampleFragment
      HeadlinesFragment firstFragment = new HeadlinesFragment();

      // In case this activity was started with special instructions from an
      // Intent,
      // pass the Intent's extras to the fragment as arguments
      firstFragment.setArguments(fragmentActivity.getIntent().getExtras());

      // Add the fragment to the 'fragment_container' FrameLayout
      fragmentActivity.getSupportFragmentManager().beginTransaction()
            .add(R.id.fragment_container, firstFragment).commit();
   }

   @Override public void onArticleSelected(int position) {
      // If the frag is not available, we're in the one-pane layout and must
      // swap frags...

      // Create fragment and give it an argument for the selected article
      ArticleFragment newFragment = new ArticleFragment();
      Bundle args = new Bundle();
      args.putInt(ArticleFragment.ARG_POSITION, position);
      newFragment.setArguments(args);
      FragmentTransaction transaction =
            fragmentActivity.getSupportFragmentManager().beginTransaction();

      // Replace whatever is in the fragment_container view with this fragment
      // Add the transaction to the back stack so the user can navigate back
      transaction.replace(R.id.fragment_container, newFragment);
      transaction.addToBackStack(null);

      // Commit the transaction
      transaction.commit();
   }


public class DoublePaneNewsView implements AbstractNewsView {

   private final FragmentActivity fragmentActivity;

   public DoublePaneNewsView(FragmentActivity fragmentActivity) {
      this.fragmentActivity = fragmentActivity;
   }

   @Override public void onCreate(Bundle savedInstanceState) {
   }

   @Override public void onArticleSelected(int position) {
      ((ArticleFragment) fragmentActivity.getSupportFragmentManager()
         .findFragmentById(R.id.article_fragment)).updateArticleView(position);
   }

}

You can find the complete source on Google code.