FragmentsTabListener.java

package bar.foo.example.lib.android;

import android.app.ActionBar;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.content.Context;

import java.lang.ref.WeakReference;

public class FragmentsTabListener<T extends Fragment> implements ActionBar.TabListener {
	private final Context context;
	private final String fragmentTag;
	private final Class<T> fragmentClass;
	private final WeakReference<OnFragmentTabChangeListener> changeListener;
	private final FragmentChangeNotifier changeNotifier;
	private T fragment;

	public FragmentsTabListener(final Context context, final String fragmentTag, final Class<T> fragmentClass) {
		this(context, fragmentTag, fragmentClass, null);
	}

	public FragmentsTabListener(final Context context, final String fragmentTag, final Class<T> fragmentClass, final OnFragmentTabChangeListener changeListener) {
		this.context = context;
		this.fragmentTag = fragmentTag;
		this.fragmentClass = fragmentClass;

		if (changeListener != null) {
			this.changeListener = new WeakReference<>(changeListener);
			changeNotifier = new EnabledFragmentChangeNotifier();

		} else {
			this.changeListener = null;
			changeNotifier = new NullFragmentChangeNotifier();
		}
	}

	@Override
	@SuppressWarnings("unchecked")
	public void onTabSelected(final ActionBar.Tab tab, final FragmentTransaction ft) {
		if (fragment == null) {
			fragment = (T) Fragment.instantiate(context, fragmentClass.getName());
			changeNotifier.notifyFragmentTabCreated(fragment);
			ft.replace(
					android.R.id.content,
					fragment,
					fragmentTag
			);
		} else {
			ft.attach(fragment);
		}
		changeNotifier.notifyFragmentTabAttached(fragment);
	}

	@Override
	public void onTabUnselected(final ActionBar.Tab tab, final FragmentTransaction ft) {
		if (fragment != null) {
			ft.detach(fragment);
			changeNotifier.notifyFragmentTabDetached(fragment);
		}
	}

	@Override
	public void onTabReselected(final ActionBar.Tab tab, final FragmentTransaction ft) {
		// Do nothing
	}

	public interface OnFragmentTabChangeListener {
		public <T extends Fragment> void onFragmentTabCreated(T newFragment);

		public <T extends Fragment> void onFragmentTabAttached(T fragment);

		public <T extends Fragment> void onFragmentTabDetached(T fragment);
	}

	private interface FragmentChangeNotifier {
		public <T extends Fragment> void notifyFragmentTabCreated(T newFragment);

		public <T extends Fragment> void notifyFragmentTabAttached(T fragment);

		public <T extends Fragment> void notifyFragmentTabDetached(T fragment);
	}

	private class EnabledFragmentChangeNotifier implements FragmentChangeNotifier {

		@Override
		public <T extends Fragment> void notifyFragmentTabCreated(final T newFragment) {
			if (hasChangeListener()) {
				changeListener.get().onFragmentTabCreated(newFragment);
			}
		}

		@Override
		public <T extends Fragment> void notifyFragmentTabAttached(final T fragment) {
			if (hasChangeListener()) {
				changeListener.get().onFragmentTabAttached(fragment);
			}
		}

		@Override
		public <T extends Fragment> void notifyFragmentTabDetached(final T fragment) {
			if (hasChangeListener()) {
				changeListener.get().onFragmentTabDetached(fragment);
			}
		}

		private boolean hasChangeListener() {
			return changeListener.get() != null;
		}
	}

	private class NullFragmentChangeNotifier implements FragmentChangeNotifier {

		@Override
		public <T extends Fragment> void notifyFragmentTabCreated(final T newFragment) {
		}

		@Override
		public <T extends Fragment> void notifyFragmentTabAttached(final T fragment) {
		}

		@Override
		public <T extends Fragment> void notifyFragmentTabDetached(final T fragment) {
		}
	}

}