[Android] Run the post method of the Otto Bus in the Main thread always. And Discussion the Observer/Observable(Callback) pattern. - Case01: Activity/Fragment using a dedicated callback. (the observer pattern) - Case02: Activity/Fragment using a common callback. (the observer pattern) - Case03: Activity/Fragment using a LocalBroadcastManager. - …

BusProvider.java

import android.content.Context;
import android.os.Handler;
import android.os.Looper;

import com.squareup.otto.Bus;

public final class BusProvider {
    private static final Object sLock = new Object();
    private static MyBus sInstance;

    public static MyBus getInstance() {

        synchronized (sLock) {
            if (sInstance == null) {
                sInstance = new MyBus(new Handler(Looper.getMainLooper()));
                Log.i("BusProvider", "The bus was ready.");
            }
            return sInstance;
        }
    }

    private BusProvider() {
        // No instances.
    }

    public static class MyBus extends Bus {
        private Handler mHandler;

        public MyBus(Handler handler) {
            super();
            mHandler = handler;
        }

        @Override
        public void post(final Object event) {
            if (Looper.myLooper() != mHandler.getLooper()) {
                mHandler.post(new Runnable() {
                    @Override
                    public void run() {
                        MyBus.super.post(event);
                    }
                });
            } else {
                super.post(event);
            }
        }
    }
}

Case01#FirstFragment.java

public class FirstFragment extends Fragment {
    OnListener mCallback;

    public interface OnListener {
        public void onFirstCallback();
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        if (activity instanceof OnListener) {
            mCallback = (OnListener) activity;
        }
    }
}

Case01#MainActivity.java

public class MainActivity extends FragmentActivity
        implements FirstFragment.OnListener, SecondFragment.OnListener {

    FirstFragment mFirstFragment;
    SecondFragment mSecondFragment;
    
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (savedInstanceState != null) {
            return;
        }
        mFirstFragment = new FirstFragment();
        mSecondFragment = new SecondFragment();
        getSupportFragmentManager().beginTransaction()
            .add(mFirstFragment, "first_frag")
            .add(mSecondFragment, "second_frag")
            .commit();
    }
    
    @Override
    public void onFirstCallback() {
    }

    @Override
    public void onSecondCallback() {
    }

}

Case01#SecondFragment.java

public class SecondFragment extends Fragment {
    OnListener mCallback;

    public interface OnListener {
        public void onFirstCallback();
    }
    
    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        if (activity instanceof OnListener) {
            mCallback = (OnListener) activity;
        }
    }
}

Case02#FirstFragment.java

public class FirstFragment extends Fragment {
    OnFragmentEvenCallback mCallback;

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        if (activity instanceof OnFragmentEvenCallback) {
            mCallback = (OnFragmentEvenCallback) activity;
        }
    }

    private void sendMessage() {
        Bundle bundle = new Bundle();
		bundle.putString("kind", "first");
		bundle.putInt("status", 0);
		
		mCallback.onFragmentEvent(bundle);
    }
}

Case02#MainActivity.java

public class MainActivity extends FragmentActivity
        implements OnFragmentEvenCallback {

    FirstFragment mFirstFragment;
    SecondFragment mSecondFragment;
    
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (savedInstanceState != null) {
            return;
        }
        mFirstFragment = new FirstFragment();
        mSecondFragment = new SecondFragment();
        getSupportFragmentManager().beginTransaction()
            .add(mFirstFragment, "first_frag")
            .add(mSecondFragment, "second_frag")
            .commit();
    }
    
    @Override
    public void onFragmentEvent(Bundle bundle) {
        String kind = bundle.getString("kind");
        if (kind.equals("first")) {
            int status = bundle.getInt("status");
            // ...
        } else 
        if (kind.equals("second")) {
            MyObject payload = (MyObject) bundle.getSerializable("payload");
            // ...
        }
    }

}

Case02#OnFragmentEvenCallback.java

public interface OnFragmentEvenCallback {
	public void onFragmentEvent(Bundle bundle);
}

Case02#SecondFragment.java

public class SecondFragment extends Fragment {
    OnFragmentEvenCallback mCallback;

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        if (activity instanceof OnFragmentEvenCallback) {
            mCallback = (OnFragmentEvenCallback) activity;
        }
    }

    private void sendMessage() {
        Bundle bundle = new Bundle();
		bundle.putString("kind", "second");
		bundle.putSerializable("payload", mMyObject);
		
		mCallback.onFragmentEvent(bundle);
    }

}

Case03#FirstFragment.java

public class FirstFragment extends Fragment {

    private void sendMessage() {
        Bundle bundle = new Bundle();
		bundle.putString("kind", "first");
		bundle.putInt("status", 0);

        Intent intent = new Intent("custom-event");
		intent.putExtras(bundle);
		LocalBroadcastManager.getInstance(getActivity()).sendBroadcast(intent);
    }
}

Case03#MainActivity.java

public class MainActivity extends FragmentActivity {

    FirstFragment mFirstFragment;
    SecondFragment mSecondFragment;
    
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (savedInstanceState != null) {
            return;
        }
        mFirstFragment = new FirstFragment();
        mSecondFragment = new SecondFragment();
        getSupportFragmentManager().beginTransaction()
            .add(mFirstFragment, "first_frag")
            .add(mSecondFragment, "second_frag")
            .commit();
    }
    
    @Override
	public void onResume() {
		super.onResume();
		LocalBroadcastManager.getInstance(this).registerReceiver(mMessageReceiver,
				new IntentFilter("custom-event"));
	}
	
	@Override
	public void onPause() {
		super.onPause();
		LocalBroadcastManager.getInstance(this.unregisterReceiver(mMessageReceiver);
	}
	
	private BroadcastReceiver mMessageReceiver = new BroadcastReceiver() {
		@Override
		public void onReceive(Context context, Intent intent) {
		    Bundle bundle = intent.getExtras();
            String kind = bundle.getString("kind");
            if (kind.equals("first")) {
                int status = bundle.getInt("status");
                // ...
            } else 
            if (kind.equals("second")) {
                MyObject payload = (MyObject) bundle.getSerializable("payload");
                // ...
            }
		}
	}
}

Case03#SecondFragment.java

public class SecondFragment extends Fragment {

    private void sendMessage() {
        Bundle bundle = new Bundle();
		bundle.putString("kind", "second");
		bundle.putSerializable("payload", mMyObject);
		
        Intent intent = new Intent("custom-event");
		intent.putExtras(bundle);
		LocalBroadcastManager.getInstance(getActivity()).sendBroadcast(intent);
    }

}

Case04#FirstFragment.java

public class FirstFragment extends Fragment {
    int mStatus;

    private void sendMessage() {
        BusProvider.getInstance(getActivity()).post(produceFirstFragmentEvent());
    }

    // The method should take no parameters and its return type will be used as the type of event
    @Produce
    public FirstFragmentEvent produceFirstFragmentEvent() {
        return new FirstFragmentEvent(mStatus);
    }
    
    
    @Override
	public void onResume() {
		super.onResume();
		BusProvider.getInstance(getActivity()).register(this);
	}
	
	@Override
	public void onPause() {
		super.onPause();
		BusProvider.getInstance(getActivity()).unregister(this);
	}
	
	@Subscribe
    public void onSecondCallback(SecondFragmentEvent event) {
        int status = event.status;
        MyObject payload = event.payload;
        // ...
    }
    
}

Case04#FirstFragmentEvent.java

public class FirstFragmentEvent {
    int status;
    
    public FirstFragmentEvent(int status) {
		this.status = status;
	}
}

Case04#MainActivity.java

public class MainActivity extends FragmentActivity {

    FirstFragment mFirstFragment;
    SecondFragment mSecondFragment;
    
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (savedInstanceState != null) {
            return;
        }
        mFirstFragment = new FirstFragment();
        mSecondFragment = new SecondFragment();
        getSupportFragmentManager().beginTransaction()
            .add(mFirstFragment, "first_frag")
            .add(mSecondFragment, "second_frag")
            .commit();
    }
    
    @Override
	public void onResume() {
		super.onResume();
		BusProvider.getInstance(getActivity()).register(this);
	}
	
	@Override
	public void onPause() {
		super.onPause();
		BusProvider.getInstance(getActivity()).unregister(this);
	}
	
	@Subscribe
    public void onFirstCallback(FirstFragmentEvent event) {
        int status = event.status;
        // ...
    }

	@Subscribe
    public void onSecondCallback(SecondFragmentEvent event) {
        int status = event.status;
        MyObject payload = event.payload;
        // ...
    }
}

Case04#MainApplication.java

public class MainApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        BusProvider.getInstance(this);
    }
}

Case04#SecondFragment.java

public class SecondFragment extends Fragment {
    Bus bus =  BusProvider.getInstance();

    private void sendMessage() {
        int status;
        MyObject payload;
        
        bus.post(new SecondFragmentEvent(status, payload));
    }

}

Case04#SecondFragmentEvent.java

public class SecondFragmentEvent {
    int status;
    MyObject payload;
    
    public SecondFragmentEvent(int status, MyObject payload) {
		this.status = status;
		this.payload = payload;
	}
}