[android] Communicating between a fragment and an activity - best practices

This question is mostly to solicit opinions on the best way to handle my app. I have three fragments being handled by one activity. Fragment A has one clickable element the photo and Fragment B has 4 clickable elements the buttons. The other fragment just displays details when the photo is clicked. I am using ActionBarSherlock.

The forward and back buttons need to change the photo to the next or previous poses, respectively. I could keep the photo and the buttons in the same fragment, but wanted to keep them separate in case I wanted to rearrange them in a tablet.

I need some advice - should I combine Fragments A and B? If not, I will need to figure out how to implement an interface for 3 clickable items.

I considered using Roboguice, but I am already extending using SherlockFragmentActivity so that's a no go. I saw mention of Otto, but I didn't see good tutorials on how to include in a project. What do you think best design practice should be?

I also need help figuring out how to communicate between a fragment and an activity. I'd like to keep some data "global" in the application, like the pose id. Is there some example code I can see besides the stock android developer's information? That is not all that helpful.

BTW, I'm already storing all the information about each pose in a SQLite database. That's the easy part.

To communicate between an Activity and Fragments, there are several options, but after lots of reading and many experiences, I found out that it could be resumed this way:

• Activity wants to communicate with child Fragment => Simply write public methods in your Fragment class, and let the Activity call them
• Fragment wants to communicate with the parent Activity => This requires a bit more of work, as the official Android link https://developer.android.com/training/basics/fragments/communicating suggests, it would be a great idea to define an interface that will be implemented by the Activity, and which will establish a contract for any Activity that wants to communicate with that Fragment. For example, if you have FragmentA, which wants to communicate with any activity that includes it, then define the FragmentAInterface which will define what method can the FragmentA call for the activities that decide to use it.
• A Fragment wants to communicate with other Fragment => This is the case where you get the most 'complicated' situation. Since you could potentially need to pass data from FragmentA to FragmentB and viceversa, that could lead us to defining 2 interfaces, FragmentAInterface which will be implemented by FragmentB and FragmentAInterface which will be implemented by FragmentA. That will start making things messy. And imagine if you have a few more Fragments on place, and even the parent activity wants to communicate with them. Well, this case is a perfect moment to establish a shared ViewModel for the activity and it's fragments. More info here https://developer.android.com/topic/libraries/architecture/viewmodel . Basically, you need to define a SharedViewModel class, that has all the data you want to share between the activity and the fragments that will be in need of communicating data among them.

The ViewModel case, makes things pretty simpler at the end, since you don't have to add extra logic that makes things dirty in the code and messy. Plus it will allow you to separate the gathering (through calls to an SQLite Database or an API) of data from the Controller (activities and fragments).

I am using Intents to communicate actions back to the main activity. The main activity is listening to these by overriding onNewIntent(Intent intent). The main activity translates these actions to the corresponding fragments for example.

So you can do something like this:

public class MainActivity extends Activity  {

    public static final String INTENT_ACTION_SHOW_FOO = "show_foo";
    public static final String INTENT_ACTION_SHOW_BAR = "show_bar";


   @Override
   protected void onNewIntent(Intent intent) {
        routeIntent(intent);
   }

  private void routeIntent(Intent intent) {
       String action = intent.getAction();
       if (action != null) {               
            switch (action) {
            case INTENT_ACTION_SHOW_FOO:
                // for example show the corresponding fragment
                loadFragment(FooFragment);
                break;
            case INTENT_ACTION_SHOW_BAR:
                loadFragment(BarFragment);
                break;               
        }
    }
}

Then inside any fragment to show the foo fragment:

Intent intent = new Intent(context, MainActivity.class);
intent.setAction(INTENT_ACTION_SHOW_FOO);
// Prevent activity to be re-instantiated if it is already running.
// Instead, the onNewEvent() is triggered
intent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP | Intent.FLAG_ACTIVITY_CLEAR_TOP);
getContext().startActivity(intent);

I made a annotation library that can do the cast for you. check this out. https://github.com/zeroarst/callbackfragment/

@CallbackFragment
public class MyFragment extends Fragment {

    @Callback
    interface FragmentCallback {
       void onClickButton(MyFragment fragment);
    }    
    private FragmentCallback mCallback;

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.bt1
                mCallback.onClickButton(this);
                break;
            case R.id.bt2
                // Because we give mandatory = false so this might be null if not implemented by the host.
                if (mCallbackNotForce != null)
                mCallbackNotForce.onClickButton(this);
                break;
        }
    }
}

It then generates a subclass of your fragment. And just add it to FragmentManager.

public class MainActivity extends AppCompatActivity implements MyFragment.FragmentCallback {

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

        getSupportFragmentManager().beginTransaction()
            .add(R.id.lo_fragm_container, MyFragmentCallbackable.create(), "MY_FRAGM")
            .commit();
    }

    Toast mToast;

    @Override
    public void onClickButton(MyFragment fragment) {
        if (mToast != null)
            mToast.cancel();
        mToast = Toast.makeText(this, "Callback from " + fragment.getTag(), Toast.LENGTH_SHORT);
        mToast.show();
    }
}

There are severals ways to communicate between activities, fragments, services etc. The obvious one is to communicate using interfaces. However, it is not a productive way to communicate. You have to implement the listeners etc.

My suggestion is to use an event bus. Event bus is a publish/subscribe pattern implementation.

You can subscribe to events in your activity and then you can post that events in your fragments etc.

Here on my blog post you can find more detail about this pattern and also an example project to show the usage.

It is implemented by a Callback interface:

First of all, we have to make an interface:

public interface UpdateFrag {
     void updatefrag();
}

In the Activity do the following code:

UpdateFrag updatfrag ;

public void updateApi(UpdateFrag listener) {
        updatfrag = listener;
}

from the event from where the callback has to fire in the Activity:

updatfrag.updatefrag();

In the Fragment implement the interface in CreateView do the following code:

 ((Home)getActivity()).updateApi(new UpdateFrag() {
        @Override
        public void updatefrag() {
              .....your stuff......
        }
 });

The suggested method for communicating between fragments is to use callbacks\listeners that are managed by your main Activity.

I think the code on this page is pretty clear: http://developer.android.com/training/basics/fragments/communicating.html

You can also reference the IO 2012 Schedule app, which is designed to be a de-facto reference app. It can be found here: http://code.google.com/p/iosched/

Also, here is a SO question with good info: How to pass data between fragments

I'm not sure I really understood what you want to do, but the suggested way to communicate between fragments is to use callbacks with the Activity, never directly between fragments. See here http://developer.android.com/training/basics/fragments/communicating.html

You can create declare a public interface with a function declaration in the fragment and implement the interface in the activity. Then you can call the function from the fragment.

There is the latest techniques to communicate fragment to activity without any interface follow the steps Step 1- Add the dependency in gradle

implementation 'androidx.fragment:fragment:1.3.0-rc01'