Basic communication between two fragments

Question

I have one activity - MainActivity. Within this activity I have two fragments, both of which I created declaratively within the xml.

I am trying to pas

Answer 1

The nicest and recommended way is to use a shared ViewModel.

https://developer.android.com/topic/libraries/architecture/viewmodel#sharing

From Google doc:

public class SharedViewModel extends ViewModel {
private final MutableLiveData<Item> selected = new MutableLiveData<Item>();

public void select(Item item) {
    selected.setValue(item);
}

public LiveData<Item> getSelected() {
    return selected;
}
}


public class MasterFragment extends Fragment {
private SharedViewModel model;
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
    itemSelector.setOnClickListener(item -> {
        model.select(item);
    });
}
}


public class DetailFragment extends Fragment {
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    SharedViewModel model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
    model.getSelected().observe(this, { item ->
       // Update the UI.
    });
}
}

ps: two fragments never communicate directly

Answer 2

Have a look at the Android developers page: http://developer.android.com/training/basics/fragments/communicating.html#DefineInterface

Basically, you define an interface in your Fragment A, and let your Activity implement that Interface. Now you can call the interface method in your Fragment, and your Activity will receive the event. Now in your activity, you can call your second Fragment to update the textview with the received value

Your Activity implements your interface (See FragmentA below)

public class YourActivity implements FragmentA.TextClicked{
    @Override
    public void sendText(String text){
        // Get Fragment B
        FraB frag = (FragB)
            getSupportFragmentManager().findFragmentById(R.id.fragment_b);
        frag.updateText(text);
    }
}

Fragment A defines an Interface, and calls the method when needed

public class FragA extends Fragment{

    TextClicked mCallback;

    public interface TextClicked{
        public void sendText(String text);
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        // This makes sure that the container activity has implemented
        // the callback interface. If not, it throws an exception
        try {
            mCallback = (TextClicked) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                + " must implement TextClicked");
        }
    }

    public void someMethod(){
        mCallback.sendText("YOUR TEXT");
    }

    @Override
    public void onDetach() {
        mCallback = null; // => avoid leaking, thanks @Deepscorn
        super.onDetach();
    }
}

Fragment B has a public method to do something with the text

public class FragB extends Fragment{

    public void updateText(String text){
        // Here you have it
    }
}

Answer 3

Take a look at https://github.com/greenrobot/EventBus or http://square.github.io/otto/

or even ... http://nerds.weddingpartyapp.com/tech/2014/12/24/implementing-an-event-bus-with-rxjava-rxbus/

Answer 4

I give my activity an interface that all the fragments can then use. If you have have many fragments on the same activity, this saves a lot of code re-writing and is a cleaner solution / more modular than making an individual interface for each fragment with similar functions. I also like how it is modular. The downside, is that some fragments will have access to functions they don't need.

    public class MyActivity extends AppCompatActivity
    implements MyActivityInterface {

        private List<String> mData; 

        @Override
        public List<String> getData(){return mData;}

        @Override
        public void setData(List<String> data){mData = data;}
    }


    public interface MyActivityInterface {

        List<String> getData(); 
        void setData(List<String> data);
    }

    public class MyFragment extends Fragment {

         private MyActivityInterface mActivity; 
         private List<String> activityData; 

         public void onButtonPress(){
              activityData = mActivity.getData()
         }

        @Override
        public void onAttach(Context context) {
            super.onAttach(context);
            if (context instanceof MyActivityInterface) {
                mActivity = (MyActivityInterface) context;
            } else {
                throw new RuntimeException(context.toString()
                        + " must implement MyActivityInterface");
            }
        }

        @Override
        public void onDetach() {
            super.onDetach();
            mActivity = null;
        }
    } 

Answer 5

There is a simple way to implement communication between fragments of an activity using architectural components. Data can be passed between fragments of an activity using ViewModel and LiveData.

Fragments involved in communication need to use the same view model objects which is tied to activity life cycle. The view model object contains livedata object to which data is passed by one fragment and the second fragment listens for changes on LiveData and receives the data sent from fragment one.

For complete example see http://www.zoftino.com/passing-data-between-android-fragments-using-viewmodel

Answer 6

Some of the other examples (and even the documentation at the time of this writing) use outdated onAttach methods. Here is a full updated example.

Notes

• You don't want the Fragments talking directly to each other or to the Activity. That ties them to a particular Activity and makes reuse difficult.
• The solution is to make an callback listener interface that the Activity will implement. When the Fragment wants to send a message to another Fragment or its parent activity, it can do it through the interface.
• It is ok for the Activity to communicate directly to its child fragment public methods.
• Thus the Activity serves as the controller, passing messages from one fragment to another.

Code

MainActivity.java

public class MainActivity extends AppCompatActivity implements GreenFragment.OnGreenFragmentListener {

    private static final String BLUE_TAG = "blue";
    private static final String GREEN_TAG = "green";
    BlueFragment mBlueFragment;
    GreenFragment mGreenFragment;

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

        // add fragments
        FragmentManager fragmentManager = getSupportFragmentManager();

        mBlueFragment = (BlueFragment) fragmentManager.findFragmentByTag(BLUE_TAG);
        if (mBlueFragment == null) {
            mBlueFragment = new BlueFragment();
            fragmentManager.beginTransaction().add(R.id.blue_fragment_container, mBlueFragment, BLUE_TAG).commit();
        }

        mGreenFragment = (GreenFragment) fragmentManager.findFragmentByTag(GREEN_TAG);
        if (mGreenFragment == null) {
            mGreenFragment = new GreenFragment();
            fragmentManager.beginTransaction().add(R.id.green_fragment_container, mGreenFragment, GREEN_TAG).commit();
        }
    }

    // The Activity handles receiving a message from one Fragment
    // and passing it on to the other Fragment
    @Override
    public void messageFromGreenFragment(String message) {
        mBlueFragment.youveGotMail(message);
    }
}

GreenFragment.java

public class GreenFragment extends Fragment {

    private OnGreenFragmentListener mCallback;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_green, container, false);

        Button button = v.findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String message = "Hello, Blue! I'm Green.";
                mCallback.messageFromGreenFragment(message);
            }
        });

        return v;
    }

    // This is the interface that the Activity will implement
    // so that this Fragment can communicate with the Activity.
    public interface OnGreenFragmentListener {
        void messageFromGreenFragment(String text);
    }

    // This method insures that the Activity has actually implemented our
    // listener and that it isn't null.
    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnGreenFragmentListener) {
            mCallback = (OnGreenFragmentListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnGreenFragmentListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mCallback = null;
    }
}

BlueFragment.java

public class BlueFragment extends Fragment {

    private TextView mTextView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_blue, container, false);
        mTextView = v.findViewById(R.id.textview);
        return v;
    }

    // This is a public method that the Activity can use to communicate
    // directly with this Fragment
    public void youveGotMail(String message) {
        mTextView.setText(message);
    }
}

XML

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp">

    <!-- Green Fragment container -->
    <FrameLayout
        android:id="@+id/green_fragment_container"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:layout_marginBottom="16dp" />

    <!-- Blue Fragment container -->
    <FrameLayout
        android:id="@+id/blue_fragment_container"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1" />

</LinearLayout>

fragment_green.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:background="#98e8ba"
              android:padding="8dp"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <Button
        android:id="@+id/button"
        android:text="send message to blue"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</LinearLayout>

fragment_blue.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:background="#30c9fb"
              android:padding="16dp"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <TextView
        android:id="@+id/textview"
        android:text="TextView"
        android:textSize="24sp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</LinearLayout>