RecyclerView - how to send data to MainActivity

Question

I have a RecyclerView adapter for user added favorites. The recyclerview is populated in a dialog box fragment.

public class FavoriteAdapter extends RecyclerView.Adapter<FavoriteViewHolder> implements ILatLong {

    private Context context;
    View view;
    MainActivity mainActivity;

    private List<Favorite> listFavorites;
    private SQLiteDB mDatabase;
    private int position;

    private ILatLong mCallback;

    public FavoriteAdapter(ILatLong mCallback){
        this.mCallback = mCallback;
    }

    public FavoriteAdapter(Context context, List<Favorite> listFavorites) {
        this.context = context;
        this.listFavorites = listFavorites;
        mDatabase = new SQLiteDB(context);
    }

    @Override
    public void onBindViewHolder(final FavoriteViewHolder holder, final int mPosition) {
        final Favorite singleFavorite = listFavorites.get(mPosition);

        view.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(context, "Clicked " + "'" + singleFavorite.getName() + "'", Toast.LENGTH_SHORT, true).show();
                //mainActivity.onLatLngData(singleFavorite.getLat(), singleFavorite.getLong());
                mCallback.onLatLngData(singleFavorite.getLat(), singleFavorite.getLong());
            }
        });

        //other stuff

    @Override
    public void onLatLngData(Double lat, Double lng) {
        try {
            mCallback = (ILatLong) context;
        }
        catch (ClassCastException e) {
            Log.d("MyDialog", "Activity doesn't implement the ILatLong interface");
        }
    }
}

Interface:

public interface ILatLong {
    void onLatLngData(Double lat, Double lng);
}

MainActivity method:

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener, ILatLong {
    @Override
    public void onLatLngData(Double lat, Double lng) {
        //use the values
    }
}

I've set up the onclicklistener in the onBindViewHolder as you can see, so I know which item is clicked, and when a recyclerview item is clicked it displays a toast with the correct item name (or whatever other attribute I want).

When I try and send the latitude and longitude back to my mainactivity method by uncommenting the line in onclick I get "java.lang.NullPointerException: Attempt to invoke virtual method ...onLatLngData(java.lang.Double, java.lang.Double)' on a null object reference".

How can I send this data back to my mainactivity? I've tried using an interface to callback to MainActivity with no luck but maybe I wasn't doing it right.

Edit: included the interface callback stuff

Answer 1

You have to do those 4 steps in order to make callback work

1. Create an interface that contains the callback function

public interface MyCallback{
    public void onCallbackClicked(String param)
}

2. Implement this interface my your main activity

... implements MyCallback{
// Other stuff here
@Override
public void onCallbackClicked(String param){
// Code here
}

3. Inside your FavoriteAdapter , create variable of type MyCallback

 MyCallback myCallback;

inside your onBindViewHolder you should have something like :

@Override
    public void onBindViewHolder(final FavoriteViewHolder holder, final int mPosition) {
        final Favorite singleFavorite = listFavorites.get(mPosition);

        view.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(myCallback != null){
                    myCallback.onCallbackClicked(/* your data here */);
                 }
            }
        });
}

4. This is an important step , pass the listener (Main activity) to the adapter , you can pass it using constructor

public FavoriteAdapter(MyCallback myCallback){
   this.myCallback = myCallback;
}

or using a setter

public void setCallback(MyCallback myCallback){
   this.myCallback = myCallback;
}

now you are good to go.

Answer 2

Thanks for the help. I managed to solve it through passing my MainActivity.

The problem was it goes MainActivty -> FavoriteDialog -> FavoriteAdapter and I think I was getting confused with the chain here. Maybe it will help someone else.

MainActivity:

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener, ILatLong {

MainActivity mActivity;

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

        mActivity = this;
    }

    // Where I call my FavoriteDialog fragment, pass mActivity in constructor
    FragmentManager fm = getSupportFragmentManager();
    FavoriteDialog dialogFavorite = new DialogFavorite(context, mActivity);
    dialogFragment.show(fm, "FavoriteDialog");
}

DialogFavorite

DialogFavorite(Context context, MainActivity mActivity) {
        mainActivity = mActivity;
    }

// Adapter, pass mainActivity
FavoriteAdapter mAdapter = new FavoriteAdapter(context, allFavorites, mainActivity);
favoriteView.setAdapter(mAdapter);

FavoriteAdapter:

public FavoriteAdapter(Context context, List<Favorite> listFavorites, MainActivity mActivity) {
    this.context = context;
    this.listFavorites = listFavorites;
    mDatabase = new SQLiteDB(context);
    this.mainActivity = mActivity;
    }

Now calling mainActivity in my FavoriteAdapter onclick works:

@Override
public void onBindViewHolder(final FavoriteViewHolder holder, final int mPosition) {
    final Favorite singleFavorite = listFavorites.get(mPosition);

    view.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mainActivity.onLatLngData(52.246218, 20.970564);
        }
    });