Populating ListView (or RecyclerView) based on Firebase cross-referenced results with a Geofire node

ExampleFragment.java

public class ExampleFragment extends Fragment implements GoogleApiClient.ConnectionCallbacks,
        GoogleApiClient.OnConnectionFailedListener,
        ResultCallback<LocationSettingsResult>,
        SharedPreferences.OnSharedPreferenceChangeListener {

    private Callbacks mCallbacks;
    private Snackbar snackbar;

    public interface Callbacks {
        /**
         * Callback for when an item has been selected.
         */
        public void onItemSelected(Item deal, View view);
    }

    /**
     * A dummy implementation of the {@link Callbacks} interface that does
     * nothing. Used only when this fragment is not attached to an activity.
     */
    private static Callbacks sDummyCallbacks = new Callbacks() {
        @Override
        public void onItemSelected(Item deal, View view) {
        }
    };

    /**
     * The desired interval for location updates. Inexact. Updates may be more or less frequent.
     */
    public static final long UPDATE_INTERVAL_IN_MILLISECONDS = 10000;
    /**
     * The fastest rate for active location updates. Exact. Updates will never be more frequent
     * than this value.
     */
    public static final long FASTEST_UPDATE_INTERVAL_IN_MILLISECONDS =
            UPDATE_INTERVAL_IN_MILLISECONDS / 2;
    public static final String GEOFIRE_CHILD = "geofire";
    public static final String DEALS_CHILD = "items";
    protected static final int REQUEST_CHECK_SETTINGS = 0x1;
    private static final String SELECTED_KEY = "selected_position";

    private final String LOG_TAG = ExampleFragment.class.getSimpleName();
    public Firebase mFirebaseRef;
    /**
     * Stores parameters for requests to the FusedLocationProviderApi.
     */
    protected LocationRequest mLocationRequest;
    /**
     * Stores the types of location services the client is interested in using. Used for checking
     * settings to determine if the device has optimal location settings.
     */
    protected LocationSettingsRequest mLocationSettingsRequest;

    //Main Screen Views
    ListView mListView;

    @InjectView(R.id.MainLayout)
    LinearLayout mMainLayout;
    @InjectView(R.id.list_container)
    MultiStateView mListContainer;
    @InjectView(R.id.header_msg)
    TextView mHeader;

    SharedPreferences settings;
    // global variables
    @Icicle
    int mPosition = ListView.INVALID_POSITION;
    GeoLocation center;
    @Icicle
    boolean mActiveGeoQuery = false;
    private GeoFire geoFire;
    private GeoQuery query;
    private View rootView;
    private ItemListAdapter mItemListAdapter;
    private int counter = 0;
    /* Data from the authenticated user */
    private AuthData mAuthData;
    /* Client used to interact with Google APIs. */
    private GoogleApiClient mGoogleApiClient;
    private Location mLastLocation;
    private double mLatitudeText;
    private double mLongitudeText;
    private Context mContext;

    public ExampleFragment() {
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        Log.d(LOG_TAG, "onCreate");
        super.onCreate(savedInstanceState);
        mContext = getActivity();

        settings = PreferenceManager.getDefaultSharedPreferences(mContext);
        setHasOptionsMenu(true);

        mFirebaseRef = new Firebase(getString(R.string.firebase_url)).child(DEALS_CHILD);
        //I initialize the object that will be used to retrieve the location
        buildGoogleApiClient();
        buildLocationSettingsRequest();
        checkLocationSettings();
        geoFire = new GeoFire(new Firebase(getString(R.string.firebase_url)).child(GEOFIRE_CHILD));

        mItemListAdapter = new ItemListAdapter(mFirebaseRef.equalTo(GEOFIRE_CHILD), getActivity(), R.layout.list_item_items);
    }

    protected synchronized void buildGoogleApiClient() {
        Log.d(LOG_TAG, "buildGoogleApiClient");
        mGoogleApiClient = new GoogleApiClient.Builder(mContext)
                .addConnectionCallbacks(this)
                .addOnConnectionFailedListener(this)
                .addApi(LocationServices.API)
                .build();
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        settings.unregisterOnSharedPreferenceChangeListener(this);
    }

    @Override
    public void onStop() {
        super.onStop();
        if (mGoogleApiClient.isConnected()) {
            mGoogleApiClient.disconnect();
        }
    }

    @Override
    public void onStart() {
        super.onStart();
        Log.d(LOG_TAG, "onStart");

        mGoogleApiClient.connect();
        //TODO: check if location is activated

        center = new GeoLocation(PrefsUtils.getPreferredLat(mContext), PrefsUtils.getPreferredLong(mContext));
        if (center.latitude != 0 && center.longitude != 0 && !mActiveGeoQuery) {
            startGeoQuery();
        } else if (mActiveGeoQuery) {
            Log.d(LOG_TAG, "geoquery already active");
        } else {
            Log.d(LOG_TAG, "center not setted");
            //I first try to set the center at the Last Known Location if retrieved
            if (Double.isNaN(mLatitudeText) && mLatitudeText != 0) {
                Log.d(LOG_TAG, "center setted from fused location");
                center = new GeoLocation(mLatitudeText, mLongitudeText);
                startGeoQuery();

            } 
        }
        checkForItems();
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.d(LOG_TAG, "onResume");

        center = new GeoLocation(PrefsUtils.getPreferredLat(mContext), PrefsUtils.getPreferredLong(mContext));
        if (center.latitude != 0 && center.longitude != 0) {
            Log.d(LOG_TAG, "onResume center setted");
            mItemListAdapter.notifyDataSetChanged();
        } else {
            Log.d(LOG_TAG, "onResume center not setted");
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        rootView = inflater.inflate(R.layout.fragment_main, container, false);

        ButterKnife.inject(this, rootView);

        mListView = (ListView) mListContainer.getContentView();
        mListContainer.setState(MultiStateView.ContentState.LOADING);
        mListContainer.setEmptyString(getString(R.string.empty_list_description));
        mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                Item cursor = (Item) mListView.getItemAtPosition(i);

                if (cursor != null) {
                    mCallbacks.onItemSelected(cursor, view);
                }
                mPosition = i;
            }
        });
        return rootView;
    }

    private void bindListView() {
        mListView.setAdapter(mItemListAdapter);
    }

    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
        Log.d(LOG_TAG, "preference " + key + " change");
        if (query != null) {
            if (key.equals(getString(R.string.search_radius_key))) {
                String newRadius = PrefsUtils.getSearchadius(mContext);
                Log.d(LOG_TAG, "new radius " + newRadius);
                query.setRadius(Double.parseDouble(newRadius));
            }
            // when the location will change it will trigger the change on the latitude and longitude preference
            // I hook up the change of the center in the query to longitude that will be fired last
            if (key.equals(getString(R.string.long_pref_key))) {
                GeoLocation newCenter = new GeoLocation(PrefsUtils.getPreferredLat(mContext), PrefsUtils.getPreferredLong(mContext));
                Log.d(LOG_TAG, "new center " + newCenter.toString());
                query.setCenter(newCenter);
            }
            if (key.equals(getString(R.string.category_pref_key))) {
                String filter = PrefsUtils.getCategoryFilter(mContext);
                Log.d(LOG_TAG, "filter: " + filter);

                mItemListAdapter.getFilter().filter(filter, new Filter.FilterListener() {
                    @Override
                    public void onFilterComplete(int count) {
                        //wait until the the filtering is completed to change the state of the main view
                        isListEmpty();
                    }
                });
                // I update the header view
                if (!filter.isEmpty()) {
                    mHeader.setVisibility(View.VISIBLE);
                    mHeader.setText(filter);
                } else {
                    mHeader.setVisibility(View.GONE);
                }
            }
        }
    }

    private void startGeoQuery() {
        double radius = Double.parseDouble(PrefsUtils.getSearchadius(mContext));
        // safety net in case that for some reasons the returned radius is 0
        if (radius < 1.0) {
            radius = 2.0;
        }
        query = geoFire.queryAtLocation(center, radius);
        Log.d(LOG_TAG, "center: " + center.toString() + ", radius: " + radius);
        query.addGeoQueryEventListener(new GeoQueryEventListener() {
            @Override
            public void onKeyEntered(String key, GeoLocation location) {
                Log.d(LOG_TAG, "Key " + key + " entered the search area at [" + location.latitude + "," + location.longitude + "]");
                Firebase tempRef = mFirebaseRef.child(key);
                tempRef.addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot snapshot) {
                        // I  add the deal only if it doesn't exist already in the adapter
                        String key = snapshot.getKey();
                        if (!mItemListAdapter.exists(key)) {
                            Log.d(LOG_TAG, "item added " + key);
                            mItemListAdapter.addSingle(snapshot);
                            mItemListAdapter.notifyDataSetChanged();
                            mListContainer.setState(MultiStateView.ContentState.CONTENT);
                        } else {
                            //...otherwise I will update the record
                            Log.d(LOG_TAG, "item updated: " + key);
                            mItemListAdapter.update(snapshot, key);
                            mItemListAdapter.notifyDataSetChanged();
                        }
                    }

                    @Override
                    public void onCancelled(FirebaseError firebaseError) {
                        Log.d(LOG_TAG, "cancelled with error:" + firebaseError.getMessage());
                    }
                });
            }

            @Override
            public void onKeyExited(String key) {
                Log.d(LOG_TAG, "deal " + key + " is no longer in the search area");
                mItemListAdapter.remove(key);
                isListEmpty();
            }

            @Override
            public void onKeyMoved(String key, GeoLocation location) {
                Log.d(LOG_TAG, String.format("Key " + key + " moved within the search area to [%f,%f]", location.latitude, location.longitude));
            }

            @Override
            public void onGeoQueryReady() {
                Log.d(LOG_TAG, "All initial data has been loaded and events have been fired!");
                isListEmpty();
                mActiveGeoQuery = true;
            }

            @Override
            public void onGeoQueryError(FirebaseError error) {
                Log.e(LOG_TAG, "There was an error with this query: " + error);
                mListContainer.setState(MultiStateView.ContentState.ERROR_GENERAL);
                mListContainer.setCustomErrorString(mContext.getString(R.string.list_error_message));
                mActiveGeoQuery = false;
            }
        });
        bindListView();
    }

    private void isListEmpty() {
        if (mItemListAdapter.getCount() == 0) {
            mListContainer.setState(MultiStateView.ContentState.EMPTY);
            if (mListView.hasFocus()) {
                mListContainer.setEmptyString(getString(R.string.empty_list_description));
            }
        } else {
            mListContainer.setState(MultiStateView.ContentState.CONTENT);
        }
    }

    private void checkForItems() {
        Handler handler = new Handler();
        handler.postDelayed(new Runnable() {
            public void run() {
                if (mListView.hasFocus()) {
                    if (mItemListAdapter.getCount() == 0) {
                        mListContainer.setState(MultiStateView.ContentState.EMPTY);
                        mListContainer.setEmptyString(getString(R.string.empty_list_description));
                    }
                }
            }
        }, 8000);
    }

    public void filterList(String query) {
        if (query.equals("")) {
            mItemListAdapter = new ItemListAdapter(mFirebaseRef.limitToFirst(20), getActivity(), R.layout.list_item_items);
        } else {
            mItemListAdapter = new ItemListAdapter(mFirebaseRef.orderByValue().startAt(query), getActivity(), R.layout.list_item_items);
        }
        bindListView();
    }

    @Override
    public void onConnected(Bundle bundle) {
        Log.d(LOG_TAG, "connected");
        mLastLocation = LocationServices.FusedLocationApi.getLastLocation(
                mGoogleApiClient);
        if (mLastLocation != null) {
            mLatitudeText = mLastLocation.getLatitude();
            mLongitudeText = mLastLocation.getLongitude();
            Log.d(LOG_TAG, "lat: " + mLatitudeText + ", long: " + mLongitudeText);
            center = new GeoLocation(PrefsUtils.getPreferredLat(mContext), PrefsUtils.getPreferredLong(mContext));
            if (!mActiveGeoQuery) {
                PrefsUtils.setPreferredLatLong(mContext, mLatitudeText, mLongitudeText);
                center = new GeoLocation(mLatitudeText, mLongitudeText);
                startGeoQuery();
                mItemListAdapter.notifyDataSetChanged();
            }
        }
    }

    @Override
    public void onConnectionSuspended(int i) {
        // The connection to Google Play services was lost for some reason. We call connect() to
        // attempt to re-establish the connection.
        Log.d(LOG_TAG, "Connection suspended");
        mGoogleApiClient.connect();
    }

    @Override
    public void onConnectionFailed(ConnectionResult connectionResult) {
        Log.d(LOG_TAG, "connection failed" + connectionResult.getErrorCode());
    }

    protected void createLocationRequest() {
        mLocationRequest = new LocationRequest();

        // Sets the desired interval for active location updates. This interval is
        // inexact. You may not receive updates at all if no location sources are available, or
        // you may receive them slower than requested. You may also receive updates faster than
        // requested if other applications are requesting location at a faster interval.
        mLocationRequest.setInterval(UPDATE_INTERVAL_IN_MILLISECONDS);

        // Sets the fastest rate for active location updates. This interval is exact, and your
        // application will never receive updates faster than this value.
        mLocationRequest.setFastestInterval(FASTEST_UPDATE_INTERVAL_IN_MILLISECONDS);

        mLocationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
    }

    protected void buildLocationSettingsRequest() {
        LocationSettingsRequest.Builder builder = new LocationSettingsRequest.Builder();
        builder.addLocationRequest(mLocationRequest);
        mLocationSettingsRequest = builder.build();
    }

    protected void checkLocationSettings() {
        PendingResult<LocationSettingsResult> result =
                LocationServices.SettingsApi.checkLocationSettings(
                        mGoogleApiClient,
                        mLocationSettingsRequest
                );
        result.setResultCallback(this);
    }

    @Override
    public void onResult(LocationSettingsResult locationSettingsResult) {
        final Status status = locationSettingsResult.getStatus();
        switch (status.getStatusCode()) {
            case LocationSettingsStatusCodes.SUCCESS:
                Log.i(LOG_TAG, "All location settings are satisfied.");
                break;
            case LocationSettingsStatusCodes.RESOLUTION_REQUIRED:
                Log.i(LOG_TAG, "Location settings are not satisfied. Show the user a dialog to" +
                        "upgrade location settings ");

                try {
                    // Show the dialog by calling startResolutionForResult(), and check the result
                    // in onActivityResult().
                    status.startResolutionForResult(getActivity(), REQUEST_CHECK_SETTINGS);
                } catch (IntentSender.SendIntentException e) {
                    Log.i(LOG_TAG, "PendingIntent unable to execute request.");
                }
                break;
            case LocationSettingsStatusCodes.SETTINGS_CHANGE_UNAVAILABLE:
                Log.i(LOG_TAG, "Location settings are inadequate, and cannot be fixed here. Dialog " +
                        "not created.");
                break;
            default:
                Log.i(LOG_TAG, "Result not recognized");
        }
    }

}

To be used in conjunction with this Adapter

I am having an issue with ItemListAdapter is abstract and cannot be instantiated.

mItemListAdapter = new ItemListAdapter(mFirebaseRef.equalTo("GeoFire"), getActivity(), R.layout.activity_post_detail);

Here is my ItemListAdapter code.

public abstract class ItemListAdapter<T> extends BaseAdapter implements Filterable {

    private static final String LOG_TAG = "FirebaseListAdapter";
    private Query mRef;
    private Class<T> mModelClass;
    private int mLayout;
    private LayoutInflater mInflater;
    private List<T> mModels;
    private List<T> mFilteredModels;
    private Map<String, T> mModelKeys;
    private Map<String, T> mFilteredKeys;
    private ChildEventListener mListener;
    private Context mContext;
    private ValueFilter valueFilter;


    /**
     * @param mRef        The Firebase location to watch for data changes. Can also be a slice of a location, using some
     *                    combination of <code>limit()</code>, <code>startAt()</code>, and <code>endAt()</code>,
     * @param mModelClass Firebase will marshall the data at a location into an instance of a class that you provide
     * @param mLayout     This is the mLayout used to represent a single list item. You will be responsible for populating an
     *                    instance of the corresponding view with the data from an instance of mModelClass.
     * @param activity    The activity containing the ListView
     */
    public ItemListAdapter(Query mRef, Class<T> mModelClass, int mLayout, Activity activity) {
        this.mRef = mRef;
        this.mModelClass = mModelClass;
        this.mLayout = mLayout;
        mInflater = activity.getLayoutInflater();
        mModels = new ArrayList<>();
        mModelKeys = new HashMap<>();
        // Look for all child events. We will then map them to our own internal ArrayList, which backs ListView
        mListener = this.mRef.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {

                T model = dataSnapshot.getValue(FireBaseListAdapter.this.mModelClass);
                mModelKeys.put(dataSnapshot.getKey(), model);

                // Insert into the correct location, based on previousChildName
                if (previousChildName == null) {
                    mModels.add(0, model);
                } else {
                    T previousModel = mModelKeys.get(previousChildName);
                    int previousIndex = mModels.indexOf(previousModel);
                    int nextIndex = previousIndex + 1;
                    if (nextIndex == mModels.size()) {
                        mModels.add(model);
                    } else {
                        mModels.add(nextIndex, model);
                    }
                }

                notifyDataSetChanged();
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                Log.d(LOG_TAG, "onChildChanged");
                // One of the mModels changed. Replace it in our list and name mapping
                String modelName = dataSnapshot.getKey();
                T oldModel = mModelKeys.get(modelName);
                T newModel = dataSnapshot.getValue(FireBaseListAdapter.this.mModelClass);
                int index = mModels.indexOf(oldModel);

                mModels.set(index, newModel);
                mModelKeys.put(modelName, newModel);

                notifyDataSetChanged();
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                Log.d(LOG_TAG, "onChildRemoved");
                // A model was removed from the list. Remove it from our list and the name mapping
                String modelName = dataSnapshot.getKey();
                T oldModel = mModelKeys.get(modelName);
                mModels.remove(oldModel);
                mModelKeys.remove(modelName);
                notifyDataSetChanged();
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String previousChildName) {
                Log.d(LOG_TAG, "onChildMoved");
                // A model changed position in the list. Update our list accordingly
                String modelName = dataSnapshot.getKey();
                T oldModel = mModelKeys.get(modelName);
                T newModel = dataSnapshot.getValue(FireBaseListAdapter.this.mModelClass);
                int index = mModels.indexOf(oldModel);
                mModels.remove(index);
                if (previousChildName == null) {
                    mModels.add(0, newModel);
                } else {
                    T previousModel = mModelKeys.get(previousChildName);
                    int previousIndex = mModels.indexOf(previousModel);
                    int nextIndex = previousIndex + 1;
                    if (nextIndex == mModels.size()) {
                        mModels.add(newModel);
                    } else {
                        mModels.add(nextIndex, newModel);
                    }
                }
                notifyDataSetChanged();
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.e("FirebaseListAdapter", "Listen was cancelled, no more updates will occur");
            }
        });
    }

What is to be entered for the Model Class in the ItemListAdapter?

I am getting NullPointerException atif (!mFireAdapter.exists(key)) mFireAdapter is FireBaseListAdapter.

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.android.pokechat.FireBaseListAdapter.update(com.google.firebase.database.DataSnapshot, java.lang.String)' on a null object reference
                                                                        at com.android.pokechat.fragment.GeoPostsFragment$3$1.onDataChange(GeoPostsFragment.java:236)

Here it shows the information correctly in my logcat.

07-19 01:49:26.640 7703-7703/com.android.pokechat D/GeoPostsFragment: Key -KMsopZitLFzcxFuPOCn entered the search area at [40.7853889,-122.4056973]
07-19 01:49:26.642 7703-7703/com.android.pokechat D/GeoPostsFragment: Key -KMu2IcjIyMx5Qpb5Kfh entered the search area at [40.7853889,-122.4056973]
07-19 01:49:26.642 7703-7703/com.android.pokechat D/GeoPostsFragment: Key -KMz3knxKsAAixlf4ijY entered the search area at [40.7853889,-122.4056973]
07-19 01:49:26.642 7703-7703/com.android.pokechat D/GeoPostsFragment: Key -KMzXh2Kf7nLdOPqnMpY entered the search area at [40.7853889,-122.4056973]

Is this is where I could be possibly wrong I am using your ExampleAdapter as mItemListAdapter

mItemListAdapter = new ItemListAdapter(mFirebaseRef.equalTo("GeoFire"),getActivity(), R.layout.activity_post_detail);

Please help me resolve the issue I am quite ahead with my project finishing.