如何在 Android 中使用 LiveData 和 ViewModel 不断更新 Firebase 实时数据库的查询

Question

I use Firebase Realtime Database and LiveData with ViewModel in Android and I would like to constantly update the query.

Here is the ViewModel class:

public class ViewModel_MainActivity extends ViewModel {


    /*
    LiveData with Database query for the Firebase node "Ratings"
    */

    private static long currentTimeMillisRating = System.currentTimeMillis();
    private static double pastMinuteForDisplayingRatings = 1;
    private static long pastTimeMillisRatings = System.currentTimeMillis() - (long) (pastMinuteForDisplayingRatings * 60 * 60 * 1000);


    private static final Query QUERY_RATINGS =
            FirebaseDatabase.getInstance(DB_SQLite_Firebase_Entries.FIREBASE_URL).getReference().child(DB_SQLite_Firebase_Entries.FIREBASE_NODE_RATINGS).orderByChild(DB_SQLite_Firebase_Entries.FIREBASE_RATINGDATEINMILLISECONDS).endAt(pastTimeMillisRatings);


    private final LiveData_FirebaseRating liveData_firebaseRating = new LiveData_FirebaseRating(QUERY_RATINGS);

    @NonNull
    public LiveData_FirebaseRating getDataSnapshotLiveData_FirebaseRating() {
        Log.e("LogTag_ViMo", "pastTimeMillisRatings: " + pastTimeMillisRatings);
        return liveData_firebaseRating;
    }


}

Here is the LiveData class:

public class LiveData_FirebaseRating extends LiveData<DataSnapshot> {
    private static final String LOG_TAG = "LiveData_FirebaseRating";

    private Query query;
    private final LiveData_FirebaseRating.MyValueEventListener listener = new LiveData_FirebaseRating.MyValueEventListener();

    DataSnapshot currentDataSnapShotFromFirebase;

    public LiveData_FirebaseRating(Query query) {
        this.query = query;
    }

    public LiveData_FirebaseRating(DatabaseReference ref) {
        this.query = ref;
    }

    public void changeQuery(Query newQuery) {
        this.query = newQuery;
    }

    public DataSnapshot getCurrentDataSnapShotFromFirebase() {
        return currentDataSnapShotFromFirebase;
    }

    public void setCurrentDataSnapShotFromFirebase(DataSnapshot currentDataSnapShotFromFirebase) {
        this.currentDataSnapShotFromFirebase = currentDataSnapShotFromFirebase;
    }

    @Override
    protected void onActive() {
        Log.d(LOG_TAG, "onActive");
        query.addValueEventListener(listener);
    }

    @Override
    protected void onInactive() {
        Log.d(LOG_TAG, "onInactive");
        query.removeEventListener(listener);
    }

    private class MyValueEventListener implements ValueEventListener {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {


            setValue(dataSnapshot);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.e(LOG_TAG, "Can't listen to query " + query, databaseError.toException());
        }
    }
}

And here is the part of the main activity, where the live data and view model are created and observed:

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



        binding = ActivityMainBinding.inflate(getLayoutInflater());
        View view = binding.getRoot();
        setContentView(view);
       
        /*
        Initiate View Model with LiveData for Firebase
         */



        rootRef_Firebase = FirebaseDatabase.getInstance(DB_SQLite_Firebase_Entries.FIREBASE_URL).getReference();
        viewModel = new ViewModelProvider(this).get(ViewModel_MainActivity.class);

        liveData_firebaseRating = viewModel.getDataSnapshotLiveData_FirebaseRating();
        liveData_firebaseRating.observe(this, new Observer<DataSnapshot>() {

            @Override
            public void onChanged(@Nullable DataSnapshot dataSnapshot) {
                liveData_firebaseRating.setCurrentDataSnapShotFromFirebase(dataSnapshot);
                if(liveData_firebaseRating.getCurrentDataSnapShotFromFirebase()!=null) {
                    //Do something with the dataSnapshot of the query
                }
            }// end onChange
        }); //end observe



    }

So my problem lies in the non-updating of the QUERY_RATINGS in the class ViewModel_MainActivity . What I want is to return all the nodes from the FIREBASE_NODE_RATINGS whose at attribute FIREBASE_RATINGDATEINMILLISECONDS is older than 1 Minute (meaning that the entry was created at least 1 Minute before the current time slot). How can I do that such that I always get these nodes?

Answer 1

Firebase Query objects are immutable, and their parameters are fixed values. So there is no way to update the existing query, nor is there a "one minute ago" value that automatically updates.

The options I can quickly think off:

• Create a new query frequently to capture the new full range of relevant nodes.
• Create a new query frequently to capture only the new nodes (by combining startAt and endAt ) and merge them with the results you already have.
• Remove the condition from the query, and perform the filtering in your application code.

Based on what I know, I'd probably start with the third option as it seems the simplest.