如何在不监听事件的情况下从firebase数据库检索数据[重复]

Question

 @Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View offersView = inflater.inflate(R.layout.fragment_offers, container, false);

    mOffersRecyclerView = offersView.findViewById(R.id.rv_offers);
    mAddCard = offersView.findViewById(R.id.fab_add_card);

    setUp();

    return offersView;
}

@Override
public void setPresenter(OffersContract.Presenter presenter) {
    mPresenter = presenter;
}

@Override
public void setUp() {
    mOffersList = mPresenter.getOffersList();
    mOffersAdapter = new OffersAdapter(getActivity(), mOffersList);

    mOffersRecyclerView.setAdapter(mOffersAdapter);
    RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());
    mOffersRecyclerView.setLayoutManager(layoutManager);
    mOffersRecyclerView.setHasFixedSize(false);

    performActions(mAddCard);
}

 @Override
public List<Offers> getOffersList() {

    return databaseHelper.getOffers();
}

DatabaseHelper->getOffers（）

public List<Offers> getOffers() {
    DatabaseReference offerReference = getDatabase().child("offers");

    offerReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            final Iterable<DataSnapshot> children = dataSnapshot.getChildren();

            for (DataSnapshot d : children) {
                offersList.add(d.getValue(Offers.class));
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    return offersList;
}

Answer 1

firebase回调是异步的，因此getoffers方法在被回调更新之前将返回null。您必须将适配器实例和列表实例传递给firebase回调，以便它从那里更新，请尝试以下机制：在活动中初始化列表

@Override
public void setUp() {
    mOffersList = new ArrayList<Offers> ();
    mOffersAdapter = new OffersAdapter(getActivity(), mOffersList);

    mOffersRecyclerView.setAdapter(mOffersAdapter);
    RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());
    mOffersRecyclerView.setLayoutManager(layoutManager);
    mOffersRecyclerView.setHasFixedSize(false);

    performActions(mAddCard);
}

更新getter

@Override
public void getOffersList(List<Offers> offers, OffersAdapter adapter) {

    return databaseHelper.getOffers(offers, adapter);
}

Helper将如下所示：

public void getOffers(List<Offers> offers,OffersAdapter adapter) {
    if (offers == null) offers = new ArrayList<Offers> ();
    DatabaseReference offerReference = getDatabase().child("offers");

    offerReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            final Iterable<DataSnapshot> children = dataSnapshot.getChildren();

            for (DataSnapshot d : children) {
                offers.add(d.getValue(Offers.class));
            }
            adapter.notifyDataSetChanged();
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}