Firebase RecyclerView无法加载新数据

每次用户将项目添加到Firebase中的某个数据库引用时，我都会尝试创建一个实时的RecyclerView . 这是我的模型类：

@IgnoreExtraProperties
public class Poll {

private String question;
private String image_URL;

public Poll() {
}

public Poll(String Question, String Image_URL){
    this.question = Question;
    this.image_URL = Image_URL;
}


public String getQuestion() {
    return question;
}

public void setQuestion(String question) {
    question = question;
}

public String getImage_URL() {
    return image_URL;
}

public void setImage_URL(String image_URL) {
    image_URL = image_URL;
}

@Exclude
public Map<String, Object> toMap(){
    HashMap<String, Object> result = new HashMap<>();
    result.put("question", question);
    result.put("image_URL", image_URL);
    return result;
}

这是我将数据保存到Firebase的类 . 它实际上是写入Firebase，所以我可以确认我正在写它：

mSubmitPollCreation.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            //TODO: Need to check if poll requirements are added, i.e. Question, Answer, ......
            //check if image has been loaded first
            if (resultImageURL == null){
                Toast.makeText(getApplicationContext(),getResources().getString(R.string.no_image_selected),Toast.LENGTH_LONG).show();
                return;
            }

            Poll poll = new Poll(mCreatePollQuestion.getText().toString(), resultImageURL);
            Map <String, Object> pollMap = poll.toMap();
            String key = mBaseRef.child("Polls").push().getKey();
            Map<String, Object> childUpdates = new HashMap<String, Object>();
            childUpdates.put("/Polls/" + key, pollMap);
            mBaseRef.updateChildren(childUpdates);

            Intent toHomeActivity = new Intent(CreateActivity.this, HomeActivity.class);
            toHomeActivity.putExtra("viewpager_position", 2);
            startActivity(toHomeActivity);
        }
    });

我希望在添加项目时，recyclerview在所有设备上实时更新 . 这是我用来使用Firebase UI库填充RecyclerView的片段 . 当我向前面的代码段中列出的Firebase添加数据/写入时，我没有在任何设备上看到任何实时更新：

public class LiveFragment extends Fragment {
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";

private RecyclerView mRecyclerview;
private DatabaseReference mBaseRef;
private DatabaseReference mPollsRef;
private LinearLayoutManager mLayoutManager;

private FirebaseRecyclerAdapter <Poll, PollHolder> mFireAdapter;


// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;

private OnFragmentInteractionListener mListener;

public LiveFragment() {
    // Required empty public constructor
}

/**
 * Use this factory method to create a new instance of
 * this fragment using the provided parameters.
 *
 * @param param1 Parameter 1.
 * @param param2 Parameter 2.
 * @return A new instance of fragment LiveFragment.
 */
// TODO: Rename and change types and number of parameters
public static LiveFragment newInstance(String param1, String param2) {
    LiveFragment fragment = new LiveFragment();
    Bundle args = new Bundle();
    args.putString(ARG_PARAM1, param1);
    args.putString(ARG_PARAM2, param2);
    fragment.setArguments(args);
    return fragment;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (getArguments() != null) {
        mParam1 = getArguments().getString(ARG_PARAM1);
        mParam2 = getArguments().getString(ARG_PARAM2);

    }
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    final View v = inflater.inflate(R.layout.fragment_new, container, false);
    mRecyclerview = (RecyclerView) v.findViewById(R.id.new_RecyclerView);
    mBaseRef = FirebaseDatabase.getInstance().getReference();
    mPollsRef = mBaseRef.child("Polls");

    if (mRecyclerview != null){
        mRecyclerview.setHasFixedSize(true);
    }

    mLayoutManager = new LinearLayoutManager(getActivity().getApplicationContext());
    mLayoutManager.setOrientation(LinearLayoutManager.VERTICAL
    );

    mRecyclerview.setLayoutManager(mLayoutManager);
    return v;
}

// TODO: Rename method, update argument and hook method into UI event
public void onButtonPressed(Uri uri) {
    if (mListener != null) {
        mListener.onFragmentInteraction(uri);
    }
}

@Override
public void onStart() {
    super.onStart();

    mFireAdapter = new FirebaseRecyclerAdapter<Poll, PollHolder>(
            Poll.class,
            R.layout.latest_item,
            PollHolder.class,
            mPollsRef
    ){

        @Override
        protected void populateViewHolder(PollHolder viewHolder, Poll model, int position) {
            viewHolder.mPollQuestion.setText(model.getQuestion());
            Picasso.with(getActivity().getApplicationContext())
                    .load(model.getImage_URL())
                    .fit()
                    .into(viewHolder.mPollImage);
            Log.v("QUESTION", model.getQuestion());
            Log.v("IMAGE", model.getImage_URL());

        }
    };

    mRecyclerview.setAdapter(mFireAdapter);
}

@Override
public void onStop() {
    super.onStop();
    if (mFireAdapter != null){
        mFireAdapter.cleanup();
    }
}

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

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


public static class PollHolder extends RecyclerView.ViewHolder {

    TextView mPollQuestion;
    ImageView mPollImage;


    public PollHolder(View itemView) {
        super(itemView);

        mPollQuestion = (TextView) itemView.findViewById(R.id.latest_item_question);
        mPollImage = (ImageView) itemView.findViewById(R.id.pollThumbNailImage);

    }
}

/**
 * This interface must be implemented by activities that contain this
 * fragment to allow an interaction in this fragment to be communicated
 * to the activity and potentially other fragments contained in that
 * activity.
 * <p>
 * See the Android Training lesson <a href=
 * "http://developer.android.com/training/basics/fragments/communicating.html"
 * >Communicating with Other Fragments</a> for more information.
 */
public interface OnFragmentInteractionListener {
    // TODO: Update argument type and name
    void onFragmentInteraction(Uri uri);
   }
}

编辑：我认为由于某些原因，写入Firebase的数据为空，因为 Question 和 ImageURL 的日志没有返回任何数据 .