Android-CardView未显示在连接到Firebase的RecyclerView上

我正在构建一个RecyclerViewin Fragment，它从Firebase数据库引入数据，并假设将它们显示在上CardView。我编写了所有代码，如下所示，但运行时出现的只是一个空的RecyclerViewFragment，该方法getItemCount()始终返回0。

card_item.xml

<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:card_view="http://schemas.android.com/apk/res-auto"
android:id="@+id/cardView"
android:layout_width="match_parent"
android:layout_height="200dp"
android:layout_margin="10dp"
android:orientation="horizontal"
card_view:cardCornerRadius="5dp"
card_view:cardElevation="5dp">

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorPrimary">

    <TextView
        android:id="@+id/primaryText"
        android:layout_width="300dp"
        android:layout_height="60dp"
        android:textColor="@color/colorPrimary"
        android:textSize="24sp" />

    <TextView
        android:id="@+id/subText"
        android:layout_width="300dp"
        android:layout_height="60dp"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_centerVertical="true"
        android:textColor="@color/colorPrimary"
        android:textSize="14sp" />

    <TextView
        android:id="@+id/rateValue"
        android:layout_width="300dp"
        android:layout_height="60dp"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_below="@+id/subText"
        android:textColor="@color/colorPrimary"
        android:textSize="24sp" />


</RelativeLayout>

fragment_profile.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#F1F1F1"
android:orientation="vertical">

<android.support.v7.widget.RecyclerView
    android:id="@+id/placesRecycler"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

PlacesModel.java

public class PlaceModel {
private String mPrimaryText, mSubText, mRateValue;

public PlaceModel() {
}

public PlaceModel(String mCardImage, String mPrimaryText,
                  String mSubText, String mRateValue) {
    //this.mCardImageURL = mCardImage;
    this.mPrimaryText = mPrimaryText;
    this.mSubText = mSubText;
    this.mRateValue = mRateValue;

}

 public void setmPrimaryText(String mPrimaryText) {
    this.mPrimaryText = mPrimaryText;
}

public void setmSubText(String mSubText) {
    this.mSubText = mSubText;
}

public void setmRateValue(String mRateValue) {
    this.mRateValue = mRateValue;
}


public String getmPrimaryText() {
    return mPrimaryText;
}

public String getmSubText() {
    return mSubText;
}

public String getmRateValue() {
    return mRateValue;
}}

PlacesAdapter.java

public class PlacesAdapter extends RecyclerView.Adapter<PlacesAdapter.PlacesViewHolder> {

private ArrayList<PlaceModel> cardContents;
private Context context;

public PlacesAdapter(Context context, ArrayList<PlaceModel> cardContents) {
    this.cardContents = cardContents;
    this.context = context;
}

@Override
public PlacesViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(context).inflate(R.layout.card_item, parent, false);
    return new PlacesViewHolder(view);
}

@Override
public void onBindViewHolder(PlacesViewHolder holder, int position) {

    PlaceModel place = cardContents.get(position);
    holder.primaryText.setText(place.getmPrimaryText());
    holder.subText.setText(place.getmSubText());
    holder.rateValue.setText(place.getmRateValue());
}


@Override
public int getItemCount() {
    return cardContents.size();
}

public class PlacesViewHolder extends RecyclerView.ViewHolder {

    CardView cardView;
    public TextView primaryText, subText, rateValue;


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


        cardView = (CardView) itemView.findViewById(R.id.cardView);
        primaryText = (TextView) itemView.findViewById(R.id.primaryText);
        subText = (TextView) itemView.findViewById(R.id.subText);
        rateValue = (TextView) itemView.findViewById(R.id.rateValue);

    }

}}

FirebaseConnector.java

  public class FirebaseConnector {
DatabaseReference db;
PlaceModel placeModel = new PlaceModel();
ArrayList<PlaceModel> cardContent = new ArrayList<>();


public FirebaseConnector(DatabaseReference db) {
    this.db = db;

}

public ArrayList<PlaceModel> retrieve() {

    db.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            fetchData(dataSnapshot);
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
            fetchData(dataSnapshot);
        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    return cardContent;
}

private void fetchData(DataSnapshot dataSnapshot) {
    cardContent.clear(); //clear card content from last usage

    for (DataSnapshot ds : dataSnapshot.getChildren()) {

        placeModel = ds.child("Place Model").getValue(PlaceModel.class);
        cardContent.add(placeModel);
     }

 }}

ProfileFragment.java

public class ProfileFragment 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 FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;
private RecyclerView placesRecycler;
private PlacesAdapter placesAdapter;
private FirebaseConnector connector;
private DatabaseReference ref;


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


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


// TODO: Rename and change types and number of parameters
public static ProfileFragment newInstance(String param1, String param2) {
    ProfileFragment fragment = new ProfileFragment();
    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
    View v = inflater.inflate(R.layout.fragment_profile, container, false);
    mAuth = FirebaseAuth.getInstance();
    //Initialize Database..
    ref = FirebaseDatabase.getInstance().getReference();
    connector = new FirebaseConnector(ref);

    //Initialize RecyclerView
    placesRecycler = (RecyclerView)v.findViewById(R.id.placesRecycler);
    placesRecycler.setLayoutManager(new LinearLayoutManager(this.getActivity()));
    //Adapter
    placesAdapter = new PlacesAdapter(this.getActivity(), connector.retrieve());
    placesRecycler.setAdapter(placesAdapter);

    Toast.makeText(getActivity(), "We have "+placesAdapter.getItemCount()+" cards", Toast.LENGTH_LONG).show();


    return v;
}

@Override
public void onStart() {
    super.onStart();
    if (mAuth.getCurrentUser() == null) {
        startActivity(new Intent(getActivity(), LoginActivity.class));
    }
}

public void updateUI() {

    if (mAuth.getCurrentUser() == null) {
        startActivity(new Intent(getActivity(), LoginActivity.class));
    }

}}

数据库试用结构 数据库试用结构