Android FirebaseRecyclerAdapter populateViewHolder() 永远不会被调用

我有一个如下所示的 firebase 数据结构。我正在尝试使用 FirebaseRecyclerAdapter 加载此数据。问题是 populateViewHolder() 没有被调用。对数据库结构的引用有效。

我可以使用 FriendsRef.addValueEventListener 读取数据。我希望使用 FirebaseRecyclerAdapter 避免什么。（代码如下）

数据结构为：

-users  
  |-<uid>
     |-friends
         |-<uid>:name1
         |-<uid>:name2

主 Activity 中的身份验证调用

private void firebaseAuthWithGoogle(GoogleSignInAccount acct) {

        AuthCredential credential = GoogleAuthProvider.getCredential(acct.getIdToken(), null);
        mAuth.signInWithCredential(credential)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        Log.d(getString(R.string.logtag), "signInWithCredential:onComplete:" + task.isSuccessful());

                        // If sign in fails, display a message to the user. If sign in succeeds
                        // the auth state listener will be notified and logic to handle the
                        // signed in user can be handled in the listener.
                        if (!task.isSuccessful()) {
//                            Log.w(TAG, "signInWithCredential", task.getException());
                            Toast.makeText(HomeActivity.this, "Authentication failed.",
                                    Toast.LENGTH_SHORT).show();
                        }
                        // ...
                    }
                });
    }

片段代码：

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

    setHasOptionsMenu(true);
    View view = inflater.inflate(R.layout.friends_fragment, container, false);
    RecyclerView friendRecyclerView = (RecyclerView) view.findViewById(R.id.friends_recycler_view);

    friendRecyclerView.setHasFixedSize(true);
    friendRecyclerView.setLayoutManager(new LinearLayoutManager(mActivity));

    DatabaseReference friendsRef = mActivity.getRootRef().child("users").child(mActivity.getAuth().getCurrentUser().getUid()).child("friends");

    FirebaseRecyclerAdapter<String, ViewHolder> recyclerViewAdapter = new FirebaseRecyclerAdapter<String, ViewHolder>(String.class, R.layout.friends_list_item, ViewHolder.class,  friendsRef) {
        @Override
        protected void populateViewHolder(ViewHolder viewHolder, String model, int position) {
            viewHolder.friendName.setText(model);
        }
    } ;

    friendRecyclerView.setAdapter(recyclerViewAdapter);

    return view;
}

@Override
protected int getContentView() {
    return R.layout.friends_fragment;
}

public static class ViewHolder extends RecyclerView.ViewHolder{
    private TextView friendName;
    public ViewHolder(View itemView) {
        super(itemView);
        friendName=(TextView) itemView.findViewById(R.id.friends_name_list_item);
    }


}

FriendsRef.addValueEventListener：

DatabaseReference friendsRef = mActivity.getRootRef().child("users").child(mActivity.getAuth().getCurrentUser().getUid()).child("friends");

        friendsRef.addValueEventListener(new ValueEventListener() {
                                             @Override
                                             public void onDataChange(DataSnapshot dataSnapshot) {

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

                                                     Log.d("tag", data.toString());
                                                 }


                                             }

只需将 xml 中的 RecyclerView 的 layout_height 更改为“match_parent”