FirebaseRecyclerAdapter 问题，删除数据时空对象引用

Question

Firebase 数据库的显示工作正常，但是当我尝试从数据库中删除项目时，应用程序崩溃，尽管删除发生了。写入字符串中的空对象引用：

String postDescription = dataSnapshot.child("desc").getValue().toString();

这是我的代码：

public class PostFragment extends Fragment {

private RecyclerView recyclerPost;

private DatabaseReference postReference;

private View view;
private LinearLayoutManager layoutManager;

public PostFragment() {
}

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

    view = inflater.inflate(R.layout.fragment_post, container, false);

    recyclerPost = view.findViewById(R.id.recycler_post);

    postReference = FirebaseDatabase.getInstance().getReference().child("Posts");
    postReference.keepSynced(true);

    layoutManager = new LinearLayoutManager(getContext());
    layoutManager.setReverseLayout(true);
    layoutManager.setStackFromEnd(true);
    recyclerPost.setHasFixedSize(true);
    recyclerPost.setLayoutManager(layoutManager);

    return view;
}

@Override
public void onStart() {
    super.onStart();
    Query query = postReference.orderByChild("timestamp");

    FirebaseRecyclerOptions<Posts> options =
            new FirebaseRecyclerOptions.Builder<Posts>()
                    .setQuery(query, Posts.class)
                    .build();

    FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Posts, PostViewHolder>(options) {

        @Override
        protected void onBindViewHolder(@NonNull final PostViewHolder holder, int position, @NonNull final Posts model) {

            final String postId = getRef(position).getKey();
            postReference.child(postId).addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    String postDescription = dataSnapshot.child("desc").getValue().toString();
                    holder.postDesc.setText(postDescription);                   
                }

                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                }
            });

            holder.delBtn.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    postReference.child(postId).removeValue();
                }
            });


        }

        @NonNull
        @Override
        public PostViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int position) {
            View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.blog_item, viewGroup, false);
            return new PostViewHolder(view);
        }

    };

    adapter.startListening();
    recyclerPost.setAdapter(adapter);
}

public class PostViewHolder extends RecyclerView.ViewHolder {

    private TextView postDesc;
    private Button delBtn;
    private View view;

    public PostViewHolder(@NonNull View itemView) {
        super(itemView);
        view = itemView;
        postDesc = (TextView) view.findViewById(R.id.post_description); 
        delBtn = (Button) view.findViewById(R.id.del_post_btn);

    }
}
}

请告诉我这个问题的解决方法。

Answer 1

您可以尝试以下方法：

            postReference.child(postId).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                  if(dataSnapshot.exists()){
                String postDescription = dataSnapshot.child("desc").getValue().toString();
                holder.postDesc.setText(postDescription);                   
            }
          }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

您可以使用方法 exists():

public boolean exists ()

Returns true if the snapshot contains a non-null value.

它将检查数据快照是否在数据库，如果它不在数据库中那么你可以添加一个小吃店或吐司。

更多信息在这里：

https://firebase.google.com/docs/reference/android/com/google/firebase/database/DataSnapshot#exists()

Answer 2

每次将 post 添加到 adapter/list 视图时，您现在都会在 onBindViewHolder 中创建一个带有 addValueEventListener 的侦听器。此侦听器从数据库中获取值，然后继续侦听更改，直到您将其删除。由于您永远不会删除这些侦听器，因此随着时间的推移它们会累加起来，事情可能会变得不同步。

最简单的解决方案是使用 addListenerForSingleValueEvent:

@Override
protected void onBindViewHolder(@NonNull final PostViewHolder holder, int position, @NonNull final Posts model) {

    final String postId = getRef(position).getKey();
    postReference.child(postId).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            String postDescription = dataSnapshot.child("desc").getValue().toString();
            holder.postDesc.setText(postDescription);                   
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            throw databaseError.toException(); // don't ignore errors
        }
    });

当您使用 addListenerForSingleValueEvent 时，侦听器会在 onDataChange 第一次触发后立即被删除。

FirebaseRecyclerAdapter 问题，删除数据时空对象引用

Issue with FirebaseRecyclerAdapter, on a null object reference when delete data

android

firebase

android-recyclerview

firebase-realtime-database

firebaseui