Dfyjkt

The display from the Firebase database works fine, but when I try to delete an item from the database, the application crashes, although the deletion occurs. Writes on a null object reference in the string:

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

Here is my code:

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);

 

Please tell me the solution to this problem.

You can try the following:

 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) 

 
 );

You can use the method exists():

public boolean exists ()

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

It will check if the datasnapshot is in the
database, if it is not in the database then you can add a snackbar or a toast.

More information here:

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

Every time a post is added to the adapter/list view, you now create a listener with addValueEventListener in your onBindViewHolder. This listener gets the value from the database, and then keeps listening for changes until you remove it. Since you never remove these listeners, over time they add up and things likely get out of sync.

The simplest solution is to use 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
 
 );

When you use addListenerForSingleValueEvent, the listener is removed right after onDataChange fires for the first time.