How know if a Firebase list is void?

Question

I use the ChildEventListener following the doc. During the data loading, I just make visible a progress drawable in a listview

• If the list is not empty, then I hide/null the progress view.

• If the list is void, I display "no item"

  // Get a reference to our posts
  Firebase ref = new Firebase("https://docs-examples.firebaseio.com/web/saving-data/fireblog/posts");
  
  ref.addChildEventListener(new ChildEventListener() {
      // Retrieve new posts as they are added to the database
      @Override
      public void onChildAdded(DataSnapshot snapshot, String previousChildKey) {
          BlogPost newPost = snapshot.getValue(BlogPost.class);
          System.out.println("Author: " + newPost.getAuthor());
          System.out.println("Title: " + newPost.getTitle());
      }
  
      //... ChildEventListener also defines onChildChanged, onChildRemoved,
      //    onChildMoved and onCanceled, covered in later sections.
  });
  

With ref.addValueEventListenerwe can just check that the datasnapshotis null to know it.

How do know that the list is void ? Is it possible ? Without first checking with addValueEventListener ?

Answer 1

You can only know that there are no items in the list by attaching a ValueEventListener.

ref.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        if (!snapshot.hasChildren()) {
            System.out.println("List is empty");
        }
    }
    @Override
    public void onCancelled(FirebaseError error) {
    }
}

But keep in mind that usually if you call addChildEventListener(), you'll end up storing the items in some internal storage such as a List. You can also simply check if that internal storage is empty (after onChildRemoved() is triggered) and print the message there.