onDataChange(), in addListenerForSingleValueEvent(), is only executed after my method returns

Question

I’m trying to check if a given email exists, with android using firebase I’m doing this way :

public boolean existeEmail(String email) {

    final boolean[] retorno = new boolean[1];
    final DatabaseReference databaseArtists = FirebaseDatabase.getInstance().getReference();
    Query query = databaseArtists.child(colecao).orderByChild("email").equalTo(email);

    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.exists())
                retorno[0] = true;
            else
                retorno[0] = false;
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    return retorno[0];
}

Problem

Is running first

return retorno[0];

And then executes:

    query.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if (dataSnapshot.exists())
            retorno[0] = true;
        else
            retorno[0] = false;
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

and is always returning false. How do I first execute this function

 query.addListenerForSingleValueEvent() 

Answer 1

Queries made to Firebasedatabase have their results "delivered" asynchronously.

To receive the result it is necessary to specify a Listener who will receive it, in this case a Valueeventlistener.
That’s what you did when using the method addListenerForSingleValueEvent().

The code inside the parentheses only implements the interface and creates a new object.
Only when the object Query has obtained the result is that the code within the methods onDataChange() or onCancelled() is executed.

As the time required to obtain the result is greater than necessary to execute the method existeEmail(), is returned retorno[0] with the value it was initialized with.

Thus, the treatment of the result has to be done within the methods of implementing the Valueeventlistener interface.

For this there are several solutions:

• write all the code in the implementation inline of the interface

  query.addListenerForSingleValueEvent(new ValueEventListener() {
      @Override
      public void onDataChange(DataSnapshot dataSnapshot) {
          if (dataSnapshot.exists())
  
              //todo o código aqui
          else
              //todo o código aqui
      }
  
      @Override
      public void onCancelled(DatabaseError databaseError) {
          //todo o código aqui
      }
  });
  

• write methods where you place this code and call them in Valueeventlistener methods

  query.addListenerForSingleValueEvent(new ValueEventListener() {
      @Override
      public void onDataChange(DataSnapshot dataSnapshot) {
  
          handleEmail(dataSnapshot.exists()):
  
      }
  
      @Override
      public void onCancelled(DatabaseError databaseError) {
          handleEmailError(databaseError);
      }
  });
  

• Implement Valueeventlistener separately

  public class EmailResultListener() extends ValueEventListener{
      @Override
      public void onDataChange(DataSnapshot dataSnapshot) {
          if (dataSnapshot.exists())
  
              //todo o código aqui, ou chamar método da classe
          else
              //todo o código aqui, ou chamar método da classe
      }
  
      @Override
      public void onCancelled(DatabaseError databaseError) {
          //todo o código aqui, ou chamar método da classe
      }
  }
  

  and use it like this query.addListenerForSingleValueEvent(new EmailResultListener());

The last two approaches allow the reuse of the code.

In short:

To receive the result it is necessary to make available to the Query object, through the method addListenerForSingleValueEvent(), an object that implements the Valueeventlistener interface.
Query will use this object to "deliver" the result by calling the method onDataChange() or, in the event of an error, onCancelled().
The implementation can be created inline (ex. 1 and 2) or in part (e.g. 3).
The code that will treat the result must be, or be called, within the onDataChange().

Answer 2

This happens because the firebase data recovery process runs in the background, or better in the background, so the return will always be false. You can enable a dialog before starting the background processing, and disabling at the end of the run.

dialog.setVisibility(View.VISIBLE);
query.addListenerForSingleValueEvent(new ValueEventListener() {
     @Override
     public void onDataChange(DataSnapshot dataSnapshot) {

        if ( dataSnapshot.exists() ){
            // Notifica usuário
            ...
            // Atualiza tela
            ...
        }

        dialog.setVisibility(View.VISIBLE);

    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
    }
});