public synchronized GcmHelper addOnMessageReceivedCallback(@NonNull GcmMessageListener messageReceivedListener){
  if (null == messageReceivedCallbacks) {
    messageReceivedCallbacks=new ConcurrentLinkedQueue<>();
  }
  messageReceivedCallbacks.add(messageReceivedListener);
  return this;
}

public synchronized GcmHelper addOnMessageReceivedCallback ( @ NonNull GcmMessageListener messageReceivedListener ) { if ( null == messageReceivedCallbacks ) { messageReceivedCallbacks = new ConcurrentLinkedQueue < > ( ) ; } messageReceivedCallbacks . add ( messageReceivedListener ) ; return this ; }
add a gcm messaging listener . this listener will be triggered when a gcm payload is received
