question: public synchronized GcmHelper addRegistrationCallback ( @ NonNull Context context , @ NonNull GcmRegistrationListener registrationListener , boolean sticky ) { if ( null == registrationCallbacks ) { registrationCallbacks = new ConcurrentLinkedQueue < > ( ) ; } registrationCallbacks . add ( registrationListener ) ; if ( sticky && ! TextUtils . isEmpty ( this . pushToken ) ) { registrationListener . onTokenAvailable ( context , pushToken , false ) ; } return this ; }
target:  Add a GCM Registration listener. This does not replace the existing listeners but adds to the list of listeners