Android :: Service Callback To Activity

Our application will expose a Service that can be called by Activities in other people's applications.In many cases, the parent applications calling Activity may be paused before our Service completes. I am looking for the best way for a Service to communicate back to the calling Activity that may have been paused.These are the known options:

(1) Require calling Activities to have a registerReceiver() with a custom action and broadcast to that from our Service. The only way to secure this registerReceiver() is with a signature-based permission.As our Service communicates with any number of unknown 3rd party apps,we can't sign our Service's parent app with all these unknown certificates. These apps would therefore be exposing an unsecured registerReceiver() on their Activity. Would ideally like to avoid requiring this.

(2) Create a PendingIntent to send results back to the activity and give it to our Service. Our Service would send data to calling Activity's onActivityResult(). Each time the result is delivered, the calling Activity will go through onPause() and onResume() but this should be OK.

(3) The calling Activities could create a Handler. The Activity would then create a Messenger pointing to that Handler and send it to our service. Our Service can then use the Messenger to deliver our message back to the calling Activity.