Convert callback function to Observables

Hii, how to convert this callback function to Observables…

interface GeoQuery {
center(): number[];
radius(): number;
updateCriteria(criteria: GeoQueryUpdateCriteria);
on(eventType: EventType, callback: (key:string, location: number[], distance: number) => void): GeoCallbackRegistration;
cancel();
}

from https://github.com/daveozoalor/geofire-firebase-not-working-in-ionic-fix