Wraps another {@link ActivityMapper} and caches the last activity it returned, to be re-used if we see the same place twice.

class CachingActivityMapper implements ActivityMapper {

 final ActivityMapper wrapped;

 Place lastPlace;
 Activity lastActivity;

 /**
  * Constructs a CachingActivityMapper object.
  *
  * @param wrapped an ActivityMapper object
  */
 CachingActivityMapper(this.wrapped);

 Activity getActivity(Place place) {
   if (place != lastPlace) {
     lastPlace = place;
     lastActivity = wrapped.getActivity(place);
   }

   return lastActivity;
 }
}