It's easy. You can download data for certain areas you expect to be in offline mode and move all routing logic to app. It has been done many times in many apps.

It's easy. You can download data for certain areas you expect to be in offline mode and move all routing logic to app. It has been done many times in many apps. (ok, it only easy conceptually in reality it's quite big chunk of conscious dosign of app as a whole and no lesser chunk of actual work )