how should I implement a continously GPS tracking function?

I want to be able to track a phone. How should I implement it to call doSomething(latitude,longitude) as frequent as possible? can I do this without a timer, so I get the GPS location as frequent as possible? Do you have an implementation of such code, or how would you do it?

on iOS, background services are time limited, often just to a few minutes, as documented here.

For android, take a look to this. Note that implementation is different for each platform.