Recognising nearby iPhones running my app

I have a requirem开发者_如何学Cent in my iPhone app that I know when somebody running the same app is close by. I'm thinking that GameKit using Bluetooth would let me do that. GPS location is not precise enough in this particular case.

However, if the user closes the app then the function won't work anymore. Even under iOS4, the task switching will close down the BonJour services so I can't have the app run in the background.

Seems to me I can only have this function if the user leaves the app up and running. Would you agree? Or is there a different approach I can take?

People are very concerned with privacy, so they would naturally expect such privacy invading functionality to be switched off when they close down the application.

If they choose to have this feature enabled, then it's a different matter. Then they expect that somebody else may locate them.

You might however consider implementing some sort of opt-in offline service, where the latest gps position is stored on a server including a timestamp. When somebody else (with the app running) moves within a certain distance of this location while it's still "fresh", then you send off a notification to the first device. That way the user would be notified that somebody may be close, and can then switch on their own device.