CoreBluetooth - iPhone advertising in background mode

Question

I am working on an iPhone and Mac OS X application, which allows you to lock and unlock your Mac via proximity. Means if you the signal strength is under a determined threshold or the connection gets lost it shall lock the mac.

I am working with Apples CoreBluetooth framework for BTLE, using the iPhone as a peripheral and the Mac as a central. So far so good. It also works very well but when I send the app to the background on the iPhone the advertising seems to change. The iPhone still advertises but without the service profile and characteristics, I use in the app. Although this is not a problem when the iPhone is still in the range of the Mac, because it's still connected and the characteristics are not used in the central, it becomes a problem after moving the iPhone out of the range. As expected the Mac locks and starts discovering to reconnect the iPhone and unlock if succeeded.

But in this discovery, I use the specified service profile and the characteristics to only get devices running my app and to identify the one for unlocking.

I tried a workaround by discovering without a service profile and identifying the correct device via its UUID, which I saved when I started to use this iPhone for locking and unlocking. This workaround also works in a small scope, because when I turn off the Bluetooth on the iPhone and turn it on again, it gets another UUID. That's a constraint I could live with, but it also changes the UUID after a few hours and then the unlocking does not work.

Maybe someone already worked on an app like that and know how to fix such a problem? Or you know a static value which I can use to identify the device?

Answer 1

So it seems, like usual, Apple has some weird and unique things going on in their framework. When you advertise from an iOS device (such as your iPhone), there are two "storage areas" for the advertisements -- a normal one that any device that is scanning can see, and an "overflow" one that can only be seen by iOS devices that are specifically scanning for it. When your app advertises in the background, all services UUIDs that you advertise go into this overflow area unfortunately, so it looks like only other iOS devices can see it -- and not your Mac. From the CBPeripheralManager docs:

Any service universally unique identifiers (UUIDs) contained in the value of the CBAdvertisementDataServiceUUIDsKey key that do not fit in the allotted space are added to a special “overflow” area; they can be discovered only by an iOS device that is explicitly scanning for them. While your app is in the background, the local name is not advertised and all service UUIDs are placed in the overflow area.