I am developing an iOS (7.0+) application for fitness (running) that uses the users GPS location, does a small calculation and transmits the data to a Bluetooth Low Energy (4.0) watch. This process needs to be happening in the background, even when the user locks their iOS device.
I have implemented the following background modes as well:
App communicates using CoreBluetooth
App registers for location updates
I have successfully been able to get everything to work well, except for after a certain amount of time (ex. 2 hours) when the device is locked, the iOS device stops updating the location because I can see that it is no longer sending updating GPS values to the Bluetooth watch. I then have to unlock the device, re-open the app and location services works as it should again.
Does anybody know how to keep location services running the whole time in the background (device locked) without it suddenly stopping the location updates after a certain amount of time? If possible, an efficient solution would be preferred that does not drain the battery too much more than it normal would for using GPS.
Most important: make sure you are setting pausesLocationUpdatesAutomatically to NO before you start the CLLocationManager (see docs).
To minimize battery usage you should also use deferred location updates (more details on SO). This allows the CPU to sleep and save battery while the GPS chip collects location updates for you. It periodically dumps them to the CPU. This may not be for you if you need to update the watch once a second, but if you can update it every 10 or 20 or 30 seconds it will save that many CPU wake ups.