How does iOS knows which location manager delegate to call on region monitoring while the app is not running?

Question

I might be confusing how region monitoring works, but this is what i have so far:

I am registering a region to monitor through my location manager, which is implemented on a singleton class, this singleton is also set as the delegate of the location manager so the implemented method is being called.

- (void)locationManager:(CLLocationManager *)manager didEnterRegion:(CLRegion *)region

This works totally as expected, if the app is active or suspended the method is being called. It also makes total sense because the class has been already loaded and when the region enter event occurs iOS sends this even to my app which calls the location manager who registered (probably has a reference to it) and in turn it calls whatever delegate was also registered along it (since the class is there ready and loaded).

The issue is, what happens when the app has been killed? Is it first launched into the background? How does iOS know what delegate method to call, and if it has already been loaded?

Answer 1

When your app has been killed and gets started for a location update there can't be a location manager delegate yet and as such there are no notifications delivered to that delegate. The system can't know which of your classes should be used as a location manager delegate or how to instantiate it.

Instead your application:didFinishLaunchingWithOptions: gets called as usual, but the UIApplicationLaunchOptionsLocationKey is set in the options dictionary. That tells your app that you need to instantiate a location manager and set it's delegate. Only after you did this the delegate gets called with the region updates.