This is the section from the CLLocationManager documentation describing the app behavior with startMonitoringSignificantLocationChanges:
If you start this service and your
application is subsequently
terminated, the system automatically
relaunches the application into the
background if a new event arrives. In
such a case, the options dictionary
passed to the
application:didFinishLaunchingWithOptions:
method of your application delegate
contains the key
UIApplicationLaunchOptionsLocationKey
to indicate that your application was
launched because of a location event.
Upon relaunch, you must still
configure a location manager object
and call this method to continue
receiving location events. When you
restart location services, the current
event is delivered to your delegate
immediately. In addition, the location
property of your location manager
object is populated with the most
recent location object even before you
start location services.
So my understanding is that if your app terminates (and I assume if you don’t call stopMonitoringSignificantLocationChanges from applicationWillTerminate) you will get woken up with a UIApplicationLaunchOptionsLocationKey parameter to application:didFinishLaunchingWithOptions. At that point you create your CLLocationManager, call startMonitoringSignificantLocationChanges and do your background location processing for a limited time. So I am fine with this bit.
The previous paragraph only talks about what happens when the app is terminated, it doesn’t suggest what you do when the application is suspended. The documentation for didFinishLaunchingWithOptions says:
The application tracks location
updates in the background, was purged,
and has now been relaunched. In this
case, the dictionary contains a key
indicating that the application was
relaunched because of a new location
event.
Suggesting that you will only receive this call when your app is launched (because of a location change) after you have been terminated.
However the paragraph on the Significant Change Service in the Location Awareness Programming Guide has the following to say:
If you leave this service running and
your application is subsequently
suspended or terminated, the service
automatically wakes up your
application when new location data
arrives. At wake-up time, your
application is put into the background
and given a small amount of time to
process the location data. Because
your application is in the background,
it should do minimal work and avoid
any tasks (such as querying the
network) that might prevent it from
returning before the allocated time
expires. If it does not, your
application may be terminated.
This suggests you are woken up with location data if your app has been suspended, but fails to mention how you are woken up:
- Does the UIApplicationDelegate get a callback telling me that I am resuming from a suspended state into a background state?
- Does the location manager (that was freeze dried when the app was suspended) start receiving locationManager:didUpdateToLocation:fromLocation callbacks?
- Do I just need to implement code in my didUpdateToLocation message which checks the application state and does minimal processing if in background mode?
In the process of writing this up, I think I may have just answered my own question, but it would be great to have my understanding of this confirmed by someone more knowledgeable.
Since I asked this question, I have done a fair bit of testing (mostly on the train between home and work) and have confirmed that the behaviour for suspended apps is as I suspected at the end of the question.
That is, your suspended app is woken up, you don’t receive any callbacks on your app delegate, instead you receive your location updates through your existing CLLocationManagerDelegate. You can detect that you are running in the background by checking the applicationState, and do limited work for the case where you are woken from a suspended state to do location processing.
I came to this conclusion with a location test harness that you are welcome to download and try out. It is a pretty simple app that allows you to turn on significant change and GPS change APIs through the UI and log all the responses that you get back.
N.B. Point six in the previous answer is not correct. Freeze dried suspended apps do receive CLLocationManagerDelegate callbacks when they are woken up from a suspended state.