NSNotificationCenter callback while app in background - ios

Question

One question and one issue:
I have the following code:
- (void) registerForLocalCalendarChanges
{
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(localCalendarStoreChanged) name:EKEventStoreChangedNotification object:store ];
}
- (void) localCalendarStoreChanged
{
// This gets call when an event in store changes
// you have to go through the calendar to look for changes
[self getCalendarEvents];
}
These methods are in a class/object called CalendarEventReporter which contains the method getCalendarEvents (in the callback).
Two things:
1) If the app is in the background the callback does not run. Is there a way to make it do that?
2) When I bring the app back into the foreground (after having changed the calendar on the device) the app crashes without any error message in the debug window or on the device. My guess is that the CalendarEventReporter object that contains the callback is being garbage-collected. Is that possible? Any other thoughts on what might be causing the crash? Or how to see any error messages?

Answer 1

1) In order for the app to run in the background you should be using one of the modes mentioned in the "Background Execution and Multitasking section here:
uses location services
records or plays audio
provides VOIP
services
background refresh
connection to external devices
like through BLE
If you are not using any of the above, it is not possible to get asynchronous events in the background.
2) In order to see the crash logs/call stack place an exception breakpoint or look into the "Device Logs" section here: Window->Organizer->Devices->"Device Name" on left->Device Logs on Xcode.

Answer 2

To answer your first question, take a look at https://developer.apple.com/library/ios/documentation/iphone/conceptual/iphoneosprogrammingguide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html
What I did to get code running in the background is to do something like
In the .h file
UIBackgroundTaskIdentifier backgroundUploadTask;
In the .m file
-(void) functionYouWantToRunInTheBackground
{
self.backgroundUploadTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
[self endBackgroundUpdateTask];
}];
//code to do something
}
-(void) endBackgroundUpdateTask
{
[[UIApplication sharedApplication] endBackgroundTask: self.backgroundUploadTask];
self.backgroundUploadTask = UIBackgroundTaskInvalid;
}
The code above I pretty much learned from objective c - Proper use of beginBackgroundTaskWithExpirationHandler
As for your second question, you should set a breakpoint where code is supposed to run when you bring the app back to the foreground. No one can figure out why an app crashes if not given enough code or information.

Answer 3

The solution to the second part of the question was to raise the scope of the object containing the callback code. I raised it to the level of the containing ViewController. This seems to work. I still can't figure out how to raise the Notification (i.e. execute the call back) if the notification comes while the app is in the background/suspended. This prevented the object containing the callback from being cleaned up.