Question

We’re currently developing an iOS app that needs to check location in the background. At first, we tried to use significant location changes, but they aren’t accurate enough/don’t trigger often enough. We considered using region monitoring, but from what I’ve read online, that isn’t always accurate either, and you also have the problem of a limited number of regions to monitor. (We may eventually try region monitoring.) At the moment, however, we’re attempting to use the standard location updates to track the user location in the background, with a plan to have it at check at intervals of 5 minutes, or so.

The app is registered for location updates in the background (using ‘App registers for location updates’ for ‘Required background modes’), and we start a background task which checks the location once, stops location updates, then uses NSThread sleepForTimeInterval: to (at the moment, while we’re in development) pause the task for 10 seconds. It then checks the location once again, stops location updates, pauses for 10 seconds, etc.

This appears to work as expected… When the app goes into the background, we receive a log/notification with our location update every 10 seconds, and when the app is reopened, the logs/notifications stop. However, the problem is that when the app then goes into the background for a second time, it appears the original background task was never cancelled, and a new one is created, so there are now two tasks running, each checking location at 10 sec on intervals. If the app is opened/sent to the background multiple times, then a background task is started for each of them.

I thought about setting a flag to say “has the app been sent to the background at least once?”, and only run the task if it’s the first time it’s sent to the background, but this seems to cause additional problems, and (as a relatively new iOS developer) I’m curious as to why the background tasks aren’t being cancelled when the app enters the foreground.

The AppDelegate.h file contains…

@interface AppDelegate : UIResponder <UIApplicationDelegate, CLLocationManagerDelegate> {
    UIWindow *window;
    UINavigationController *navigationController;

    UIBackgroundTaskIdentifier bgTask;
    BOOL inBackground;
}

The AppDelegate.m file contains…

- (void)applicationDidEnterBackground:(UIApplication *)application {
    inBackground = YES;

    bgTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
        [[UIApplication sharedApplication] endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        while (inBackground == YES) {
            NSLog(@"%@", @"Check location...");
            [locationManager startUpdatingLocation];

            [NSThread sleepForTimeInterval:10];
        }

        [[UIApplication sharedApplication] endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    });
}

- (void)applicationWillEnterForeground:(UIApplication *)application
{
    // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
    inBackground = NO;

    [[UIApplication sharedApplication] endBackgroundTask:bgTask];
    bgTask = UIBackgroundTaskInvalid;
}

The location updates are working as expected, I just can’t work out why the background tasks aren’t being cancelled/ended when the app enters the foreground. I do wonder if it’s anything to do with the NSThread sleepForTimeInterval:, but I’m not sure if it is, or how to fix it (if indeed, it is). Thanks, in advance, for any help!

Answer 1

I’m fairly certain that your instance variable bgTask is being reallocated when the app comes back into the foreground, so the value doesn’t contain the identifier you’re looking to kill. Consider saving this identifier in NSUserDefaults or something a little more permanent and retrieving it later.