Periodic iOS background location updates

I'm writing an application that requires background location updates with high accuracy and low frequency. The solution seems to be a background NSTimer task that starts the location manager's updates, which then immediately shuts down. This question has been asked before:

How do I get a background location update every n minutes in my iOS application?

Getting user location every n minutes after app goes to background

iOS Not the typical background location tracking timer issue

iOS long-running background timer with "location" background mode

iOS full-time background-service based on location tracking

but I have yet to get a minimum example working. After trying every permutation of the above accepted answers, I put together a starting point. Entering background:

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    self.bgTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
        NSLog(@"ending background task");
        [[UIApplication sharedApplication] endBackgroundTask:self.bgTask];
        self.bgTask = UIBackgroundTaskInvalid;
    }];


    self.timer = [NSTimer scheduledTimerWithTimeInterval:60
                                                  target:self.locationManager
                                                selector:@selector(startUpdatingLocation)
                                                userInfo:nil
                                                 repeats:YES];
}

and the delegate method:

- (void)locationManager:(CLLocationManager *)manager 
    didUpdateToLocation:(CLLocation *)newLocation 
           fromLocation:(CLLocation *)oldLocation {

    NSLog(@"%@", newLocation);

    NSLog(@"background time: %f", [UIApplication sharedApplication].backgroundTimeRemaining);
    [self.locationManager stopUpdatingLocation];

}

The current behavior is that the backgroundTimeRemaining decrements from 180 seconds to zero (while logging location), and then the expiration handler executes and no further location updates are generated. How do I modify the above code in order to receive periodic location updates in the background indefinitely?

Update: I'm targeting iOS 7 and there appears to be some evidence that background tasks behave differently:

Start Location Manager in iOS 7 from background task

Solution 1:

It seems that stopUpdatingLocation is what triggers the background watchdog timer, so I replaced it in didUpdateLocation with:

[self.locationManager setDesiredAccuracy:kCLLocationAccuracyThreeKilometers];
[self.locationManager setDistanceFilter:99999];

which appears to effectively power down the GPS. The selector for the background NSTimer then becomes:

- (void) changeAccuracy {
    [self.locationManager setDesiredAccuracy:kCLLocationAccuracyBest];
    [self.locationManager setDistanceFilter:kCLDistanceFilterNone];
}

All I'm doing is periodically toggling the accuracy to get a high-accuracy coordinate every few minutes and because the locationManager hasn't been stopped, backgroundTimeRemaining stays at its maximum value. This reduced battery consumption from ~10% per hour (with constant kCLLocationAccuracyBest in the background) to ~2% per hour on my device.

Solution 2:

I did write an app using Location services, app must send location every 10s. And it worked very well.

Just use the "allowDeferredLocationUpdatesUntilTraveled:timeout" method, following Apple's doc.

Steps are as follows:

Required: Register background mode for update Location.

1. Create LocationManger and startUpdatingLocation, with accuracy and filteredDistance as whatever you want:

-(void) initLocationManager    
{
    // Create the manager object
    self.locationManager = [[[CLLocationManager alloc] init] autorelease];
    _locationManager.delegate = self;
    // This is the most important property to set for the manager. It ultimately determines how the manager will
    // attempt to acquire location and thus, the amount of power that will be consumed.
    _locationManager.desiredAccuracy = 45;
    _locationManager.distanceFilter = 100;
    // Once configured, the location manager must be "started".
    [_locationManager startUpdatingLocation];
}

2. To keep app run forever using "allowDeferredLocationUpdatesUntilTraveled:timeout" method in background, you must restart updatingLocation with new parameter when app moves to background, like this:

- (void)applicationWillResignActive:(UIApplication *)application {
     _isBackgroundMode = YES;

    [_locationManager stopUpdatingLocation];
    [_locationManager setDesiredAccuracy:kCLLocationAccuracyBest];
    [_locationManager setDistanceFilter:kCLDistanceFilterNone];
    _locationManager.pausesLocationUpdatesAutomatically = NO;
    _locationManager.activityType = CLActivityTypeAutomotiveNavigation;
    [_locationManager startUpdatingLocation];
 }

3. App gets updatedLocations as normal with "locationManager:didUpdateLocations:" callback:

-(void) locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations
{
//  store data
    CLLocation *newLocation = [locations lastObject];
    self.userLocation = newLocation;

   //tell the centralManager that you want to deferred this updatedLocation
    if (_isBackgroundMode && !_deferringUpdates)
    {
        _deferringUpdates = YES;
        [self.locationManager allowDeferredLocationUpdatesUntilTraveled:CLLocationDistanceMax timeout:10];
    }
}

4. But you should handle the data in then "locationManager:didFinishDeferredUpdatesWithError:" callback for your purpose

- (void) locationManager:(CLLocationManager *)manager didFinishDeferredUpdatesWithError:(NSError *)error {

     _deferringUpdates = NO;

     //do something 
}

5. NOTE: I think we should reset parameters of LocationManager each time app switches between background/forground mode.

Solution 3:

1. If you have the UIBackgroundModes in your plist with location key then you don't need to use beginBackgroundTaskWithExpirationHandler method. That's redundant. Also you're using it incorrectly (see here) but that's moot since your plist is set.

2. With UIBackgroundModes location in the plist the app will continue to run in the background indefinitely only as long as CLLocationManger is running. If you call stopUpdatingLocation while in the background then the app will stop and won't start again.

   Maybe you could call beginBackgroundTaskWithExpirationHandler just before calling stopUpdatingLocation and then after calling startUpdatingLocation you could call the endBackgroundTask to keep it backgrounded while the GPS is stopped, but I've never tried that - it's just an idea.

   Another option (which I haven't tried) is to keep the location manager running while in the background but once you get an accurate location change the desiredAccuracy property to 1000m or higher to allow the GPS chip to get turned off (to save battery). Then 10 minutes later when you need another location update, change the desiredAccuracy back to 100m to turn on the GPS until you get an accurate location, repeat.

3. When you call startUpdatingLocation on the location manager you must give it time to get a position. You should not immediately call stopUpdatingLocation. We let it run for a maximum of 10 seconds or until we get a non-cached high accuracy location.

4. You need to filter out cached locations and check the accuracy of the location you get to make sure it meets your minimum required accuracy (see here). The first update you get may be 10 mins or 10 days old. The first accuracy you get may be 3000m.

5. Consider using the significant location change APIs instead. Once you get the significant change notification, you could start CLLocationManager for a few seconds to get a high accuracy position. I'm not certain, I've never used the significant location change services.