I am looking into using deferred location updates for an iOS activity tracker, which allows location services in background. I've implemented the suggested code snippets (see below). In Xcode debugging, deferred locations attempt to start a few times until location data comes in at about 1 per second. After that, it claims to succeed in starting deferrals, and the callback for the finish trigger also succeeds after the specified time period expires. However during the time, the location handler still runs once per second. I've read that this is because the phone hasn't deemed itself ready to enter the background, and that testing in Xcode does this. Note, AppDelegate's "didEnterBackground" eventhandler got called immediately when turning off the screen, and resumed when reopening app.
I ran the same code with the phone disconnected as another test, near the window with GPS, screen off, or switching to entirely different apps, and it still never actually defers the updates. I can tell because the networked update still comes in once every 30 seconds, instead of the interval of 120 seconds which is desired in the code sample below.
What else is needed to actually get deferrals to work, since there is no error occurring in starting them and they do get their finish callback? Why do location updates continue at 1 per second even when the app goes to background?
Iphone 5s, IOS 7.1.1
// .h file (partial)
@interface MotionTracker : NSObject<CLLocationManagerDelegate, UIAccelerometerDelegate>
@property (strong, nonatomic) CLLocationManager *locationManager;
@end
// .m file (parial)
- (id) init {
if(self = [super init]){
_locationManager = [[CLLocationManager alloc] init];
_locationManager.delegate = self;
_locationManager.distanceFilter = kCLDistanceFilterNone;
_locationManager.desiredAccuracy = kCLLocationAccuracyBest;
// if set to YES (default), app stops logging location at some point and doesn't resume in any timely fashion, all data points lost in between
_locationManager.pausesLocationUpdatesAutomatically = NO;
_locationManager.activityType = CLActivityTypeFitness;
}
return self;
}
// called early in program after login confirmed
- (void) startCollectingLocation {
[_locationManager startUpdatingLocation];
}
- (void)locationManager:(CLLocationManager *)manager
didUpdateLocations:(NSArray *)locations {
// logs to file when device is not in debug
// always returns 1
NSLog(@"Location update count: %d",[locations count]);
// some code here to handle location updates
// - collect key location day in NSDictionary
// - every N seconds send Network call to server to save (have tried 30 seconds, 15 minutes, 30 minute network intervals). Have also tried turning off network calls completely.
// deferred updates starter
if (!self.deferringUpdates) {
if([CLLocationManager deferredLocationUpdatesAvailable]){
[_locationManager allowDeferredLocationUpdatesUntilTraveled:500 timeout:(NSTimeInterval)120]; // (have also tried large numbers, and "Infinite"
self.deferringUpdates = YES;
NSLog(@"Deferred updates start");
} else {
NSLog(@"Deferred updates not available");
}
}
}
- (void)locationManager:(CLLocationManager *)manager didFinishDeferredUpdatesWithError:(NSError *)error {
if(!error){
_deferringUpdates = NO;
NSLog(@"Deferred updates: finished");
} else {
_deferringUpdates = NO;
NSLog(@"Deferred updates: %@", [error localizedDescription]);
}
}
