sash sash - 1 month ago 17
iOS Question

Start Location Manager in iOS 7 from background task

It seems that in iOS 7 an app can not start Location Manager (by calling startUpdatingLocation) from the background task anymore.

In iOS 6 I used approach described here: http://stackoverflow.com/a/6465280 to run background location update every n minutes. The idea was to run background task with a timer and start Location Manager when the timer triggers it. After that turn off Location Manager and start another background task.

After updating to iOS 7 this approach does not work anymore. After starting Location Manager an app does not receive any locationManager:didUpdateLocations. Any ideas?

Answer

I found the problem/solution. When it is time to start location service and stop background task, background task should be stopped with a delay (I used 1 second). Otherwise location service wont start. Also Location Service should be left ON for a couple of seconds (in my example it is 3 seconds).

Another important notice, max background time in iOS 7 is now 3 minutes instead of 10 minutes.

Updated on October 29 '16

There is a cocoapod APScheduledLocationManager that allows to get background location updates every n seconds with desired location accuracy.

let manager = APScheduledLocationManager(delegate: self)
manager.startUpdatingLocation(interval: 170, acceptableLocationAccuracy: 100)

The repository also contains an example app written in Swift 3.

Updated on May 27 '14

Objective-C example:

1) In ".plist" file set UIBackgroundModes to "location".

2) Create instance of ScheduledLocationManager anywhere you want.

@property (strong, nonatomic) ScheduledLocationManager *slm;

3) Set it up

self.slm = [[ScheduledLocationManager alloc]init];
self.slm.delegate = self;
[self.slm getUserLocationWithInterval:60]; // replace this value with what you want, but it can not be higher than kMaxBGTime

4) Implement delegate methods

-(void)scheduledLocationManageDidFailWithError:(NSError *)error
{
    NSLog(@"Error %@",error);
}

-(void)scheduledLocationManageDidUpdateLocations:(NSArray *)locations
{
    // You will receive location updates every 60 seconds (value what you set with getUserLocationWithInterval)
    // and you will continue to receive location updates for 3 seconds (value of kTimeToGetLocations).
    // You can gather and pick most accurate location
    NSLog(@"Locations %@",locations);
}

Here is implementation of ScheduledLocationManager:

ScheduledLocationManager.h

#import <Foundation/Foundation.h>
#import <CoreLocation/CoreLocation.h>

@protocol ScheduledLocationManagerDelegate <NSObject>

-(void)scheduledLocationManageDidFailWithError:(NSError*)error;
-(void)scheduledLocationManageDidUpdateLocations:(NSArray*)locations;

@end

@interface ScheduledLocationManager : NSObject <CLLocationManagerDelegate>

-(void)getUserLocationWithInterval:(int)interval;

@end

ScheduledLocationManager.m

#import "ScheduledLocationManager.h"

int const kMaxBGTime = 170; // 3 min - 10 seconds (as bg task is killed faster)
int const kTimeToGetLocations = 3; // time to wait for locations

@implementation ScheduledLocationManager
{
    UIBackgroundTaskIdentifier bgTask;
    CLLocationManager *locationManager;
    NSTimer *checkLocationTimer;
    int checkLocationInterval;
    NSTimer *waitForLocationUpdatesTimer;
}

- (id)init
{
    self = [super init];
    if (self) {
        locationManager = [[CLLocationManager alloc] init];
        locationManager.delegate = self;
        locationManager.desiredAccuracy = kCLLocationAccuracyBest;
        locationManager.distanceFilter = kCLDistanceFilterNone;

        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationDidEnterBackground:) name:UIApplicationDidEnterBackgroundNotification object:nil];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationDidBecomeActive:) name:UIApplicationDidBecomeActiveNotification object:nil];
    }
    return self;
}

-(void)getUserLocationWithInterval:(int)interval
{
    checkLocationInterval = (interval > kMaxBGTime)? kMaxBGTime : interval;
    [locationManager startUpdatingLocation];
}

- (void)timerEvent:(NSTimer*)theTimer
{
    [self stopCheckLocationTimer];
    [locationManager startUpdatingLocation];

    // in iOS 7 we need to stop background task with delay, otherwise location service won't start
    [self performSelector:@selector(stopBackgroundTask) withObject:nil afterDelay:1];
}

-(void)startCheckLocationTimer
{
    [self stopCheckLocationTimer];
    checkLocationTimer = [NSTimer scheduledTimerWithTimeInterval:checkLocationInterval target:self selector:@selector(timerEvent:) userInfo:NULL repeats:NO];
}

-(void)stopCheckLocationTimer
{
    if(checkLocationTimer){
        [checkLocationTimer invalidate];
        checkLocationTimer=nil;
    }
}

-(void)startBackgroundTask
{
    [self stopBackgroundTask];
    bgTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
        //in case bg task is killed faster than expected, try to start Location Service
        [self timerEvent:checkLocationTimer];
    }];
}

-(void)stopBackgroundTask
{
    if(bgTask!=UIBackgroundTaskInvalid){
        [[UIApplication sharedApplication] endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }
}

-(void)stopWaitForLocationUpdatesTimer
{
    if(waitForLocationUpdatesTimer){
        [waitForLocationUpdatesTimer invalidate];
        waitForLocationUpdatesTimer =nil;
    }
}

-(void)startWaitForLocationUpdatesTimer
{
    [self stopWaitForLocationUpdatesTimer];
    waitForLocationUpdatesTimer = [NSTimer scheduledTimerWithTimeInterval:kTimeToGetLocations target:self selector:@selector(waitForLoactions:) userInfo:NULL repeats:NO];
}

- (void)waitForLoactions:(NSTimer*)theTimer
{
    [self stopWaitForLocationUpdatesTimer];

    if(([[UIApplication sharedApplication ]applicationState]==UIApplicationStateBackground ||
        [[UIApplication sharedApplication ]applicationState]==UIApplicationStateInactive) &&
       bgTask==UIBackgroundTaskInvalid){
        [self startBackgroundTask];
    }

    [self startCheckLocationTimer];
    [locationManager stopUpdatingLocation];
}

#pragma mark - CLLocationManagerDelegate methods

- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations
{
    if(checkLocationTimer){
        //sometimes it happens that location manager does not stop even after stopUpdationLocations
        return;
    }

    if (self.delegate && [self.delegate respondsToSelector:@selector(scheduledLocationManageDidUpdateLocations:)]) {
        [self.delegate scheduledLocationManageDidUpdateLocations:locations];
    }

    if(waitForLocationUpdatesTimer==nil){
        [self startWaitForLocationUpdatesTimer];
    }
}

- (void)locationManager:(CLLocationManager *)manager didFailWithError:(NSError *)error
{
    if (self.delegate && [self.delegate respondsToSelector:@selector(scheduledLocationManageDidFailWithError:)]) {
        [self.delegate scheduledLocationManageDidFailWithError:error];
    }
}

#pragma mark - UIAplicatin notifications

- (void)applicationDidEnterBackground:(NSNotification *) notification
{
    if([self isLocationServiceAvailable]==YES){
        [self startBackgroundTask];
    }
}

- (void)applicationDidBecomeActive:(NSNotification *) notification
{
    [self stopBackgroundTask];
    if([self isLocationServiceAvailable]==NO){
        NSError *error = [NSError errorWithDomain:@"your.domain" code:1 userInfo:[NSDictionary dictionaryWithObject:@"Authorization status denied" forKey:NSLocalizedDescriptionKey]];

        if (self.delegate && [self.delegate respondsToSelector:@selector(scheduledLocationManageDidFailWithError:)]) {
            [self.delegate scheduledLocationManageDidFailWithError:error];
        }
    }
}

#pragma mark - Helpers

-(BOOL)isLocationServiceAvailable
{
    if([CLLocationManager locationServicesEnabled]==NO ||
       [CLLocationManager authorizationStatus]==kCLAuthorizationStatusDenied ||
       [CLLocationManager authorizationStatus]==kCLAuthorizationStatusRestricted){
        return NO;
    }else{
        return YES;
    }
}

@end