26 Sep 2014
Abstracting iOS 8 new Location Manager

In the new iOS release, CLLocationManager, the class responsible for retrieving location, gets a major overhaul, by introducing specific permission methods. While you can get some details on these changes by looking at the documentation, I wanted to introduce a wrapper class we are using at Guestful for wrapping all the Location-retrieval logic, especially the backward-compatibility code. This solution is based off Michael Babiy’s code, which exposes a method for enabling a Singleton CLLocationManager class. The method exposed here takes this even further, by allowing any number of “observers” on this class.

The Code

We start off by exposing a protocol for our observers to implement, in our GFLocationManager.h file:

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

@protocol GFLocationManagerDelegate

- (void)locationManagerDidUpdateLocation:(CLLocation *)location;

@end

@interface GFLocationManager : NSObject<CLLocationManagerDelegate>

+ (GFLocationManager *)sharedInstance;
- (void) addLocationManagerDelegate:(id<GFLocationManagerDelegate>) delegate;
- (void) removeLocationManagerDelegate:(id<GFLocationManagerDelegate>) delegate;

@end

The interface for our GFLocationManager is pretty straightforward: we provide with a sharedInstance method (singleton, duh!) and two methods to add/remove our observer. Don’t forget to implement the protocol CLLocationManagerDelegate.

On the GFLocationManager.m side, we initiate our manager by following the new iOS 8 requirements, and simply broadcast any new location updates to the observers:

#import "GFLocationManager.h"

@interface GFLocationManager()

@property (strong, nonatomic) CLLocationManager* manager;
@property (strong, nonatomic) NSMutableArray *observers;

@end

@implementation GFLocationManager
static int errorCount = 0;
#define MAX_LOCATION_ERROR 3

//This is a singleton the objective-c way
+ (GFLocationManager*) sharedInstance {
    static GFLocationManager *sharedInstance = nil;
    static dispatch_once_t token;
    dispatch_once(&token, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}

- (instancetype) init {
    self = [super init];
    if(self) {
        
	//Must check authorizationStatus before initiating a CLLocationManager
        CLAuthorizationStatus status = [CLLocationManager authorizationStatus];
        if (status == kCLAuthorizationStatusRestricted && status == kCLAuthorizationStatusDenied) {
        } else {
            _manager = [[CLLocationManager alloc] init];
            _manager.delegate = self;
            _manager.desiredAccuracy = kCLLocationAccuracyBest;
        }
        if (status == kCLAuthorizationStatusNotDetermined) {
            //Must check if selector exists before messaging it
            if ([_manager respondsToSelector:@selector(requestWhenInUseAuthorization)]) {
                [_manager requestWhenInUseAuthorization];
            }
        }
        
        _observers = [[NSMutableArray alloc] init];
    }
    return self;
}

- (void) addLocationManagerDelegate:(id<GFLocationManagerDelegate>)delegate {
    if (![self.observers containsObject:delegate]) {
        [self.observers addObject:delegate];
    }
    [self.manager startUpdatingLocation];
}

- (void) removeLocationManagerDelegate:(id<GFLocationManagerDelegate>)delegate {
    if ([self.observers containsObject:delegate]) {
        [self.observers removeObject:delegate];
    }
}


#pragma mark - Location Manager Delegate

- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations
{
    [self.manager stopUpdatingLocation];
    for(id<GFLocationManagerDelegate> observer in self.observers) {
        if (observer) {
            [observer locationManagerDidUpdateLocation:[locations lastObject]];
        }
    }
}

-(void) locationManager:(CLLocationManager *)manager didFailWithError:(NSError *)error
{
    errorCount += 1;
    if(errorCount >= MAX_LOCATION_ERROR) {
        [self.manager stopUpdatingLocation];
	errorCount = 0;
    }
}

@end

Note a few things:

The location manager will stop trying to update location if it fails three consecutive times. This is just a heuristic value; I would be interested to know if there are cases where this won’t be enough.
We kick off a retrieval of location every time an observer requests it. This works because CLLocationManager takes care of caching the value

How will you use this?

In your viewController, start by implementing the GFLocationManagerDelegate protocol:

- (void) locationManagerDidUpdateLocation:(CLLocation *)location {
    self.currentLocation = location;
}

Then, you can add/remove self in the viewWillAppear and viewWillDisappear methods of your viewController:

-(void) viewWillAppear:(BOOL)animated
{
    [[GFLocationManager sharedInstance] addLocationManagerDelegate:self];
}

- (void) viewWillDisappear:(BOOL)animated {
    [[GFLocationManager sharedInstance] removeLocationManagerDelegate:self];
}

Now, everytime the location is retrieved, the locationManagerDidUpdateLocation will be called.

###Improvements

Introduce time-based refreshing of the location: if addLocationManagerDelegate has not been called in 5 min, schedule it again.

PS: I know I am not supposed to tell you what to do, but, you know, you could always follow me on Twitter

26 Sep 2014 Abstracting iOS 8 new Location Manager

The Code

How will you use this?

26 Sep 2014
Abstracting iOS 8 new Location Manager