UPDATED: Singletons in Objective C - An Example of CLLocationManager with ARC support

In software engineering, the singleton pattern is a design pattern used to implement the mathematical concept of a singleton, by restricting the instantiation of a class to one object.

In Cocoa, singleton objects act as global variables which can be allocated only once and can not be deleted. When it comes to global variables, as developers, we are often told that it is bad design and should be avoid. However, they are essential (see this article by Matt Gallagher for more detail) and helpful especially when exactly one object is needed to coordinate actions across the system.

An Example of CLLocationManager

Using CLLocationManager as a singleton object will be a perfect example if you are designing a location-based application and want to access the user location across the whole system. First we need to create a class called LocationController and set it to be the singleton.
In LocationController.h

//
//  LocationController.h
//
//  Created by Jinru on 12/19/09.
//  Copyright 2009 Arizona State University. All rights reserved.
//

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

// protocol for sending location updates to another view controller
@protocol LocationControllerDelegate
@required
- (void)locationUpdate:(CLLocation*)location;
@end

@interface LocationController : NSObject  {

	CLLocationManager* locationManager;
	CLLocation* location;
	__weak id delegate;
}

@property (nonatomic, strong) CLLocationManager* locationManager;
@property (nonatomic, strong) CLLocation* location;
@property (nonatomic, weak) id  delegate;

+ (LocationController*)sharedInstance; // Singleton method

@end

Then in LocationController.m

//
//  LocationController.m
//
//  Created by Jinru on 12/19/09.
//  Copyright 2009 Arizona State University. All rights reserved.
//

#import "LocationController.h"

static LocationController* sharedCLDelegate = nil;

@implementation LocationController
@synthesize locationManager, location, delegate;

- (id)init
{
 	self = [super init];
	if (self != nil) {
		self.locationManager = [[CLLocationManager alloc] init];
		self.locationManager.delegate = self;
		self.locationManager.desiredAccuracy = kCLLocationAccuracyBest;
	}
	return self;
}

#pragma mark -
#pragma mark CLLocationManagerDelegate Methods
- (void)locationManager:(CLLocationManager*)manager
	didUpdateToLocation:(CLLocation*)newLocation
		   fromLocation:(CLLocation*)oldLocation
{
	/* ... */
}

- (void)locationManager:(CLLocationManager*)manager
	   didFailWithError:(NSError*)error
{
       /* ... */

}

#pragma mark -
#pragma mark Singleton Object Methods
// THE FOLLOWING CODE IS NO LONGER SUPPORTED IN ARC
+ (LocationController*)sharedInstance {
    @synchronized(self) {
        if (sharedCLDelegate == nil) {
            [[self alloc] init];
        }
    }
    return sharedCLDelegate;
}

+ (id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if (sharedCLDelegate == nil) {
            sharedCLDelegate = [super allocWithZone:zone];
            return sharedCLDelegate;  // assignment and return on first allocation
        }
    }
    return nil; // on subsequent allocation attempts return nil
}

- (id)copyWithZone:(NSZone *)zone
{
    return self;
}

- (id)retain {
    return self;
}

- (unsigned)retainCount {
    return UINT_MAX;  // denotes an object that cannot be released
}

- (void)release {
    //do nothing
}

- (id)autorelease {
    return self;
}
@end

Now your LocationController class is ready to be used as a singleton in your code. To access its instance just use:

[LocationController sharedInstance];

And you are done.

UPDATE

With the new ARC support in iOS 4 and MacOS 10.6, the implementation of a singleton class needs to be updated. Simply because in ARC, you cannot call or override methods such as retain, release, autorelease and retaincount. Here is how you should implement the LocationController as a singleton class now.

#pragma mark - Singleton implementation in ARC
+ (LocationController *)sharedLocationController
{
static LocationController *sharedLocationControllerInstance = nil;
static dispatch_once_t predicate;
dispatch_once(&predicate, ^{
sharedLocationControllerInstance = [[self alloc] init];
});
return sharedLocationControllerInstance;
}

To access your singleton class is the same:

LocationController* locationController = [LocationController sharedLocationController];

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