Singletons in Objective-C

One of my most used design patterns when developing for iOS is the singleton pattern. It’s an extremely powerful way to share data between different parts of code without having to pass the data around manually. More about the singleton pattern and other patterns can be found in this excellent book:

Background

Singleton classes are an important concept to understand because they exhibit an extremely useful design pattern. This idea is used throughout the iPhone SDK, for example, UIApplication has a method called sharedApplication which when called from anywhere will return the UIApplication instance which relates to the currently running application.

How to implement

You can implement a singleton class in Objective-C using the following code:

MyManager.h

#import <foundation/Foundation.h>

@interface MyManager : NSObject {
    NSString *someProperty;
}

@property (nonatomic, retain) NSString *someProperty;

+ (id)sharedManager;

@end

MyManager.m

#import "MyManager.h"

@implementation MyManager

@synthesize someProperty;

#pragma mark Singleton Methods

+ (id)sharedManager {
    static MyManager *sharedMyManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedMyManager = [[self alloc] init];
    });
    return sharedMyManager;
}

- (id)init {
  if (self = [super init]) {
      someProperty = [[NSString alloc] initWithString:@"Default Property Value"];
  }
  return self;
}

- (void)dealloc {
  // Should never be called, but just here for clarity really.
}

@end

What this does is it defines a static variable (but only global to this translation unit)) called sharedMyManager which is then initialised once and only once in sharedManager. The way we ensure that it’s only created once is by using the dispatch_once method from Grand Central Dispatch (GCD). This is thread safe and handled entirely by the OS for you so that you don’t have to worry about it at all.

However, if you would rather not use GCD then you should use the following code for sharedManager:

Non-GCD based code

+ (id)sharedManager {
    static MyManager *sharedMyManager = nil;
    @synchronized(self) {
        if (sharedMyManager == nil)
            sharedMyManager = [[self alloc] init];
    }
    return sharedMyManager;
}

Then you can reference the singleton from anywhere by calling the following function:

MyManager *sharedManager = [MyManager sharedManager];

I’ve used this extensively throughout my code for things such as creating a singleton to handle CoreLocation or CoreData functions.

Non-ARC code

Not that I recommend it, but if you are not using Automatic Reference Counting (ARC), then you should use the following code:

MyManager.h non-ARC

#import "MyManager.h"

static MyManager *sharedMyManager = nil;

@implementation MyManager

@synthesize someProperty;

#pragma mark Singleton Methods
+ (id)sharedManager {
  @synchronized(self) {
      if(sharedMyManager == nil)
          sharedMyManager = [[super allocWithZone:NULL] init];
  }
  return sharedMyManager;
}
+ (id)allocWithZone:(NSZone *)zone {
  return [[self sharedManager] retain];
}
- (id)copyWithZone:(NSZone *)zone {
  return self;
}
- (id)retain {
  return self;
}
- (unsigned)retainCount {
  return UINT_MAX; //denotes an object that cannot be released
}
- (oneway void)release {
  // never release
}
- (id)autorelease {
  return self;
}
- (id)init {
  if (self = [super init]) {
      someProperty = [[NSString alloc] initWithString:@"Default Property Value"];
  }
  return self;
}
- (void)dealloc {
  // Should never be called, but just here for clarity really.
  [someProperty release];
  [super dealloc];
}

@end