Singleton Object in iOS

Question

As we know that a singleton object can be instantiated only once and we use singletons in objective C to have a global access to shared resources. We also know that singletons are instantiated using the following method.

    + (instancetype)sharedManager
{
    static PhotoManager *sharedPhotoManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedPhotoManager = [[[self class] alloc] init];
        sharedPhotoManager->_photosArray = [NSMutableArray array];
    });
    return sharedPhotoManager;
}

But I can also do this -

PhotoManager *sharedManager = [[PhotoManager alloc] init];

Now this way I can also create another instance of the same singleton class then how come the class is singleton if it is having two instances.

Please clarify.

score 2 · Accepted Answer · answered Aug 21 '15 at 15:00

You can forbid calls of init with such trick: add - (instancetype)init NS_UNAVAILABLE; definition to your singleton interface.

Instead of [[PhotoManager alloc] init]; use [[[self class] alloc] init];

PhotoManager *sharedManager = [[PhotoManager alloc] init]; won't compile.

There is my example:

@interface SomeSingleton : NSObject

+ (instancetype)sharedInstance;
- (instancetype)init NS_UNAVAILABLE;

@end

@implementation SomeSingleton

+ (instancetype)sharedInstance {
    static dispatch_once_t onceToken;
    static SomeSingleton *instance;
    dispatch_once(&onceToken, ^{
        instance = [[[self class] alloc] init];
    });

    return instance;
}

- (instancetype)init {
    self = [super init];

    return self;
}

@end

As result SomeSingleton *s1 = [SomeSingleton sharedInstance]; works, but SomeSingleton *s2 = [[SomeSingleton alloc] init]; leads to the compile error.

I think in class methods you can even omit the call to `class` since in the class method, `self` already refers to the class. — Lukas Spieß, Dec 09 '15 at 17:39

score 0 · Answer 2 · answered Aug 21 '15 at 14:40

0

You have singleton behaviour for object only when you use sharedManager: class method. All other ways you instantiate object are not guaranteed to produce singleton object.

answered Aug 21 '15 at 14:40

John Tracid

3,836
3
22
33

score 0 · Answer 3 · answered Aug 21 '15 at 14:51

Objective-C allows you to do lots of things that may not be intended. Like calling a private method if you know the method name.

If you are obsessed to make sure your class is only used as singleton, maybe something like this would be of use:

static PhotoManager *sharedPhotoManager = nil;

+ (instancetype)sharedManager
{
    if (!sharedPhotoManager) {
        sharedPhotoManager = [[PhotoManager alloc] init];
    }
    return sharedPhotoManager;
}

- (instancetype)init {
    if (sharedPhotoManager) {
        // init method is allowed to return different object
        return sharedPhotoManager;
    }
    self = [super init];
    if (self) {

    }
    return self;
}

score -1 · Answer 4 · edited May 23 '17 at 12:15

-1

When you write

PhotoManager *sharedManager = [[PhotoManager alloc] init];

you are not getting your sharedInstance, you are creating a new one. The way you should use it is

[PhotoManager sharedInstance];

instead of

PhotoManager *sharedManager = [[PhotoManager alloc] init];

When you create a singleton class you has never use alloc init, you always should use sharedInstance method to keep the same instance in your app

By the way... as T_77 suggest you, you should use dispatch_once instead of your implementation to create an instance.

 + (instancetype)sharedInstance
 {
     static dispatch_once_t once;
     static id sharedInstance;
     dispatch_once(&once, ^{
         sharedInstance = [[self alloc] init];
     });
     return sharedInstance;
 }

edited May 23 '17 at 12:15

Community

1
1

answered Aug 21 '15 at 14:40

Fran Martin

2,369
22
19

and I still got down vote :P – Teja Nandamuri Aug 21 '15 at 14:48
@T_77, probably because you don't answer the question, why it is called singleton if indeed it is possible to create more than one object. – vikingosegundo Aug 21 '15 at 14:54
you are right. I will keep it in mind. :) – Teja Nandamuri Aug 21 '15 at 14:55
@SonGoku68 Please read the question carefully. I know the method to get a shared instance. But my question is how to prevent creating another instance of same class if we can alloc init. – Aisha Aug 22 '15 at 01:06

Singleton Object in iOS

4 Answers4