Create singleton using GCD's dispatch_once in Objective-C

Question

If you can target iOS 4.0 or above

Using GCD, is it the best way to create singleton in Objective-C (thread safe)?

+ (instancetype)sharedInstance
{
          


        

Answer 1

You ask whether this is the "best way to create singleton".

A few thoughts:

1. First, yes, this is a thread-safe solution. This dispatch_once pattern is the modern, thread-safe way to generate singletons in Objective-C. No worries there.

2. You asked, though, whether this is the "best" way to do it. One should acknowledge, though, that the instancetype and [[self alloc] init] is potentially misleading when used in conjunction with singletons.

   The benefit of instancetype is that it's an unambiguous way of declaring that the class can be subclassed without resorting to a type of id, like we had to do in yesteryear.

   But the static in this method presents subclassing challenges. What if ImageCache and BlobCache singletons were both subclasses from a Cache superclass without implementing their own sharedCache method?

   ImageCache *imageCache = [ImageCache sharedCache];  // fine
   BlobCache *blobCache = [BlobCache sharedCache];     // error; this will return the aforementioned ImageCache!!!
   

   For this to work, you'd have to make sure subclasses implement their own sharedInstance (or whatever you call it for your particular class) method.

   Bottom line, your original sharedInstance looks like it will support subclasses, but it won't. If you intend to support subclassing, at the very least include documentation that warns future developers that they must override this method.

3. For best interoperability with Swift, you probably want to define this to be a property, not a class method, e.g.:

   @interface Foo : NSObject
   @property (class, readonly, strong) Foo *sharedFoo;
   @end
   

   Then you can go ahead and write a getter for this property (the implementation would use the dispatch_once pattern you suggested):

   + (Foo *)sharedFoo { ... }
   

   The benefit of this is that if a Swift user goes to use it, they'd do something like:

   let foo = Foo.shared
   

   Note, there is no (), because we implemented it as a property. Starting Swift 3, this is how singletons are generally accessed. So defining it as a property helps facilitate that interoperability.

   As an aside, if you look at how Apple is defining their singletons, this is the pattern that they've adopted, e.g. their NSURLSession singleton is defined as follows:

   @property (class, readonly, strong) NSURLSession *sharedSession;
   

4. Another, very minor Swift interoperability consideration was the name of the singleton. It's best if you can incorporate the name of the type, rather than sharedInstance. For example, if the class was Foo, you might define the singleton property as sharedFoo. Or if the class was DatabaseManager, you might call the property sharedManager. Then Swift users could do:

   let foo = Foo.shared
   let manager = DatabaseManager.shared
   

   Clearly, if you really want to use sharedInstance, you could always declare the Swift name should you want to:

   @property (class, readonly, strong) Foo* sharedInstance NS_SWIFT_NAME(shared);
   

   Clearly, when writing Objective-C code, we shouldn't let Swift interoperability outweigh other design considerations, but still, if we can write code that gracefully supports both languages, that's preferable.

5. I agree with others who point out that if you want this to be a true singleton where developers can’t/shouldn’t (accidentally) instantiate their own instances, the unavailable qualifier on init and new is prudent.