Shared instance swift [duplicate]

Question

How would you write the following in swift?

static ClassName* singleCommon = nil;
+ (ClassName*)sharedInstance {
    @synchronized(singleCommon) {
        if(!singleCommon) singleCommon = [[ClassName alloc] init];
    }
    return singleCommon;
}

Answer 1

A good solution is:

class Singleton {
    class var sharedInstance: Singleton {
        struct Static {
            static var instance: Singleton?
            static var token: dispatch_once_t = 0
        }

        dispatch_once(&Static.token) {
            Static.instance = Singleton()
        }

        return Static.instance!
    }
}

Solution and explanation coming from: http://code.martinrue.com

Answer 2

I Usually use sharedInstance in Swift like this:

private let _sharedInstance = SomeClass()

class SomeClass: NSObject {

    class var sharedInstance: SomeClass {
        get {
            return _sharedInstance
        }
    }

}

Here is a blogpost discussing this question: http://thatthinginswift.com/singletons/