Singleton in Swift

http://krakendev.io/blog/the-right-way-to-write-a-singleton

class TheOneAndOnlyKraken {
    static let sharedInstance = TheOneAndOnlyKraken()
}

https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html

In Swift, you can simply use a static type property, which is guaranteed to be lazily initialized only once, even when accessed across multiple threads simultaneously:

class Singleton {
    static let sharedInstance = Singleton()
}

If you need to perform additional setup beyond initialization, you can assign the result of the invocation of a closure to the global constant:

class Singleton {
    static let sharedInstance: Singleton = {
        let instance = Singleton()
        // setup code
        return instance
    }()
}

class TestClass {
    // both of below will init once
    
    // init at first
    let a: Int = {
        print("aaa")
        return 1+2
    }()
    
    // lazy init
    lazy var b: Int = {
        print("bbb")
        return 2+3
    }()
}