Lazy loading property in Extension (Swift)

you can use computed property, combined with associatedObject. in this way, you can mimic a stored property. so the lazy var simulation will be:

// global var's address used as key
var #PropertyKey# : UInt8 = 0
var #varName# : #type# {
    get {
        if let v = objc_getAssociatedObject(self, & #PropertyKey#) as #type# {
            return v
        }else {
            // the val not exist, init it and set it. then return it

            // this way doesn't support nil option value. 
            // if you need nil option value, you need another associatedObject to indicate this situation.
        }
    }
    set {
        objc_setAssociatedObject(self, & #PropertyKey#, newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN_NONATOMIC))
    }
}

If you don't need to refer to self you can use a static var:

extension String {
    static var count = 0
    static var laughingOutLoud : String = {
        count++

        return "LOL: \(count)"
    }()
}

String.laughingOutLoud // outputs "LOL: 1"
String.laughingOutLoud // outputs "LOL: 1"
String.laughingOutLoud // outputs "LOL: 1"

(You don't need count in your implementation; that's just there to show it's only executed once.)