Swift Lock

func synchronized(_ lock: AnyObject, closure: () -> ()) {
    objc_sync_enter(lock)
    closure()
    objc_sync_exit(lock)
}

class Obj {
    var _str = "123"
    var str: String {
        get {
            return _str
        }
        set {
            synchronized(self) {
                _str = newValue
            }
        }
    }
}