swift-可选链

class Room {
    let name: String
    init(name: String) { self.name = name }
}

class Residence{
    var rooms = [Room]()
    
    var numberOfRooms : Int {
        return rooms.count
    }
    subscript(i: Int) -> Room{
        
        get{
            return rooms[i]
        }
        set{
            rooms[i] = newValue
        }
        
    }
    func prineNumberOfRooms(){
        print("The number of rooms is \(numberOfRooms)")
    }
    var address : Address?
    
}

class Address {
    
    var buildingName: String?
    var buildingNumber : String?
    var street: String?
    func buildingIdentifier() -> String? {
        if buildingName != nil {
            return buildingName
        }else if buildingName != nil && street != nil {
            return "\(String(describing: buildingName)) \(String(describing: street))"
        }else{
            return nil
        }
    }

}

class Person {
    var residence: Residence?
}

let john  = Person()

//let roomcount = john.residence!.numberOfRooms

if let roomcount = john.residence?.numberOfRooms {
    print("John's residence has\(roomcount)room(s)")
}else{
    print("Unable to retrieve the number of rooms.")
}


let someAddress = Address()
someAddress.buildingNumber = "29"
someAddress.street = "Acacia Road"
john.residence? .address = someAddress


func createAdddress() -> Address{
    print("Function was called.")
    let someaddress = Address()
    someaddress.buildingNumber = "32"
    someaddress.street = "NaShanStreet"
    return someaddress
}

john.residence?.address = createAdddress()