第十七章 可选链

强制解析！可选类型会引起运行时错误。采用可选？来解析，可选链是一个链式访问，逐层访问。

class Person {

    var residence: Residence?

}

class Residence {

    var rooms = [Room]()

    var numberOfRooms: Int {

    return rooms.count

}

subscript(i: Int) -> Room {

    get {

        return rooms[i]

    }

    set {

        rooms[i] = newValue

    }

}

func printNumberOfRooms() {

    print("The number of rooms is \(rooms.count)")

}

var address: Address?

}

class Room {

    var name: String

    init(name: String) {

        self.name = name

    }

}

class Address {

    var buildingName: String?

    var buildingNumber: String?

    var street: String?

    func buildingIdentifier() -> String? {

        if buildingName != nil {

            return buildingName

        } else if buildingNumber != nil && street != nil {

            return "\(buildingNumber) \(street)"

        } else {

            return nil

        }

    }

}

let john = Person()

if let residence = john.residence {

    print(residence.printNumberOfRooms())

}

if let address = john.residence?.address {

    if let addressID = address.buildingIdentifier() {

        print(addressID)

    } else {

        print("Unable to retrieve address identifier")

    }

 } else {

    print("Unable to retrieve the address.")

}

john.residence = Residence()

if let residence = john.residence {

    print(residence.printNumberOfRooms())

}

if let address = john.residence?.address {

    if let addressID = address.buildingIdentifier() {

        print(addressID)

    } else {

        print("Unable to retrieve address identifier")

    }

} else {

    print("Unable to retrieve the address.")

}

john.residence?.address = Address()

john.residence?.address?.buildingName = "AiDing Dasha"

if let building = john.residence?.address?.buildingIdentifier() {

    print(building)

}

john.residence?.rooms.append(Room(name: "Room 1001"))

john.residence?.rooms.append(Room(name: "Room 1002"))

john.residence?[0] = Room(name: "Room 2001")

if let room = john.residence?[0] {

    print(room.name)

}