Room { get { return rooms[i] } set { rooms[i] = newValue } } func printNumberOfRooms() { print("The number of rooms is \(numberOfRooms)") } var address: Address? } class Room { let name: String init(name: String) { self.name = name } } class Address { var buildingName: String? var build"> Room { get { return rooms[i] } set { rooms[i] = newValue } } func printNumberOfRooms() { print("The number of rooms is \(numberOfRooms)") } var address: Address? } class Room { let name: String init(name: String) { self.name = name } } class Address { var buildingName: String? var build"> Room { get { return rooms[i] } set { rooms[i] = newValue } } func printNumberOfRooms() { print("The number of rooms is \(numberOfRooms)") } var address: Address? } class Room { let name: String init(name: String) { self.name = name } } class Address { var buildingName: String? var build"> 
//class Person {
//    var residence: Residence?
//}
//
//class Residence {
//    var numberOfRooms = 1
//}
//
//let john = Person()
//
////let roomCount = john.residence!.numberOfRooms
//// 这会引发运行时错误
//
//if let roomCount = john.residence?.numberOfRooms {
//    print("John's residence has \\(roomCount) room(s).")
//} else {
//    print("Unable to retrieve the number of rooms.")
//}
//// 打印“Unable to retrieve the number of rooms.”

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 \\(numberOfRooms)")
    }
    var address: Address?
}

class Room {
    let 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 let buildingNumber = buildingNumber, let street = street {
            return "\\(buildingNumber) \\(street)"
        } else {
            return nil
        }
    }
}

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

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

func createAddress() -> Address {
    print("Function was called.")

    let someAddress = Address()
    someAddress.buildingNumber = "29"
    someAddress.street = "Acacia Road"

    return someAddress
}
john.residence?.address = createAddress()

if john.residence?.printNumberOfRooms() != nil {
    print("It was possible to print the number of rooms.")
} else {
    print("It was not possible to print the number of rooms.")
}
// 打印“It was not possible to print the number of rooms.”

if (john.residence?.address = someAddress) != nil {
    print("It was possible to set the address.")
} else {
    print("It was not possible to set the address.")
}
// 打印“It was not possible to set the address.”

if let firstRoomName = john.residence?[0].name {
    print("The first room name is \\(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
}
// 打印“Unable to retrieve the first room name.”

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

if let firstRoomName2 = john.residence?[0].name {
    print("The first room name is \\(firstRoomName2).")
} else {
    print("Unable to retrieve the first room name.")
}

let johnsHouse = Residence()
johnsHouse.rooms.append(Room(name: "Living Room"))
johnsHouse.rooms.append(Room(name: "Kitchen"))
john.residence = johnsHouse

if let firstRoomName = john.residence?[0].name {
    print("The first room name is \\(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
}
// 打印“The first room name is Living Room.”

var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]
testScores["Dave"]?[0] = 91
testScores["Bev"]?[0] += 1
testScores["Brian"]?[0] = 72
// "Dave" 数组现在是 [91, 82, 84]，"Bev" 数组现在是 [80, 94, 81]

if let johnsStreet = john.residence?.address?.street {
    print("John's street name is \\(johnsStreet).")
} else {
    print("Unable to retrieve the address.")
}
// 打印“Unable to retrieve the address.”

let johnsAddress = Address()
johnsAddress.buildingName = "The Larches"
johnsAddress.street = "Laurel Street"
john.residence?.address = johnsAddress

if let johnsStreet = john.residence?.address?.street {
    print("John's street name is \\(johnsStreet).")
} else {
    print("Unable to retrieve the address.")
}
// 打印“John's street name is Laurel Street.”

if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
    print("John's building identifier is \\(buildingIdentifier).")
}
// 打印“John's building identifier is The Larches.”

if let beginsWithThe =
    john.residence?.address?.buildingIdentifier()?.hasPrefix("The") {
        if beginsWithThe {
            print("John's building identifier begins with \\"The\\".")
        } else {
            print("John's building identifier does not begin with \\"The\\".")
        }
}
// 打印“John's building identifier begins with "The".”

通过可选链访问下标

<aside> 🌼 通过可选链访问下标时，? 要加在可选项的后面，而不是下标的后面，如下： john.residence?[0] = Room(name: "Bathroom")

</aside>

访问可选类型的下标

<aside> 🦄 如果通过下标访问的值是一个可选类型，则继续访问它的下标时，需要在其后面加 ?，也就是说 ? 要加载一个可选类型的后面。如通过字典键访问到的值是一个数组，继续访问数组中的元素。

</aside>

var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]
testScores["Dave"]?[0] = 91
testScores["Bev"]?[0] += 1
testScores["Brian"]?[0] = 72
// "Dave" 数组现在是 [91, 82, 84]，"Bev" 数组现在是 [80, 94, 81]