一種可以在當(dāng)前值可能為nil的可選值上請求和調(diào)用屬性魁衙、方法及下標(biāo)的方法

1 使用可選鏈?zhǔn)秸{(diào)用代替強(qiáng)制展開

可選鏈?zhǔn)秸{(diào)用的返回結(jié)果與原本的返回結(jié)果具有相同的類型蛋叼，但是被包裝成了一個(gè)可選值碉熄。

class Person {
    var residence: Residence?
}
class Residence {
    var numberOfRooms = 1
}
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.")
}
john.residence = Residence()
if let roomCount = john.residence?.numberOfRooms {
    print("John's residence has \(roomCount) room(s).")
} else {
    print("Unable to retrieve the number of rooms.")
}

2 為可選鏈?zhǔn)秸{(diào)用定義模型類

class Person2 {
    var residence: Residence2?
}
class Residence2 {
    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 buildingNumber != nil && street != nil {
            return "\(buildingNumber) \(street)"
        } else {
            return nil
        }
    }
}
func createAddress() -> Address {
    print("Function was called.")
    let someAddress = Address()
    someAddress.buildingNumber = "29"
    someAddress.street = "Acacia Road"
    return someAddress
}

3 通過可選鏈?zhǔn)秸{(diào)用訪問屬性

let andy = Person2()
andy.residence?.address = createAddress()       // andy.residence結(jié)果為nil，可選鏈?zhǔn)秸{(diào)用失敗時(shí)饰序，等號右側(cè)的代碼不會被執(zhí)行泉褐。

4 通過可選鏈?zhǔn)秸{(diào)用調(diào)用方法

if let firstRoomName = andy.residence?[0].name {
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
}
let andyHouse = Residence2()
andyHouse.rooms.append(Room(name: "Living Room"))
andyHouse.rooms.append(Room(name: "Kitchen"))
andy.residence = andyHouse
if let firstRoomName = andy.residence?[0].name {
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
}

5 訪問可選類型的下標(biāo)

var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]
testScores["Dave"]?[0] = 91
testScores["Bev"]?[0] += 1
testScores["Brian"]?[0] = 72

6 連接多層可選鏈?zhǔn)秸{(diào)用

if let andyStreet = andy.residence?.address?.street {
    print("John's street name is \(andyStreet).")
} else {
    print("Unable to retrieve the address.")
}
let andyAddress = Address()
andyAddress.buildingName = "東方明珠"
andyAddress.street = "南京東路100號"
andy.residence?.address = andyAddress
if let andyStreet = andy.residence?.address?.street {
    print("John's street name is \(andyStreet).")
} else {
    print("Unable to retrieve the address.")
}

7 在方法的可選返回值上進(jìn)行可選鏈?zhǔn)秸{(diào)用

if let buildingIdentifier = andy.residence?.address?.buildingIdentifier() {
    print("John's building identifier is \(buildingIdentifier).")
}
if let beginsWithThe =
    andy.residence?.address?.buildingIdentifier()?.hasPrefix("東") {
    if beginsWithThe {
        print("John's building identifier begins with \"東\".")
    } else {
        print("John's building identifier does not begin with \"東\".")
    }
}

playground文件在andyRon/LearnSwift