1、Swift入门学习笔记（第一版），对Swift的基础知识点进行梳理总结。知识点一直在变，只是作为参考，以苹果官方文档为准~

2、在学习完基本的知识点以后会结合官方文档及相关资料，在此版本的基础上进行添加更改。

十七、可选链

1、写于前-概念梳理

1.1、可选类型使用时需要解包，解包时需要判断是否为nil，常用if语句判断，再去访问其属性或方法，否则会运行错误。当可选类型访问其属性还是可选时，即多层可选，那么if就显得比较麻烦。Swift提供了可选链（可空链式调用）

1.2、可空链式调用：一种可以请求和调用属性、方法和下标的过程，请求和调用目标可能为空。不为空调用成功，为空返回空(nil)。多个连续调用链接在一起，有任意一个结点为空(nil)将导致整个链调用失败。

1.3、多层链接：
a、访问的值不是可空，通过可选链将会放回可空值
b、访问的值已经是可空，通过可选链调用不会变的“更”可空
简言之：
a、可选链访问一个Int值，将返回Int？
b、可选链访问Int？值，你该不会变的更可空（仍然是可空）

通过拆分官方文档来说明：

class Person {
    var residence:Residence?
}

class Residence {
    var address:Address?
}

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

2、可空链式调用访问属性

let Zane = Person()
if let number = Zane.residence?.address?.buildingNumber {
    print("The buildingNumber of Zane's residence is \(number)")
}else {
    print("Unable to get the buildingNumber")
}

Output：

Unable to get the buildingNumber

可选链任意一个属性为空则返回nil

2.1、设置属性值

let myResidence = Residence()
Zane.residence = myResidence

let someAddress = Address()
someAddress.buildingNumber = "218"
Zane.residence?.address = someAddress

if let number = Zane.residence?.address?.buildingNumber {
    print("The buildingNumber of Zane's residence is \(number)")
}else {
    print("Unable to get the buildingNumber")
}

Output：

Output：
The buildingNumber of Zane's residence is 218

2.2、访问函数

通过可空链来调用Address的buildingIdentifier()方法，返回的是String？类型，因此打印的是可选类型

print(Zane.residence?.address?.buildingIdentifier())

Output：

Optional("218")

if let buildingIdentifier = Zane.residence?.address?.buildingIdentifier() {
    print("Zane's building identifier is \(buildingIdentifier)")
}

Output：

Zane's building identifier is 218

3、下标可空值访问

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 \(numberOfRooms)")
    }
}

有一个存储Room类的数组，及房间数属性
通过下标读写数组元素

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

3.1、可选链调用方法

let Zane = Person()
if Zane.residence?.printNumberOfRooms() != nil {
    print("It was possible to print the number of rooms")
} else {
    print("Unable to print the number of rooms")
}

Output：

Unable to print the number of rooms

3.2、可选链访问下标

访问下标时，问号放在方括号前面
访问Zane.residence中rooms数组中第一个房间名称

if let firstRoomName = Zane.residence?.rooms[0].name {
//if let firstRoomName = Zane.residence?[0].name {      //等同
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to get the first room name")
}

Output：

Unable to get the first room name

定义属性

let myResidence = Residence()
myResidence.rooms.append(Room(name: "Living Room"))
myResidence.rooms.append(Room(name: "BathRoom"))
Zane.residence = myResidence

if let firstRoomName = Zane.residence?.rooms[0].name {
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to get the first room name")
}

if Zane.residence?.printNumberOfRooms() != nil {
    print("It was possible to print the number of rooms")
} else {
    print("Unable to print the number of rooms")
}

Output：

The first room name is Living Room.
The number of rooms is 2
It was possible to print the number of rooms