1，swift为什么有可选类型值？
如果一个常量，或变量，它的值可能存在，也可能不存在，那么可以定义这个值为可选类型值。这样做的好处就是，它不存在的时候，自动为nil(swift里面nil跟OC中不一样，swift的nil指可选类型值不存在，OC中的nil指对象为空)
我的理解是，可选类型值，是普通的常量或变量做了封装，加了个壳，如果要用这个值，就需要解包。
解包有两种方式，（1）隐式解包 （2）显式解包
（1）隐式解包，就是在定义可选类型值的时候，加上“！”进行强制解包
如，@IBOutlet var dotImageView: UIImageView!
这样用的时候，就不用再解包，可以直接使用，如
dotImageView.transform = CGAffineTransformMakeTranslation(-view.frame.width, 0)
（2）显示解包，即在用的时候，加！
如 @IBOutlet var dotImageView: UIImageView?
dotImageView!.transform = CGAffineTransformMakeTranslation(-view.frame.width, 0)
（3）用的时候，也可以加？去解包
意思是解包获得的值可能是nil
2，解包用的时候，分常见二种情况
（1）接着调用解包后的对象的属性，方法，子脚步等
（2）需要用到解包后的值
class Person {
var residence: Residence?
}
class Residence {
var numberOfRooms = 1
}
let john = Person()
john.residence = Residence()
//用到解包后的属性 可？ 可！（！解包在运行时候，可能导致崩溃）
john.residence?.numberOfRooms = 1
let roomCount = john.residence?.numberOfRooms
if roomCount != nil {
//已经确定不为空，就可强制解包！，不能用？
var count = 1 + roomCount!
print("count=\(roomCount)")
print("count=\(count)")
}else {
print("nil")
}
（3）多层解包情况
class Person {
var residence: Residence?
}
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 {
return buildingNumber
} else {
return nil
} }
}
class Residence {
var rooms: Array= []
var numberOfRooms: Int {
return rooms.count
}
subscript(i: Int) -> Room {
return rooms[i]
}
func printNumberOfRooms() {
print("The number of rooms is \(numberOfRooms)")
}
var address: Address?
}
//通过可选链调用属性
let john = Person()
if let roomCount = john.residence?.numberOfRooms {
print("count=\(roomCount)")
}else{
print("unable get roomCount")
}
//通过可选链调用方法
john.residence?.printNumberOfRooms()
//使用可选链调用子脚本
if let firstRoomName = john.residence?[0].name {
print("The first room name is \(firstRoomName).")
} else {
print("Unable to retrieve the first room name.")
}
let johnsHouse = Residence()
johnsHouse.rooms.append(Room(name:"aa"))
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.")
}
//连接多层链接
if let johnsStreet = john.residence?.address?.street {
print("John's street name is \(johnsStreet).")
} else {
print("Unable to retrieve the address.")
}
let john1 = Person()
let johnsAddress = Address()
johnsAddress.buildingName = "The Larches"
johnsAddress.street = "Laurel Street"
john1.residence?.address = johnsAddress
if let johnsStreet = john1.residence?.address?.street {
print("John's street name is \(johnsStreet).")
} else {
print("Unable to retrieve the address.")
}
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