swift学习笔记之-可选链式调用

//可选链式调用

import UIKit

/*可选链式调用（Optional Chaining）

1.在可选值上请求和调用该可选值的属性、方法及下标的方法，如果可选值有值，那么调用就会成功，返回可选类型值，如果可选值是nil，那么调用将返回nil，多个调用可以连接在一起形成一个调用链，如果其中任何一个节点为nil，整个调用链都会失败，即返回nil；（而强制展开它的值是"!"，有值则返回该值，无值则会触发运行时错误）

2.通过在想调用的属性、方法、或下标的可选值（optional value）后面放一个问号"?"

3.可选链式调用返回的是可选类型值，它的返回结果与原本的返回结果具有相同的类型，但是被包装成了一个可选值

访问可选类型的下标：

var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]

testScores["Dave"]?[0] = 91

在方法的可选返回值上进行可选链式调用,在方法的圆括号后面加上问号

let beginsWithThe = john.residence?.address?.buildingIdentifier()?.hasPrefix("The")

在方法的圆括号后面加上问号是因为你要在buildingIdentifier()方法的可选返回值上进行可选链式调用，而不是方法本身。

*/

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 buildingNumber != nil && street != nil {

return "\(buildingNumber) \(street)"

} else {

return nil

}

}

}

let john = Person()

if let roomCount = john.residence?.numberOfRooms {      //因为john.residence为nil，调用失败，返回nil

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                   //因为john.residence当前为nil，调用失败，等号右侧的代码不会被执行，赋值失败

if john.residence?.printNumberOfRooms() != nil {        //调用方法失败，返回nil，如果成功则返回void?

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 {        //通过可选链式调用访问数组下标

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.”

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 johnsStreet = john.residence?.address?.street {      //连接多层可选链式调用

print("John's street name is \(johnsStreet).")

} else {

print("Unable to retrieve the address.")

}

// 打印 “Unable to retrieve the address.”

var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]

testScores["Dave"]?[0] = 91

testScores["Bev"]?[0]++

testScores["Brian"]?[0] = 72        //testScores字典中没有"Brian"这个键，所以第三个调用失败。

// "Dave" 数组现在是 [91, 82, 84]，"Bev" 数组现在是 [80, 94, 81]

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") {   //在方法的可选返回值上进行可选链式调用,在方法的圆括号后面加上问号

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".”