• 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".”

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  • 原文地址:https://www.cnblogs.com/susufufu/p/5705626.html
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