16.可选链 Optional Chaining Swift官方文档——版纳的笔记

//: Playground - noun: a place where people can play

import UIKit

// # 可选链代替强制展开
/*
class Person {
    var residence: Residence?
}
class Residence {
    var numberOfRooms = 1
}
let john = Person()
// 可选链：if let 的powerful 用法,可选链返回的是int?
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.")
}
*/

// # 为可选链定义模型类
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
        }
    }
}

// # 通过可选链访问属性
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.")
}
let someAddress = Address()
someAddress.buildingNumber = "29"
someAddress.street = "Acacia Road"
john.residence?.address = someAddress // 这样会失败,因为左侧为nil,事实上,=右手侧的代码不会运行

// # 通过可选链调用方法
// 如果调用返回值为空的方法,返回类型将会是Void?,作用是使用if语句检查能否调用该方法
if john.residence?.printNumberOfRooms() != nil {
    print("It was possible to print the number of rooms.")
} else {
    print("It was not possible to print the number of rooms.")
}
// trick:任何通过可选链设置属性的尝试都会返回一个 Void? 类型值，可以与 nil 比较来检查属性是否设置成功
if (john.residence?.address = someAddress) != nil {
    print("It was possible to set the address.")
} else {
    print("It was not possible to set the address.")
}

// # 通过可选链访问下标
if let firstRoomName = john.residence?[0].name {
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
} // 注意:小心下标越界
let johnsHouse = Residence()
johnsHouse.rooms.append(Room(name: "Living Room"))
johnsHouse.rooms.append(Room(name: "Kitchen"))
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.")
}
// 访问可选类型的下标
var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]
testScores["Dave"]?[0] = 91
testScores["Bev"]?[0] += 1
testScores["Brian"]?[0] = 72

// # 链的多层连接
// 不管多少层,可选链取回来的都是可选项
if let johnsStreet = john.residence?.address?.street {
    print("John's street name is \(johnsStreet)")
} else {
    print("Unable to retrieve the address.")
}
let johnsAddress = Address()
johnsAddress.buildingName = "The Larches"
johnsAddress.street = "Laurel Street"
john.residence?.address = johnsAddress
if let johnsStreet = john.residence?.address?.street {
    print("John's street name is \(johnsStreet)")
} else {
    print("Unable to retrieve the address.")
}
// 用可选返回值链接方法
if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
    print("John's building identifier is \(buildingIdentifier)")
}
if let beginsWithThe = john.residence?.address?.buildingIdentifier()?.hasPrefix("The") {
    if beginsWithThe {
        print("John's building identifier begins with The.")
    } else {
        print("John's building identifier does not begin with The.")
    }
}
// trick:String's .hasPrefix method