级别： ★☆☆☆☆
标签：「iOS」「Swift 」「可选链接」「多级可选」
作者： 沐灵洛
审校： QiShare团队

可选链接

可选链接：查询和调用当前可能为nil的可选项的属性，方法和下标的过程。若当前optional包含值，则属性，方法或下标调用成功；若optional是nil，则属性，方法或下标调用会返回nil。多个可选链接可以链在一起，如果链中的任何可选链接返回nil，则整个链都会正常失败。

可选链接作为强制解包的替代方法

可选链接表示：在要调用属性，方法或下标的可选项后放置?。与可选项后放置!进行强制解包相似，但主要区别在于使用?时，当可选项为nil，链接便会返回nil，来正常失败；若为!，可选项为nil时，强制解包会触发运行时错误。 即使要查询的属性，方法或下标返回的是非可选值，可选链接调用的结果也始终是可选的。

class Person {
    var residence: Residence?
}
class Residence {
    var numberOfRooms = 1
}
let john = Person()
//`!`方式访问属性
let roomCount = john.residence!.numberOfRooms
//`?`方式访问属性
if let roomCount = john.residence?.numberOfRooms {
    print("John's residence has \(roomCount) room(s).")
} else {
    print("Unable to retrieve the number of rooms.")
}
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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 let buildingNumber = buildingNumber, let street = street {
            return "\(buildingNumber) \(street)"
        } else if buildingName != nil {
            return buildingName
        } else {
            return nil
        }
    }
}
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通过可选链接访问属性

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.")
}
//赋值
let someAddress = Address()
someAddress.buildingNumber = "29"
someAddress.street = "Acacia Road"
john.residence?.address = someAddress
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通过可选链接访问方法

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.")
}
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通过可选链接访问下标

if let firstRoomName = john.residence?[0].name {
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
}
//赋值
john.residence?[0] = Room(name: "Bathroom")
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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
//调用后： "Dave" : [91, 82, 84] , "Bev": [80, 94, 81]
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多级可选链接

多级可选链接：将多个级别的可选链接链接在一起。采用这种方式访问和调用属性，方法和下标时： 1.返回值不是可选的，也会因为可选链接而成为可选类型。 2.返回值本身是可选的，不会因为可选链接变得更加可选。

if let johnsStreet = john.residence?.address?.street {
    print("John's street name is \(johnsStreet).")
} else {
    print("Unable to retrieve the address.")
}
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####可选链接上有optional类型的返回值的方法

if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
    print("John's building identifier is \(buildingIdentifier).")
}
//方法的可选返回值后继续调用
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\".")
    }
}
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参考资料： swift 5.1官方编程指南

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