Swift4 可选链，例子代码

时间：2022-01-13 18:55:28

可选链

苹果官方指南 Optional Chaining

苹果官方指南翻译 可选链

可选链是一个调用和查询可选属性、方法和下标的过程，它可能为 nil 。

可选链代替强制展开

可选链会在可选项为 nil 时失败，而强制展开则在可选项为 nil 时触发运行时错误。

可选链调用的结果一定是一个可选值。

class Person {
    var residence: Residence?
}
 
class Residence {
    var numberOfRooms = 1
}

let john = Person()

let roomCount = john.residence!.numberOfRooms
// this triggers a runtime error

if let roomCount = john.residence?.numberOfRooms {
    print("John's residence has \(roomCount) room(s).")
} else {
    print("Unable to retrieve the number of rooms.")
}
// Prints "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.")
}
// Prints "John's residence has 1 room(s)."

为可选链定义模型类

可以使用可选链来调用属性、方法和下标不止一个层级。

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

可以使用可选链来访问可选值里的属性，并且检查这个属性的访问是否成功。

接上例代码例子。

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.")
}
// Prints "Unable to retrieve the number of rooms."

let someAddress = Address()
someAddress.buildingNumber = "29"
someAddress.street = "Acacia Road"
john.residence?.address = someAddress

func createAddress() -> Address {
    print("Function was called.")
    
    let someAddress = Address()
    someAddress.buildingNumber = "29"
    someAddress.street = "Acacia Road"
    
    return someAddress
}
john.residence?.address = createAddress()

通过可选链调用方法

可以使用可选链来调用可选项里的方法，并且检查调用是否成功。甚至可以在没有定义返回值的方法上这么做。

func printNumberOfRooms() {
    print("The number of rooms is \(numberOfRooms)")
}

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.")
}
// Prints "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.")
}
// Prints "It was not possible to set the address."

通过可选链访问下标

可以使用可选链来给可选项下标取回或设置值，并且检查下标的调用是否成功。

通过可选链访问一个可选项的下标时，你需要把问号放在下标括号的前边，而不是后边。可选链的问号一定是紧跟在可选项表达式的后边的。

if let firstRoomName = john.residence?[0].name {
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
}

john.residence?[0] = Room(name: "Bathroom")

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.")
}
// Prints "The first room name is Living Room."

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
// the "Dave" array is now [91, 82, 84] and the "Bev" array is now [80, 94, 81]

链的多层连接

如果访问的值不是可选项，它会因为可选链而变成可选项；
如果访问的值已经是可选的，它不会因为可选链而变得更加可选。

if let johnsStreet = john.residence?.address?.street {
    print("John's street name is \(johnsStreet).")
} else {
    print("Unable to retrieve the address.")
}
// Prints "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.")
}
// Prints "John's street name is Laurel Street."

用可选返回值链接方法

可以通过可选链来调用返回可选类型的方法，并且如果需要的话可以继续对方法的返回值进行链接。

if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
    print("John's building identifier is \(buildingIdentifier).")
}
// Prints "John's building identifier is The Larches."

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\".")
        }
}
// Prints "John's building identifier begins with "The"."
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