fix 2.17 optional chainning
This commit is contained in:
stanzhai
@ -1,7 +1,5 @@
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> 翻译：Jasonbroker
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> 校对：numbbbbb
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> 翻译：Jasonbroker
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> 校对：numbbbbb, stanzhai
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# Optional Chaining
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-----------------
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@ -18,7 +16,7 @@
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可选链（Optional Chaining）是一种可以请求和调用属性、方法及子脚本的过程，它的可选性体现于请求或调用的目标当前可能为空（`nil`）。如果可选的目标有值，那么调用就会成功；相反，如果选择的目标为空（`nil`），则这种调用将返回空（`nil`）。多次请求或调用可以被链接在一起形成一个链，如果任何一个节点为空（`nil`）将导致整个链失效。
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> 注意：
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> 注意：
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Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift 可以使用在任意类型中，并且失败与否可以被检测到。
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<a name="optional_chaining_as_an_alternative_to_forced_unwrapping"></a>
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@ -34,35 +32,42 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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首先定义两个类`Person`和`Residence`。
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class Person {
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var residence: Residence?
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}
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```swift
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class Person {
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var residence: Residence?
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}
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class Residence {
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var numberOfRooms = 1
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}
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class Residence {
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var numberOfRooms = 1
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}
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```
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`Residence`具有一个`Int`类型的`numberOfRooms`，其值为 1。`Person`具有一个可选`residence`属性，它的类型是`Residence？`。
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如果你创建一个新的`Person`实例，它的`residence`属性由于是被定义为可选型的，此属性将默认初始化为空：
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let john = Person()
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```swift
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let john = Person()
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```
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如果你想使用感叹号（`!`）强制解析获得这个人`residence`属性`numberOfRooms`属性值，将会引发运行时错误，因为这时没有可以供解析的`residence`值。
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let roomCount = john.residence!.numberOfRooms
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//将导致运行时错误
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```swift
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let roomCount = john.residence!.numberOfRooms
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//将导致运行时错误
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```
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当`john.residence`不是`nil`时，会运行通过，且会将`roomCount` 设置为一个`int`类型的合理值。然而，如上所述，当`residence`为空时，这个代码将会导致运行时错误。
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可选链提供了一种另一种获得`numberOfRooms`的方法。利用可选链，使用问号来代替原来`!`的位置：
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if let roomCount = john.residence?.numberOfRooms {
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println("John's residence has \(roomCount) room(s).")
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} else {
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println("Unable to retrieve the number of rooms.")
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}
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// 打印 "Unable to retrieve the number of rooms.
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```swift
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if let roomCount = john.residence?.numberOfRooms {
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println("John's residence has \(roomCount) room(s).")
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} else {
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println("Unable to retrieve the number of rooms.")
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}
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// 打印 "Unable to retrieve the number of rooms.
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```
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这告诉 Swift 来链接可选`residence?`属性，如果`residence`存在则取回`numberOfRooms`的值。
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@ -72,16 +77,20 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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你可以自己定义一个`Residence`实例给`john.residence`，这样它就不再为空了：
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john.residence = Residence()
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```swift
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john.residence = Residence()
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```
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`john.residence` 现在有了实际存在的实例而不是nil了。如果你想使用和前面一样的可选链来获得`numberOfRoooms`，它将返回一个包含默认值 1 的`Int?`：
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if let roomCount = john.residence?.numberOfRooms {
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println("John's residence has \(roomCount) room(s).")
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} else {
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println("Unable to retrieve the number of rooms.")
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}
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// 打印 "John's residence has 1 room(s)"。
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```swift
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if let roomCount = john.residence?.numberOfRooms {
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println("John's residence has \(roomCount) room(s).")
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} else {
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println("Unable to retrieve the number of rooms.")
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}
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// 打印 "John's residence has 1 room(s)"。
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```
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<a name="defining_model_classes_for_optional_chaining"></a>
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##为可选链定义模型类
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@ -92,25 +101,29 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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`Person`类定义与之前相同。
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class Person {
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var residence: Residence?
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}
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```swift
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class Person {
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var residence: Residence?
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}
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```
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`Residence`类比之前复杂些。这次，它定义了一个变量 `rooms`，它被初始化为一个`Room[]`类型的空数组：
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class Residence {
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var rooms = Room[]()
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var numberOfRooms: Int {
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return rooms.count
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}
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subscript(i: Int) -> Room {
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return rooms[i]
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}
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func printNumberOfRooms() {
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println("The number of rooms is \(numberOfRooms)")
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}
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var address: Address?
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}
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```swift
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class Residence {
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var rooms = Room[]()
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var numberOfRooms: Int {
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return rooms.count
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}
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subscript(i: Int) -> Room {
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return rooms[i]
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}
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func printNumberOfRooms() {
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println("The number of rooms is \(numberOfRooms)")
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}
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var address: Address?
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}
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```
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因为`Residence`存储了一个`Room`实例的数组，它的`numberOfRooms`属性值不是一个固定的存储值，而是通过计算而来的。`numberOfRooms`属性值是由返回`rooms`数组的`count`属性值得到的。
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@ -121,28 +134,32 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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最后，`Residence`定义了一个可选属性叫`address`（`address?`）。`Address`类的属性将在后面定义。
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用于`rooms`数组的`Room`类是一个很简单的类，它只有一个`name`属性和一个设定`room`名的初始化器。
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class Room {
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let name: String
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init(name: String) { self.name = name }
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}
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```swift
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class Room {
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let name: String
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init(name: String) { self.name = name }
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}
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```
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这个模型中的最终类叫做`Address`。它有三个类型是`String?`的可选属性。前面两个可选属性`buildingName`和 `buildingNumber`作为地址的一部分，是定义某个建筑物的两种方式。第三个属性`street`，用于命名地址的街道名：
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class Address {
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var buildingName: String?
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var buildingNumber: String?
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var street: String?
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func buildingIdentifier() -> String? {
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if buildingName {
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return buildingName
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} else if buildingNumber {
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return buildingNumber
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} else {
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return nil
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}
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}
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}
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```swift
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class Address {
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var buildingName: String?
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var buildingNumber: String?
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var street: String?
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func buildingIdentifier() -> String? {
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if buildingName {
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return buildingName
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} else if buildingNumber {
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return buildingNumber
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} else {
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return nil
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}
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}
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}
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```
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`Address`类还提供了一个`buildingIdentifier`的方法，它的返回值类型为`String?`。这个方法检查`buildingName`和`buildingNumber`的属性，如果`buildingName`有值则将其返回，或者如果`buildingNumber`有值则将其返回，再或如果没有一个属性有值，返回空。
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@ -153,13 +170,15 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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使用上述定义的类来创建一个人实例，并再次尝试后去它的`numberOfRooms`属性：
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let john = Person()
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if let roomCount = john.residence?.numberOfRooms {
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println("John's residence has \(roomCount) room(s).")
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} else {
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println("Unable to retrieve the number of rooms.")
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}
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// 打印 "Unable to retrieve the number of rooms。
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```swift
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let john = Person()
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if let roomCount = john.residence?.numberOfRooms {
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println("John's residence has \(roomCount) room(s).")
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} else {
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println("Unable to retrieve the number of rooms.")
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}
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// 打印 "Unable to retrieve the number of rooms。
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```
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由于`john.residence`是空，所以这个可选链和之前一样失败了，但是没有运行时错误。
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@ -170,53 +189,61 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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`Residence`的`printNumberOfRooms`方法会打印`numberOfRooms`的当前值。方法如下：
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func printNumberOfRooms(){
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println(“The number of rooms is \(numberOfRooms)”)
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}
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```swift
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func printNumberOfRooms(){
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println(“The number of rooms is \(numberOfRooms)”)
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}
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```
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这个方法没有返回值。但是，没有返回值类型的函数和方法有一个隐式的返回值类型`Void`（参见Function Without Return Values）。
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如果你利用可选链调用此方法，这个方法的返回值类型将是`Void?`，而不是`Void`，因为当通过可选链调用方法时返回值总是可选类型（optional type）。即使这个方法本身没有定义返回值，你也可以使用`if`语句来检查是否能成功调用`printNumberOfRooms`方法：如果方法通过可选链调用成功，`printNumberOfRooms`的隐式返回值将会是`Void`，如果没有成功，将返回`nil`：
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if john.residence?.printNumberOfRooms() {
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println("It was possible to print the number of rooms.")
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} else {
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println("It was not possible to print the number of rooms.")
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}
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// 打印 "It was not possible to print the number of rooms."。
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```swift
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if john.residence?.printNumberOfRooms() {
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println("It was possible to print the number of rooms.")
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} else {
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println("It was not possible to print the number of rooms.")
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}
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// 打印 "It was not possible to print the number of rooms."。
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```
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<a name="calling_subscripts_through_optional_chaining"></a>
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##使用可选链调用子脚本
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你可以使用可选链来尝试从子脚本获取值并检查子脚本的调用是否成功，然而，你不能通过可选链来设置子代码。
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> 注意：
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> 注意：
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当你使用可选链来获取子脚本的时候，你应该将问号放在子脚本括号的前面而不是后面。可选链的问号一般直接跟在表达语句的后面。
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下面这个例子用在`Residence`类中定义的子脚本来获取`john.residence`数组中第一个房间的名字。因为`john.residence`现在是`nil`，子脚本的调用失败了。
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if let firstRoomName = john.residence?[0].name {
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println("The first room name is \(firstRoomName).")
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} else {
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println("Unable to retrieve the first room name.")
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}
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// 打印 "Unable to retrieve the first room name."。
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```swift
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if let firstRoomName = john.residence?[0].name {
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println("The first room name is \(firstRoomName).")
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} else {
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println("Unable to retrieve the first room name.")
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}
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// 打印 "Unable to retrieve the first room name."。
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```
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在子代码调用中可选链的问号直接跟在`john.residence`的后面，在子脚本括号的前面，因为`john.residence`是可选链试图获得的可选值。
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如果你创建一个`Residence`实例给`john.residence`，且在他的`rooms`数组中有一个或多个`Room`实例，那么你可以使用可选链通过`Residence`子脚本来获取在`rooms`数组中的实例了：
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let johnsHouse = Residence()
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johnsHouse.rooms += Room(name: "Living Room")
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johnsHouse.rooms += Room(name: "Kitchen")
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john.residence = johnsHouse
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```swift
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let johnsHouse = Residence()
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johnsHouse.rooms += Room(name: "Living Room")
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johnsHouse.rooms += Room(name: "Kitchen")
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john.residence = johnsHouse
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if let firstRoomName = john.residence?[0].name {
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println("The first room name is \(firstRoomName).")
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} else {
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println("Unable to retrieve the first room name.")
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}
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// 打印 "The first room name is Living Room."。
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if let firstRoomName = john.residence?[0].name {
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println("The first room name is \(firstRoomName).")
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} else {
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println("Unable to retrieve the first room name.")
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}
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// 打印 "The first room name is Living Room."。
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```
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<a name="linking_multiple_levels_of_chaining"></a>
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##连接多层链接
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@ -234,12 +261,14 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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下面的例子试图获取`john`的`residence`属性里的`address`的`street`属性。这里使用了两层可选链来联系`residence`和`address`属性，他们两者都是可选类型：
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if let johnsStreet = john.residence?.address?.street {
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println("John's street name is \(johnsStreet).")
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} else {
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println("Unable to retrieve the address.")
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}
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// 打印 "Unable to retrieve the address.”。
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```swift
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if let johnsStreet = john.residence?.address?.street {
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println("John's street name is \(johnsStreet).")
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} else {
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println("Unable to retrieve the address.")
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}
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// 打印 "Unable to retrieve the address.”。
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```
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`john.residence`的值现在包含一个`Residence`实例，然而`john.residence.address`现在是`nil`，因此`john.residence?.address?.street`调用失败。
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@ -247,17 +276,21 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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如果你为`Address`设定一个实例来作为`john.residence.address`的值，并为`address`的`street`属性设定一个实际值，你可以通过多层可选链来得到这个属性值。
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let johnsAddress = Address()
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johnsAddress.buildingName = "The Larches"
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johnsAddress.street = "Laurel Street"
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john.residence!.address = johnsAddress
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```swift
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let johnsAddress = Address()
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johnsAddress.buildingName = "The Larches"
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johnsAddress.street = "Laurel Street"
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john.residence!.address = johnsAddress
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```
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if let johnsStreet = john.residence?.address?.street {
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println("John's street name is \(johnsStreet).")
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} else {
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println("Unable to retrieve the address.")
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}
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// 打印 "John's street name is Laurel Street."。
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```swift
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if let johnsStreet = john.residence?.address?.street {
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println("John's street name is \(johnsStreet).")
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} else {
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println("Unable to retrieve the address.")
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}
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// 打印 "John's street name is Laurel Street."。
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```
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值得注意的是，“`!`”符号在给`john.residence.address`分配`address`实例时的使用。`john.residence`属性是一个可选类型，因此你需要在它获取`address`属性之前使用`!`解析以获得它的实际值。
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@ -268,24 +301,21 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
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下面的例子通过可选链调用了`Address`类中的`buildingIdentifier` 方法。这个方法的返回值类型是`String?`。如上所述，这个方法在可选链调用后最终的返回值类型依然是`String?`：
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if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
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println("John's building identifier is \(buildingIdentifier).")
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}
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// 打印 "John's building identifier is The Larches."。
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```swift
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if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
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println("John's building identifier is \(buildingIdentifier).")
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}
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// 打印 "John's building identifier is The Larches."。
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```
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如果你还想进一步对方法返回值执行可选链，将可选链问号符放在方法括号的后面：
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if let upper = john.residence?.address?.buildingIdentifier()?.uppercaseString {
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println("John's uppercase building identifier is \(upper).")
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}
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// 打印 "John's uppercase building identifier is THE LARCHES."。
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```swift
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if let upper = john.residence?.address?.buildingIdentifier()?.uppercaseString {
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println("John's uppercase building identifier is \(upper).")
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}
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// 打印 "John's uppercase building identifier is THE LARCHES."。
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```
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> 注意：
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> 注意：
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||||
在上面的例子中，你将可选链问号符放在括号后面是因为你想要链接的可选值是`buildingIdentifier`方法的返回值，不是`buildingIdentifier`方法本身。
|
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|
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|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
Reference in New Issue
Block a user