fix 2.17 optional chainning

source/chapter2/17_Optional_Chaining.md

@@ -1,7 +1,5 @@
 > 翻译：Jasonbroker  
-
+> 校对：numbbbbb, stanzhai  
-> 校对：numbbbbb
-
 
 # Optional Chaining
 -----------------
@@ -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
 
 首先定义两个类`Person`和`Residence`。
 
-	class Person {
+```swift
-        var residence: Residence?
+class Person {
-	}
+    var residence: Residence?
+}
 
-	class Residence {
+class Residence {
-        var numberOfRooms = 1
+    var numberOfRooms = 1
-	}
+}
+```
 
 `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？`。
 
 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：
 
-	let john = Person()
+```swift
+let john = Person()
+```
 
 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。
 
-	let roomCount = john.residence!.numberOfRooms
+```swift
-	//将导致运行时错误
+let roomCount = john.residence!.numberOfRooms
+//将导致运行时错误
+```
 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。
 
 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：
 
-	if let roomCount = john.residence?.numberOfRooms {
+```swift
-    	println("John's residence has \(roomCount) room(s).")
+if let roomCount = john.residence?.numberOfRooms {
-	} else {
+    println("John's residence has \(roomCount) room(s).")
-		println("Unable to retrieve the number of rooms.")
+} else {
-	}
+	println("Unable to retrieve the number of rooms.")
-	// 打印 "Unable to retrieve the number of rooms.
+}
-
+// 打印 "Unable to retrieve the number of rooms.
+```
 
 这告诉 Swift 来链接可选`residence?`属性，如果`residence`存在则取回`numberOfRooms`的值。
 
@@ -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
 
 你可以自己定义一个`Residence`实例给`john.residence`，这样它就不再为空了：
 
-	john.residence = Residence()
+```swift
+john.residence = Residence()
+```
 
 `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?`：
 
-	if let roomCount = john.residence?.numberOfRooms {
+```swift
-	    println("John's residence has \(roomCount) room(s).")
+if let roomCount = john.residence?.numberOfRooms {
-	} else {
+    println("John's residence has \(roomCount) room(s).")
-	    println("Unable to retrieve the number of rooms.")
+} else {
-	}
+    println("Unable to retrieve the number of rooms.")
-	// 打印 "John's residence has 1 room(s)"。
+}
+// 打印 "John's residence has 1 room(s)"。
+```
 
 <a name="defining_model_classes_for_optional_chaining"></a>
 ##为可选链定义模型类
@@ -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
 
 `Person`类定义与之前相同。
 
-	class Person {
+```swift
-	    var residence: Residence?
+class Person {
-	}
+    var residence: Residence?
+}
+```
 
 `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：
 
-	class Residence {
+```swift
-	    var rooms = Room[]()
+class Residence {
-	    var numberOfRooms: Int {
+    var rooms = Room[]()
-	    return rooms.count
+    var numberOfRooms: Int {
-	    }
+    return rooms.count
-	    subscript(i: Int) -> Room {
+    }
-	        return rooms[i]
+    subscript(i: Int) -> Room {
-	    }
+        return rooms[i]
-	    func printNumberOfRooms() {
+    }
-	        println("The number of rooms is \(numberOfRooms)")
+    func printNumberOfRooms() {
-	    }
+        println("The number of rooms is \(numberOfRooms)")
-	    var address: Address?
+    }
-	}
+    var address: Address?
+}
+```
 
 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。
 
@@ -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
 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。
 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。
 
-	class Room {
+```swift
-	    let name: String
+class Room {
-	    init(name: String) { self.name = name }
+    let name: String
-	}
+    init(name: String) { self.name = name }
+}
+```
 
 
 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：
 
-	class Address {
+```swift
-	    var buildingName: String?
+class Address {
-	    var buildingNumber: String?
+    var buildingName: String?
-	    var street: String?
+    var buildingNumber: String?
-	    func buildingIdentifier() -> String? {
+    var street: String?
-	        if buildingName {
+    func buildingIdentifier() -> String? {
-	            return buildingName
+        if buildingName {
-	        } else if buildingNumber {
+            return buildingName
-	            return buildingNumber
+        } else if buildingNumber {
-	        } else {
+            return buildingNumber
-	            return nil
+        } else {
-	        }
+            return nil
-	    }
+        }
-	}
+    }
+}
+```
 
 `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。
 
@@ -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
 
 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：
 
-	let john = Person()
+```swift
-	if let roomCount = john.residence?.numberOfRooms {
+let john = Person()
-	    println("John's residence has \(roomCount) room(s).")
+if let roomCount = john.residence?.numberOfRooms {
-	} else {
+    println("John's residence has \(roomCount) room(s).")
-	    println("Unable to retrieve the number of rooms.")
+} else {
-	}
+    println("Unable to retrieve the number of rooms.")
-	// 打印 "Unable to retrieve the number of rooms。
+}
+// 打印 "Unable to retrieve the number of rooms。
+```
 
 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。
 
@@ -170,20 +189,24 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
 
 `Residence`的`printNumberOfRooms`方法会打印`numberOfRooms`的当前值。方法如下：
 
-	func printNumberOfRooms(){
+```swift
-		println(“The number of rooms is \(numberOfRooms)”)
+func printNumberOfRooms(){
-	}
+	println(“The number of rooms is \(numberOfRooms)”)
+}
+```
 
 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）。
 
 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`：
 
-	if john.residence?.printNumberOfRooms() {
+```swift
-	    println("It was possible to print the number of rooms.")
+if john.residence?.printNumberOfRooms() {
-	} else {
+    println("It was possible to print the number of rooms.")
-	    println("It was not possible to print the number of rooms.")
+} else {
-	}
+    println("It was not possible to print the number of rooms.")
-	// 打印 "It was not possible to print the number of rooms."。
+}
+// 打印 "It was not possible to print the number of rooms."。
+```
 
 <a name="calling_subscripts_through_optional_chaining"></a>
 ##使用可选链调用子脚本
@@ -195,28 +218,32 @@ Swift 的可选链和 Objective-C 中的消息为空有些相像，但是 Swift
 
 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。
 
-	if let firstRoomName = john.residence?[0].name {
+```swift
-	    println("The first room name is \(firstRoomName).")
+if let firstRoomName = john.residence?[0].name {
-	} else {
+    println("The first room name is \(firstRoomName).")
-	    println("Unable to retrieve the first room name.")
+} else {
-	}
+    println("Unable to retrieve the first room name.")
-	// 打印 "Unable to retrieve the first room name."。
+}
+// 打印 "Unable to retrieve the first room name."。
+```
 
 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。
 
 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：
 
-	let johnsHouse = Residence()
+```swift
-	johnsHouse.rooms += Room(name: "Living Room")
+let johnsHouse = Residence()
-	johnsHouse.rooms += Room(name: "Kitchen")
+johnsHouse.rooms += Room(name: "Living Room")
-	john.residence = johnsHouse
+johnsHouse.rooms += Room(name: "Kitchen")
+john.residence = johnsHouse
 
-	if let firstRoomName = john.residence?[0].name {
+if let firstRoomName = john.residence?[0].name {
-	    println("The first room name is \(firstRoomName).")
+    println("The first room name is \(firstRoomName).")
-	} else {
+} else {
-	    println("Unable to retrieve the first room name.")
+    println("Unable to retrieve the first room name.")
-	}
+}
-	// 打印 "The first room name is Living Room."。
+// 打印 "The first room name is Living Room."。
+```
 
 <a name="linking_multiple_levels_of_chaining"></a>
 ##连接多层链接
@@ -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
 
 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，它们两者都是可选类型：
 
-	if let johnsStreet = john.residence?.address?.street {
+```swift
-	    println("John's street name is \(johnsStreet).")
+if let johnsStreet = john.residence?.address?.street {
-	} else {
+    println("John's street name is \(johnsStreet).")
-	    println("Unable to retrieve the address.")
+} else {
-	}
+    println("Unable to retrieve the address.")
-	// 打印 "Unable to retrieve the address.”。
+}
+// 打印 "Unable to retrieve the address.”。
+```
 
 `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。
 
@@ -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
 
 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。
 
-	let johnsAddress = Address()
+```swift
-	johnsAddress.buildingName = "The Larches"
+let johnsAddress = Address()
-	johnsAddress.street = "Laurel Street"
+johnsAddress.buildingName = "The Larches"
-	john.residence!.address = johnsAddress
+johnsAddress.street = "Laurel Street"
+john.residence!.address = johnsAddress
+```
 
-	if let johnsStreet = john.residence?.address?.street {
+```swift
-	    println("John's street name is \(johnsStreet).")
+if let johnsStreet = john.residence?.address?.street {
-	} else {
+    println("John's street name is \(johnsStreet).")
-	    println("Unable to retrieve the address.")
+} else {
-	}
+    println("Unable to retrieve the address.")
-	// 打印 "John's street name is Laurel Street."。
+}
+// 打印 "John's street name is Laurel Street."。
+```
 
 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。
 
@@ -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
 
 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?`：
 
-	if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
+```swift
-	    println("John's building identifier is \(buildingIdentifier).")
+if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
-	}
+    println("John's building identifier is \(buildingIdentifier).")
-	// 打印 "John's building identifier is The Larches."。
+}
+// 打印 "John's building identifier is The Larches."。
+```
 
 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：
 
-	if let upper = john.residence?.address?.buildingIdentifier()?.uppercaseString {
+```swift
-	    println("John's uppercase building identifier is \(upper).")
+if let upper = john.residence?.address?.buildingIdentifier()?.uppercaseString {
-	}
+    println("John's uppercase building identifier is \(upper).")
-	// 打印 "John's uppercase building identifier is THE LARCHES."。
+}
+// 打印 "John's uppercase building identifier is THE LARCHES."。
+```
 
 > 注意：  
 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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