Swift 可選鏈

查詢，調用屬性，下標和方法上的一個可選可能 'nil' 的過程被定義為可選的鏈。可選鏈返回兩個值

如果可選包含一個值，然後調用其相關屬性，方法和下標返回值
如果可選包含一個“nil”值，所有的相關屬性，方法和下標返回nil

由於多種查詢方法，屬性和下標故障組合在一起，以一種鏈將影響到整個鏈，並導致產生 'nil' 的值。

可選鏈作為一種替代強製解包裹

可選鏈與可選值後指定“?”調用一個屬性，方法或下標當可選的值返回一些值。

可選鏈 '?'	訪問的方法，屬性和下標可選鏈 “!” 強製解包
? 放在可選的值之後調用屬性，方法或下標	！放在可選的值之後調用屬性，方法或標來強製解包的值
失敗時，可選的值為 'nil'	強製解包觸發運行時錯誤時的可選值是 'nil'

程序用於可選鏈 '!'

class ElectionPoll {
   var candidate: Pollbooth?
}
class Pollbooth {   
   var name = "MP"
}
  
let cand = ElectionPoll()
   
let candname = cand.candidate!.name

當我們使用 playground 運行上麵的程序，得到以下結果。

fatal error: unexpectedly found nil while unwrapping an Optional value
0  swift                    0x0000000103410b68 llvm::sys::PrintStackTrace(__sFILE*) + 40
1  swift                    0x0000000103411054 SignalHandler(int) + 452
2  libsystem_platform.dylib 0x00007fff9176af1a _sigtramp + 26
3  libsystem_platform.dylib 0x000000000000000b _sigtramp + 1854492939
4  libsystem_platform.dylib 0x00000001074a0214 _sigtramp + 1976783636
5  swift                    0x0000000102a85c39 llvm::JIT::runFunction(llvm::Function*, std::__1::vector > const&) + 329
6  swift                    0x0000000102d320b3 llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*, std::__1::vector, std::__1::allocator >, std::__1::allocator, std::__1::allocator > > > const&, char const* const*) + 1523
7  swift                    0x000000010296e6ba swift::RunImmediately(swift::CompilerInstance&, std::__1::vector, std::__1::allocator >, std::__1::allocator, std::__1::allocator > > > const&, swift::IRGenOptions&, swift::SILOptions const&) + 1066
8  swift                    0x000000010275764b frontend_main(llvm::ArrayRef, char const*, void*) + 5275
9  swift                    0x0000000102754a6d main + 1677
10 libdyld.dylib            0x00007fff8bb9e5c9 start + 1
11 libdyld.dylib            0x000000000000000c start + 1950751300
Stack dump:
0.	Program arguments: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -interpret - -target x86_64-apple-darwin14.0.0 -target-cpu core2 -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -module-name main 
/bin/sh: line 47: 15672 Done                    cat  <<'SWIFT'
import Foundation

上述程序中聲明“ election poll” 作為類名，並包含了作為隸屬函數“candidate”。子類被聲明為 “poll booth” 和 “name” 作為被初始化為 'MP' 的隸屬度函數。對超類的調用是通過創建一個實例，“cand”可選的初始化 "!“。由於這些值在它的基類冇有聲明，“nil” 值被存儲，從而通過強製解包處理過程返回一個致命的錯誤。

程序用於可選鏈 '?'

class ElectionPoll {
   var candidate: Pollbooth?
}
class Pollbooth {
   var name = "MP"
}
   
let cand = ElectionPoll()
   
if let candname = cand.candidate?.name {
   println("Candidate name is \(candname)")
} 
else {
   println("Candidate name cannot be retreived")
}

當我們使用 playground 運行上麵的程序，得到以下結果。

Candidate name cannot be retreived

上述程序中聲明“ election poll” 作為類名，並包含了作為隸屬函數“candidate”。子類被聲明為 “poll booth” 和 “name” 作為被初始化為 'MP' 的隸屬度函數。對超類的調用是通過創建一個實例，“cand”可選的初始化“?”。由於基類 'nil' 的值不聲明被存儲並打印在控製台，由其他程序塊處理。

定義模型類的可選鏈接和訪問屬性

Swift 語言還提供可選鏈的概念，聲明多個子類的模型類。這個概念將是定義複雜的模型和訪問屬性，方法和下標子屬性非常有用。

class rectangle {
   var print: circle?
}

class circle {
   var area = [radius]()
   var cprint: Int {
      return area.count
   }
   
   subscript(i: Int) -> radius {
      get {
         return area[i]
      }
      set {
         area[i] = newValue
      }
   }
   func circleprint() {
      println("The number of rooms is \(cprint)")
   }
   var rectarea: circumference?
}

class radius {
   let radiusname: String
   init(radiusname: String) { self.radiusname = radiusname }
}

class circumference {
   var circumName: String?
   var circumNumber: String?
   var street: String?
   
   func buildingIdentifier() -> String? {
      if circumName != nil {
         return circumName
      } else if circumNumber != nil {
         return circumNumber
      } else {
         return nil
      }
   }
}

let rectname = rectangle()

if let rectarea = rectname.print?.cprint {
   println("Area of rectangle is \(rectarea)")
}  else {
   println("Rectangle Area is not specified")
}

當我們使用 playground 運行上麵的程序，得到以下結果。

Rectangle Area is not specified

通過可選鏈調用方法

class rectangle {
   var print: circle?
}
class circle {
   var area = [radius]()
   var cprint: Int {
      return area.count
   }
   subscript(i: Int) -> radius {
      get {
         return area[i]
      }
      set {
         area[i] = newValue
      }
   }
   
   func circleprint() {
      println("Area of Circle is: \(cprint)")
   }
   var rectarea: circumference?
}

class radius {
   let radiusname: String
   init(radiusname: String) { self.radiusname = radiusname }
}

class circumference {
   var circumName: String?
   var circumNumber: String?
   var circumarea: String?
   
   func buildingIdentifier() -> String? {
      if circumName != nil {
         return circumName
      } else if circumNumber != nil {
         return circumNumber
      } else {
         return nil
      }
   }
}

let circname = rectangle()

if circname.print?.circleprint() != nil {
   println("Area of circle is specified)")
} else {
   println("Area of circle is not specified")
}

當我們使用 playground 運行上麵的程序，得到以下結果。

Area of circle is not specified

該函數circleprint()在circle()子類中聲明通過創建名為 “circname” 的一個實例來調用。函數會返回一個值如果它包含一定的值則返回一些用戶定義的打印信息。否則會被檢查的語句 'if circname.print?.circleprint() != nil“

下標訪問通過可選鏈

可選鏈設置和檢索下標值以驗證是否要調用該下標返回一個值。 '?' 被放置在下標括號之前來訪問特定的標可選值。

程序 1

class rectangle {
   var print: circle?
}

class circle {
   var area = [radius]()
   var cprint: Int {
      return area.count
   }
   
   subscript(i: Int) -> radius {
      get {
         return area[i]
      }
      
      set {
         area[i] = newValue
      }
   }
   func circleprint() {
      println("The number of rooms is \(cprint)")
   }
   var rectarea: circumference?
}


class radius {
   let radiusname: String
   init(radiusname: String) { self.radiusname = radiusname }
}

class circumference {
   var circumName: String?
   var circumNumber: String?
   var circumarea: String?
   
   func buildingIdentifier() -> String? {
      if circumName != nil {
         return circumName
      } else if circumNumber != nil {
         return circumNumber
      } else {
         return nil
      }
   }
}


let circname = rectangle()

if let radiusName = circname.print?[0].radiusname {
   println("The first room name is \(radiusName).")
} else {
   println("Radius is not specified.")
}

當我們使用 playground 運行上麵的程序，得到以下結果。

Radius is not specified.

在以上程序未指定成員函數 “radiusName” 的實例值。因此，程序調用該函數將返回唯一內容，以返回必須定義特定成員函數值的一部分。

程序 2

class rectangle {
   var print: circle?
}

class circle {
   var area = [radius]()
   var cprint: Int {
      return area.count
   }
   
   subscript(i: Int) -> radius {
      get {
         return area[i]
      }
      set {
         area[i] = newValue
      }
   }
   func circleprint() {
      println("The number of rooms is \(cprint)")
   }
   var rectarea: circumference?
}

class radius {
   let radiusname: String
   init(radiusname: String) { self.radiusname = radiusname }
}

class circumference {
   var circumName: String?
   var circumNumber: String?
   var circumarea: String?
   
   func buildingIdentifier() -> String? {
      if circumName != nil {
         return circumName
      } else if circumNumber != nil {
         return circumNumber
      } else {
         return nil
      }
   }
}

let circname = rectangle()
circname.print?[0] = radius(radiusname: "Diameter")

let printing = circle()
printing.area.append(radius(radiusname: "Units"))
printing.area.append(radius(radiusname: "Meter"))
circname.print = printing

if let radiusName = circname.print?[0].radiusname {
   println("Radius is measured in \(radiusName).")
} else {
   println("Radius is not specified.")
}

當我們使用 playground 運行上麵的程序，得到以下結果。

Radius is measured in Units.

在上麵的程序被指定為成員函數 “radiusName” 的實例值。因此，程序現在調用函數將有返回值。

可選類型訪問下標

class rectangle {
   var print: circle?
}

class circle {
   var area = [radius]()
   var cprint: Int {
      return area.count
   }
   
   subscript(i: Int) -> radius {
      get {
         return area[i]
      }
      set {
         area[i] = newValue
      }
   }
   
   func circleprint() {
      println("The number of rooms is \(cprint)")
   }
   var rectarea: circumference?
}


class radius {
   let radiusname: String
   init(radiusname: String) { self.radiusname = radiusname }
}

class circumference {
   var circumName: String?
   var circumNumber: String?
   var circumarea: String?
   
   func buildingIdentifier() -> String? {
      if circumName != nil {
         return circumName
      } else if circumNumber != nil {
         return circumNumber
      } else {
         return nil
      }
   }
}

let circname = rectangle()
circname.print?[0] = radius(radiusname: "Diameter")

let printing = circle()
printing.area.append(radius(radiusname: "Units"))
printing.area.append(radius(radiusname: "Meter"))
circname.print = printing

var area = ["Radius": [35, 45, 78, 101], "Circle": [90, 45, 56]]
area["Radius"]?[1] = 78
area["Circle"]?[1]--

println(area["Radius"]?[0])
println(area["Radius"]?[1])
println(area["Radius"]?[2])
println(area["Radius"]?[3])


println(area["Circle"]?[0])
println(area["Circle"]?[1])
println(area["Circle"]?[2])

當我們使用 playground 運行上麵的程序，得到以下結果。

Optional(35)
Optional(78)
Optional(78)
Optional(101)
Optional(90)
Optional(44)
Optional(56)

對於下標可選的值可以通過參考其下標值來訪問。它可訪問為 subscript[0], subscript[1]等。radius 的默認值下標首先被指定為 [35，45，78，101] 和 Circle[90，45，56]。然後，下標值作為 Radius[0] 至78，以及Circle[1]〜45。

鏈接鏈的多層次性

多個子類也可以憑借其超類的方法，屬性和下標通過可選鏈接聯係。

可選的多個鏈可以鏈接：

如果檢索類型是不可選的，可選鏈會返回一個可選值。例如，如果字符串通過可選鏈就會返回字符串？值

class rectangle {
   var print: circle?
}

class circle {
   var area = [radius]()
   var cprint: Int {
      return area.count
   }
   subscript(i: Int) -> radius {
      get {
         return area[i]
      }
      set {
         area[i] = newValue
      }
   }
   func circleprint() {
      println("The number of rooms is \(cprint)")
   }
   var rectarea: circumference?
}


class radius {
   let radiusname: String
   init(radiusname: String) { self.radiusname = radiusname }
}

class circumference {
   var circumName: String?
   var circumNumber: String?
   var circumarea: String?
   
   func buildingIdentifier() -> String? {
      if circumName != nil {
         return circumName
      } else if circumNumber != nil {
         return circumNumber
      } else {
         return nil
      }
   }
}


let circname = rectangle()

if let radiusName = circname.print?[0].radiusname {
   println("The first room name is \(radiusName).")
} else {
   println("Radius is not specified.")
}

當我們使用 playground 運行上麵的程序，得到以下結果。

Radius is not specified.

在以上程序未指定隸屬函數 “radiusName” 的實例值。因此，程序調用該函數將返回唯一內容，我們必須定義返回值的特定隸屬作為函數值的一部分。

如果檢索式已經是可選的，那麼可選鏈也將返回一個可選值。例如，如果字符串 ? 通過可選的鏈也將訪問返回字符串 ?值。

class rectangle {
   var print: circle?
}

class circle {
   var area = [radius]()
   var cprint: Int {
      return area.count
   }
   subscript(i: Int) -> radius {
      get {
         return area[i]
      }
      set {
         area[i] = newValue
      }
   }
   func circleprint() {
      println("The number of rooms is \(cprint)")
   }
   var rectarea: circumference?
}


class radius {
   let radiusname: String
   init(radiusname: String) { self.radiusname = radiusname }
}

class circumference {
   var circumName: String?
   var circumNumber: String?
   var circumarea: String?
   
   func buildingIdentifier() -> String? {
      if circumName != nil {
         return circumName
      } else if circumNumber != nil {
         return circumNumber
      } else {
         return nil
      }
   }
}


let circname = rectangle()

if let radiusName = circname.print?[0].radiusname {
   println("The first room name is \(radiusName).")
} else {
   println("Radius is not specified.")
}

當我們使用 playground 運行上麵的程序，得到以下結果。

Radius is measured in Units.

在上麵的程序被指定為隸屬函數 “radiusName” 的實例值。因此，程序調用函數將返回值。

鏈上的方法可選返回值

可選鏈是用來定義方法來訪問子類。

class rectangle {
   var print: circle?
}

class circle {
   var area = [radius]()
   var cprint: Int {
      return area.count
   }
   
   subscript(i: Int) -> radius {
      get {
         return area[i]
      }
      set {
         area[i] = newValue
      }
   }
   func circleprint() {
      println("The number of rooms is \(cprint)")
   }
   var rectarea: circumference?
}

class radius {
   let radiusname: String
   init(radiusname: String) { self.radiusname = radiusname }
}

class circumference {
   var circumName: String?
   var circumNumber: String?
   var circumarea: String?
   
   func buildingIdentifier() -> String? {
      if circumName != nil {
         return circumName
      } else if circumNumber != nil {
         return circumNumber
      } else {
         return nil
      }
   }
}

let circname = rectangle()
circname.print?[0] = radius(radiusname: "Diameter")

let printing = circle()
printing.area.append(radius(radiusname: "Units"))
printing.area.append(radius(radiusname: "Meter"))
circname.print = printing

if let radiusName = circname.print?[0].radiusname {
   println("Radius is measured in \(radiusName).")
} else {
   println("Radius is not specified.")
}

當我們使用 playground 運行上麵的程序，得到以下結果。

Area of circle is not specified