Swift 可选链|极客教程

Swift 可选链

查询、调用可能为’nil’的可选值上的属性、下标和方法的过程被定义为可选链。可选链返回两个值：

如果可选值包含一个’value’，则调用其相关的属性、方法和下标返回值
如果可选值包含’nil’，则其相关的属性、方法和下标返回nil

由于多个查询到方法、属性和下标被组合在一起，对一个链的失败将影响整个链，并且导致’nil’值的结果。

可选链作为强制解包的替代方式

可选链在可选值后指定’?’，在可选值返回一些值时调用属性、方法或下标。

Optional Chaining ‘?’ :访问方法、属性和下标的可选链接’!’
可选值后面加上’?’来调用属性、方法或下标：可选值后面加上’!’来强制解包属性、方法或下标的值
如果可选项为’nil’，则会优雅地失败：如果可选项为’nil’，则强制解包会触发运行时错误

可选链接与’!’的程序

class ElectionPoll {
   var candidate: Pollbooth?
}

lass Pollbooth {
   var name = "MP"
}

let cand = ElectionPoll()
let candname = cand.candidate!.name

当我们使用playground运行上面的程序时，会得到以下结果 –

fatal error: unexpectedly found nil while unwrapping an Optional value
0 Swift 4 0x0000000103410b68
llvm::sys::PrintStackTrace(__sFILE*) + 40
1 Swift 4 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 4 0x0000000102a85c39
llvm::JIT::runFunction(llvm::Function*, std::__1::vector > const&) + 329
6 Swift 4 0x0000000102d320b3
llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*,
std::__1::vector<std::__1::basic_string, std::__1::allocator >,
std::__1::allocator<std::__1::basic_string, std::__1::allocator > > > const&,
char const* const*) + 1523
7 Swift 4 0x000000010296e6ba Swift 4::RunImmediately(Swift
4::CompilerInstance&, std::__1::vector<std::__1::basic_string,
std::__1::allocator >, std::__1::allocator<std::__1::basic_string,
std::__1::allocator > > > const&, Swift 4::IRGenOptions&, Swift 4::SILOptions
const&) + 1066
8 Swift 4 0x000000010275764b frontend_main(llvm::ArrayRef,
char const*, void*) + 5275
9 Swift 4 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 4 -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 4'
import Foundation
</std::__1::basic_string</std::__1::basic_string</std::__1::basic_string</std::
__1::basic_string

以上程序将 “选举调查” 声明为类名，并包含 “候选人” 作为成员函数。子类被声明为 “投票站”，并将 “名称” 声明为其成员函数，其初始值为 “MP”。通过创建一个可选实例 “cand” 来初始化对父类的调用，可选实例通过可选绑定操作符 “!” 初始化。由于基类中的值未声明，因此存储了 “nil” 值，从而通过强制解包过程返回了致命错误。

使用 “？” 的可选链编程示例

class ElectionPoll {
   var candidate: Pollbooth?
}

class Pollbooth {
   var name = "MP"
}
let cand = ElectionPoll()

if let candname = cand.candidate?.name {
   print("Candidate name is \(candname)")
} else {
   print("Candidate name cannot be retreived")
}

当我们在playground上运行上面的程序时，我们得到以下结果 –

Candidate name cannot be retreived

上面的程序将“选举投票”声明为类名，并包含“候选人”作为成员函数。子类声明为“投票亭”，其成员函数为“名称”，并初始化为“MP”。通过创建可选的“？”实例“cand”来初始化对超类的调用。由于值未在其基类中声明，“nil”值将被存储并由else处理程序块在控制台中打印。

为可选链接和访问属性定义模型类

Swift 4语言还提供了可选链接的概念，以声明多个子类作为模型类。这个概念对于定义复杂的模型和访问属性、方法和子属性的方法非常有用。

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() {
      print("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 {
   print("Area of rectangle is \(rectarea)")
} else {
   print("Rectangle Area is not specified")
}

当我们使用游乐场运行上述程序时，我们会得到以下结果 –

Rectangle Area is not specified

通过可选链调用方法

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() {
      print("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 {
   print("Area of circle is specified)")
} else {
   print("Area of circle is not specified")
}

当我们在playground上运行上述程序时，我们得到以下结果 –

Area of circle is not specified

在circle()子类中声明的circleprint()函数通过创建名为’circname’的实例来调用。如果函数包含某个值，它将返回一个值，否则将通过检查语句’if circname.print?.circleprint() != nil’返回一条用户定义的打印消息。

通过可选链接访问下标

可选链接用于设置和检索下标值，以验证调用该下标是否返回一个值。在下标括号之前添加’?’以访问特定下标的可选值。

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() {
      print("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 {
   print("The first room name is \(radiusName).")
} else {
   print("Radius is not specified.")
}

当我们使用playground运行上面的程序时，我们得到以下结果−

Radius is not specified.

在上面的程序中，未指定成员函数“radiusName”的实例值。因此，程序调用函数将只返回else部分，而要返回值，我们必须为特定的成员函数定义值。

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() {
      print("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 {
   print("Radius is measured in \(radiusName).")
} else {
   print("Radius is not specified.")
}

当我们使用playground运行上述程序时，得到以下结果 –

Radius is measured in Units.

在上面的程序中，指定了成员函数 ‘radiusName’ 的实例值。因此，对该函数的程序调用现在将返回值。

访问可选类型的下标

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() {
      print("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]--

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

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

当我们使用playground运行上面的程序时，我们得到以下结果-

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

可通过引用它们的下标值来访问下标的可选值。可以使用subscript[0]，subscript[1]等来访问。‘半径’的默认下标值首先被赋值为[35, 45, 78, 101]，而‘圆’的默认下标值为[90, 45, 56]。然后将下标值更改为radius[0] = 78和Circle[1] = 45。

链接多级链接

多个子类也可以通过可选链接与其父类的方法、属性和下标进行链接。

可以链接多个可选链接 −

如果检索类型不是可选类型，可选链接将返回可选值。例如，通过可选链接获得String将返回String?的值。

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() {
      print("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 {
   print("The first room name is \(radiusName).")
} else {
   print("Radius is not specified.")
}

当我们在playground中运行上述程序时，我们会得到以下结果-

Radius is not specified.

在上面的程序中，没有指定成员函数“radiusName”的实例值。因此，程序调用该函数将仅返回else部分，而要返回值，我们必须定义特定成员函数的值。

如果检索类型已经是可选的，则可选链接还将返回一个可选值。例如，如果通过可选链接访问String?，它将返回String?的值。

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() {
      print("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 {
   print("Radius is measured in \(radiusName).")
} else {
   print("Radius is not specified.")
}

运行以上程序时，我们使用playground，我们得到以下结果−

Radius is measured in Units.

在上面的程序中，指定了“radiusName”的成员函数的实例值。因此，对函数的程序调用将返回值。

使用可选返回值链接方法

可选链接可用于访问定义的子类方法。

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() {
      print("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 {
   print("Area of circle is specified)")
} else {
   print("Area of circle is not specified")
}

当我们在playground上运行上述程序时，我们得到以下结果 –