Swift可选链(Optional Chaining)

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2023-03-14

在可选的nil上查询，调用属性，下标和方法的过程叫作可选链。可选链接返回两个值 -

如果可选项包含值，则调用相关属性，方法和下标返回值。
如果可选项包含nil值，则所有相关属性，方法和下标返回nil。

由于对方法，属性和下标的多个查询组合在一起，因此对一个链的失败将影响整个链的结果为nil值。

可选链作为强制解包的替代方法

在带有?的可选值后指定可选链，在可选值返回某些值时调用属性，方法或下标。

可选链`?`	访问方法，属性和下标可选链`!`强行展开
`?`在可选值之后放置调用属性，方法或下标	`!`放在可选值之后调用属性，方法或下标来强制展开值
当可选项为`nil`时，优雅地失败	当可选项为`nil`时，强制解包会触发运行时错误

使用 ! 进行可选链的程序

class ElectionPoll {
   var candidate: Pollbooth?
}

lass Pollbooth {
   var name = "MP"
}

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

当使用playground运行上述程序时，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

上述程序将“electionpoll”声明为类名，并包含candidate成员函数。子类被声明为pollbooth，有一个名称为name成员函数，它被初始化为MP。通过使用可选的!创建一个实例cand来初始化对超类的调用。由于值未在基类中声明，因此存储nil值，从而通过强制解包过程返回致命错误。

使用 ? 的可选链程序

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")
}

当使用playground运行上述程序时，得到以下结果 -

Candidate name cannot be retreived

上述程序将electionpoll声明为类名，并使用candidate作为成员函数。子类名称为pollbooth，使用name作为其成员函数，它被初始化为MP。通过使用可选的?创建一个实例cand来初始化对超类的调用。由于值未在其基类中声明nil，因此在else处理程序块存储并在控制台中打印。

为可选链和访问属性定义模型类

Swift 4语言提供了可选链的概念，将多个子类声明为模型类。这个概念对于定义复杂模型和访问属性，方法和下标子属性非常有用。

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() {
      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")
}

当使用playground运行上述程序时，得到以下结果 -

Rectangle Area is not specified

通过可选链接调用方法

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")
}

当使用playground运行上述程序时，得到以下结果 -

Area of circle is not specified

通过创建名为circname的实例来调用circle()子类中声明的函数circleprint()。如果函数包含某个值，则该函数将返回一个值，否则它将通过检查代码语句:if circname.print?.circleprint() != nil返回一些用户定义的打印消息。

通过可选链访问下标

可选链用于设置和检索下标值，以验证对下标的调用是否返回一个值。?放在下标大括号之前，以访问特定下标上的可选值。

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.")
}

当使用playground运行上述程序时，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，对函数的调用将仅返回其他部分，而返回必须是定义特定成员函数的值。

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() {
      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，对函数的程序调用将会返回一个值。

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])

当使用playground运行上述程序时，得到以下结果 -

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

可以通过引用下标值来访问下标的可选值。它可以通过下标[0]，下标[1]等来访问。“半径”的默认下标值首先分配为[35,45,78,101]和 Circle 为：[90,45,56]。然后将下标值更改Radius [0]为78，并将Circle[1]更改为45。

链的多级链接

多个子类也可以通过可选链与其超类方法，属性和下标链接。

可选的多个链可以链接 -

如果检索类型不是可选的，则可选链接将返回可选值。例如，如果String通过可选链它将返回String?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.")
}

当使用playground运行上述程序时，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，对函数的程序调用将仅返回其他部分，而返回必须定义特定成员函数的值。

如果检索类型已经是可选的，则可选链接也将返回可选值。例如，如果String?通过可选链访问它会返回String?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，对函数的程序调用将会返回一个值。

使用可选返回值链方法

可选链也用于访问子类定义的方法。

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")
}

当使用playground运行上述程序时，得到以下结果 -

Area of circle is not specified