Swift - 可选链

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可选链是指对可能为 nil 的可选值查询、调用属性、下标和方法的过程。可选链返回两个值：

  • 如果可选值包含一个值，则调用其相关的属性、方法和下标将返回相应的值。

  • 如果可选值包含 nil 值，则其所有相关的属性、方法和下标都将返回 nil。

由于对方法、属性和下标的多个查询被组合在一起，因此一个链的失败会影响整个链，并导致返回 nil 值。

可选链作为强制解包的替代方案

可选链在可选值后使用“?”来调用属性、方法或下标，前提是可选值返回某些值。

可选链“?” 访问方法、属性和下标 可选链“!”强制解包。
“?”放在可选值之后，用于调用属性、方法或下标。 “!”放在可选值之后，用于调用属性、方法或下标，强制解包值。
当可选值为 nil 时，优雅地失败。 当可选值为 nil 时，强制解包会触发运行时错误。

使用“!”的可选链程序

示例

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

当我们在 playground 中运行上述程序时，我们将得到以下结果：

main/main.swift:10: Fatal error: Unexpectedly found nil while unwrapping an Optional value
Current stack trace:
0    libswiftCore.so                    0x00007fd880a40dc0 _swift_stdlib_reportFatalErrorInFile + 112
1    libswiftCore.so                    0x00007fd88070a191 <unavailable> + 1442193
2    libswiftCore.so                    0x00007fd880709eb6 <unavailable> + 1441462
3    libswiftCore.so                    0x00007fd880709caa <unavailable> + 1440938
4    libswiftCore.so                    0x00007fd8807096d0 _assertionFailure(_:_:file:line:flags:) + 315
6    swift-frontend                     0x000055a564ac0b3d <unavailable> + 26479421
7    swift-frontend                     0x000055a563df4db9 <unavailable> + 13061561
8    swift-frontend                     0x000055a563bc54c6 <unavailable> + 10769606
9    swift-frontend                     0x000055a563bc19b6 <unavailable> + 10754486
10   swift-frontend                     0x000055a563bc10a7 <unavailable> + 10752167
11   swift-frontend                     0x000055a563bc341e <unavailable> + 10761246
12   swift-frontend                     0x000055a563bc273d <unavailable> + 10757949
13   swift-frontend                     0x000055a563a94a39 <unavailable> + 9521721
14   libc.so.6                          0x00007fd880017d90 <unavailable> + 171408
15   libc.so.6                          0x00007fd880017dc0 __libc_start_main + 128
16   swift-frontend                     0x000055a563a94295 <unavailable> + 9519765
Stack dump:
0.	Program arguments: /opt/swift/bin/swift-frontend -frontend -interpret main.swift -disable-objc-interop -color-diagnostics -new-driver-path /opt/swift/bin/swift-driver -empty-abi-descriptor -resource-dir /opt/swift/lib/swift -module-name main
1.	Swift version 5.7.3 (swift-5.7.3-RELEASE)
2.	Compiling with the current language version
3.	While running user code "main.swift"
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
/opt/swift/bin/swift-frontend(+0x551a103)[0x55a56869a103]
/opt/swift/bin/swift-frontend(+0x551802e)[0x55a56869802e]
/opt/swift/bin/swift-frontend(+0x551a48a)[0x55a56869a48a]
/lib/x86_64-linux-gnu/libc.so.6(+0x42520)[0x7fd880030520]
/opt/swift/lib/swift/linux/libswiftCore.so(+0x160195)[0x7fd88070a195]
/opt/swift/lib/swift/linux/libswiftCore.so(+0x15feb6)[0x7fd880709eb6]
/opt/swift/lib/swift/linux/libswiftCore.so(+0x15fcaa)[0x7fd880709caa]
/opt/swift/lib/swift/linux/libswiftCore.so($ss17_assertionFailure__4file4line5flagss5NeverOs12StaticStringV_A2HSus6UInt32VtF+0x13b)[0x7fd88070980b]
[0x7fd87ece717e]
/opt/swift/bin/swift-frontend(+0x1940b3d)[0x55a564ac0b3d]
/opt/swift/bin/swift-frontend(+0xc74db9)[0x55a563df4db9]
/opt/swift/bin/swift-frontend(+0xa454c6)[0x55a563bc54c6]
/opt/swift/bin/swift-frontend(+0xa419b6)[0x55a563bc19b6]
/opt/swift/bin/swift-frontend(+0xa410a7)[0x55a563bc10a7]
/opt/swift/bin/swift-frontend(+0xa4341e)[0x55a563bc341e]
/opt/swift/bin/swift-frontend(+0xa4273d)[0x55a563bc273d]
/opt/swift/bin/swift-frontend(+0x914a39)[0x55a563a94a39]
/lib/x86_64-linux-gnu/libc.so.6(+0x29d90)[0x7fd880017d90]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80)[0x7fd880017e40]
/opt/swift/bin/swift-frontend(+0x914295)[0x55a563a94295]
Illegal instruction (core dumped)

上述程序声明“election poll”作为类名，并包含“candidate”作为成员函数。子类声明为“poll booth”，其成员函数“name”初始化为“MP”。对超类的调用是通过创建带有可选“!”的实例“cand”来初始化的。由于其基类中未声明值，因此存储 nil 值，并通过强制解包过程返回致命错误。

使用“?”的可选链程序

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")
}
输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

Candidate name cannot be retreived

上述程序声明“election poll”作为类名，并包含“candidate”作为成员函数。子类声明为“poll booth”，其成员函数“name”初始化为“MP”。对超类的调用是通过创建带有可选“?”的实例“cand”来初始化的。由于其基类中未声明值，因此存储 nil 值，并由 else 处理程序块在控制台中打印。

为可选链定义模型类和访问属性

Swift 4 语言还提供可选链的概念，可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、方法和下标的子属性非常有用。

示例

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")
}

输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

Rectangle Area is not specified

通过可选链调用方法

示例

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")
}

输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

Area of circle is not specified

通过创建名为“circname”的实例来调用在 circle() 子类中声明的 circleprint() 函数。如果函数包含某些值，则该函数将返回值；否则，它将通过检查语句“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，以验证对该下标的调用是否返回值。“?”放在下标括号之前，用于访问特定下标上的可选值。

示例 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.")
}

输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

Radius is not specified.

在上述程序中，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 else 部分，而要返回值，则必须为特定成员函数定义值。

示例 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.")
}

输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

Radius is measured in Units.

在上述程序中，指定了成员函数“radiusName”的实例值。因此，对函数的程序调用现在将返回值。

访问可选类型的下标

示例

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])

输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

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

可以通过引用其下标值来访问下标的可选值。可以访问为 subscript[0]、subscript[1] 等。首先将“radius”的默认下标值赋值为 [35, 45, 78, 101]，并将“Circle”赋值为 [90, 45, 56]]。然后将下标值更改为 Radius[0] 为 78，Circle[1] 为 45。

链接多级链

多个子类也可以通过可选链与其超类的方法、属性和下标链接。

可以链接可选值的多个链

示例

如果检索类型不是可选的，则可选链将返回一个可选值。例如，如果通过可选链检索 String，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.")
}

输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

Radius is not specified.

在上述程序中，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 else 部分，而要返回值，则必须为特定成员函数定义值。

如果检索类型已经是可选的，则可选链也将返回一个可选值。例如，如果通过可选链访问 String?，它将返回 String? 值。

示例

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.")
}

输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

Radius is measured in Units.

在上述程序中，指定了成员函数“radiusName”的实例值。因此，对函数的程序调用现在将返回值。

对具有可选返回值的方法进行链式操作

可选链也用于访问子类定义的方法。

示例

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")
}

输出

当我们在 playground 中运行上述程序时，我们将得到以下结果：

Area of circle is not specified
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