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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 -
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。
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 value0 Swift 4 0x0000000103410b68llvm::sys::PrintStackTrace(__sFILE*) + 401 Swift 4 0x0000000103411054 SignalHandler(int) + 4522 libsystem_platform.dylib 0x00007fff9176af1a _sigtramp + 263 libsystem_platform.dylib 0x000000000000000b _sigtramp + 18544929394 libsystem_platform.dylib 0x00000001074a0214 _sigtramp + 19767836365 Swift 4 0x0000000102a85c39llvm::JIT::runFunction(llvm::Function*, std::__1::vector > const&) + 3296 Swift 4 0x0000000102d320b3llvm::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*) + 15237 Swift 4 0x000000010296e6ba Swift 4::RunImmediately(Swift4::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::SILOptionsconst&) + 10668 Swift 4 0x000000010275764b frontend_main(llvm::ArrayRef,char const*, void*) + 52759 Swift 4 0x0000000102754a6d main + 167710 libdyld.dylib 0x00007fff8bb9e5c9 start + 111 libdyld.dylib 0x000000000000000c start + 1950751300Stack 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。
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
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。
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 = printingif 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。
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 = printingvar area = ["Radius": [35, 45, 78, 101], "Circle": [90, 45, 56]]area["Radius"]?[1] = 78area["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
。
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 = printingif 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。
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