swift类型转换 – Swift最新教程

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在Swift 语言中“类型转换”用于验证实例类型，它用于检查实例类型是否属于某个特定的超类或子类，或者它是在自己的层次结构中定义的。

Swift类型转换提供了两个操作符：“is”来检查值的类型和，“as”将类型值转换为不同的类型，类型转换还检查实例类型是否符合特定的协议一致性标准。

定义类层次结构

类型转换用于检查实例的类型，以确定它是否属于特定的类类型。此外，它还检查类的层次结构及其子类，以检查和转换这些实例，使其成为相同的层次结构。

class Subjects {
    var physics: String
    init(physics: String) {
       self.physics = physics
    }
 }
 
 class Chemistry: Subjects {
    var equations: String
    init(physics: String, equations: String) {
       self.equations = equations
       super.init(physics: physics)
    }
 }
 
 class Maths: Subjects {
    var formulae: String
    init(physics: String, formulae: String) {
       self.formulae = formulae
       super.init(physics: physics)
    }
 }
 
 let sa = [ Chemistry(physics: "solid physics", equations: "Hertz"),
    Maths(physics: "流体动力学", formulae: "Giga Hertz")]
 
 let samplechem = Chemistry(physics: "solid physics", equations: "Hertz")
 print("物理实例: \(samplechem.physics)")
 print("实例方程: \(samplechem.equations)")
 
 let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
 print("物理实例: \(samplemaths.physics)")
 print("表达式实例: \(samplemaths.formulae)")

类型检查

使用’is’操作符进行类型检查。’is’类型检查操作符检查实例是否属于特定的子类类型，如果属于该实例则返回’true’，否则将返回’false’。

class Subjects {
    var physics: String
    init(physics: String) {
       self.physics = physics
    }
 }
 
 class Chemistry: Subjects {
    var equations: String
    init(physics: String, equations: String) {
       self.equations = equations
       super.init(physics: physics)
    }
 }
 
 class Maths: Subjects {
    var formulae: String
    init(physics: String, formulae: String) {
       self.formulae = formulae
       super.init(physics: physics)
    }
 }
 
 let sa = [
    Chemistry(physics: "固体物理", equations: "Hertz"),
    Maths(physics: "流体动力学", formulae: "Giga Hertz"),
    Chemistry(physics: "热物理", equations: "Decibels"),
    Maths(physics: "宇宙物理学", formulae: "MegaHertz"),
    Maths(physics: "微分方程", formulae: "余弦级数")]
 
 let samplechem = Chemistry(physics: "固体物理", equations: "Hertz")
 print("物理实例: \(samplechem.physics)")
 print("实例方程: \(samplechem.equations)")
 
 let samplemaths = Maths(physics: "流体动力学", formulae: "Giga Hertz")
 print("物理实例: \(samplemaths.physics)")
 print("表达式实例: \(samplemaths.formulae)")
 
 var chemCount = 0
 var mathsCount = 0
 for item in sa {
    if item is Chemistry {
       ++chemCount
    } else if item is Maths {
       ++mathsCount
    }
 }
 
 print("Subjects, chemistry: \(chemCount), math: \(mathsCount)")

向下类型转换

可以使用两个操作符(as?和as!)”as?当该值返回nil时，返回一个可选值，它用于检查成功的向下强制转换。

“as!当向下强制转换返回空值时，将返回可选链接中讨论的强制展开，它用于在向下转换失败时触发运行时错误：

class Subjects {
    var physics: String
    init(physics: String) {
       self.physics = physics
    }
 }
 
 class Chemistry: Subjects {
    var equations: String
    init(physics: String, equations: String) {
       self.equations = equations
       super.init(physics: physics)
    }
 }
 
 class Maths: Subjects {
    var formulae: String
    init(physics: String, formulae: String) {
       self.formulae = formulae
       super.init(physics: physics)
    }
 }
 
 let sa = [
    Chemistry(physics: "AA", equations: "Hertz"),
    Maths(physics: "BB", formulae: "Giga Hertz"),
    Chemistry(physics: "CC", equations: "Decibels"),
    Maths(physics: "DD", formulae: "MegaHertz"),
    Maths(physics: "EE", formulae: "Cosine Series")]
 
 let samplechem = Chemistry(physics: "AA", equations: "Hertz")
 print("物理实例: \(samplechem.physics)")
 print("方程实例: \(samplechem.equations)")
 
 let samplemaths = Maths(physics: "BB", formulae: "Giga Hertz")
 print("物理实例: \(samplemaths.physics)")
 print("表达式: \(samplemaths.formulae)")
 
 var chemCount = 0
 var mathsCount = 0
 
 for item in sa {
    if let print = item as? Chemistry {
       print("Chemistry: '\(print.physics)', \(print.equations)")
    } else if let example = item as? Maths {
       print("Maths: '\(example.physics)', \(example.formulae)")
    }
 }

类型转换：any和任何对象

关键字“Any”用于表示属于任何类型(包括函数类型)的实例。

class Subjects {
    var physics: String
    init(physics: String) {
       self.physics = physics
    }
 }
 
 class Chemistry: Subjects {
    var equations: String
    init(physics: String, equations: String) {
       self.equations = equations
       super.init(physics: physics)
    }
 }
 
 class Maths: Subjects {
    var formulae: String
    init(physics: String, formulae: String) {
       self.formulae = formulae
       super.init(physics: physics)
    }
 }
 
 let sa = [
    Chemistry(physics: "固体物理", equations: "Hertz"),
    Maths(physics: "流体动力学", formulae: "Giga Hertz"),
    Chemistry(physics: "热力", equations: "Decibels"),
    Maths(physics: "宇宙学", formulae: "MegaHertz"),
    Maths(physics: "微分方程", formulae: "Cosine Series")]
 
 let samplechem = Chemistry(physics: "固体物理", equations: "Hertz")
 print("a: \(samplechem.physics)")
 print("b: \(samplechem.equations)")
 
 let samplemaths = Maths(physics: "流体动力学", formulae: "Giga Hertz")
 print("a: \(samplemaths.physics)")
 print("b: \(samplemaths.formulae)")
 
 var chemCount = 0
 var mathsCount = 0
 
 for item in sa {
    if let print = item as? Chemistry {
       print("Chemistry: '\(print.physics)', \(print.equations)")
    } else if let example = item as? Maths {
       print("Maths: '\(example.physics)', \(example.formulae)")
    }
 }
 
 var exampleany = [Any]()
 
 exampleany.append(12)
 exampleany.append(3.14159)
 exampleany.append("Any实例")
 exampleany.append(Chemistry(physics: "固体物理", equations: "Hertz"))
 
 for print in exampleany {
    switch print {
       case let someInt as Int:
          print("Integer: \(someInt)")
       case let someDouble as Double where someDouble > 0:
          print("Pi: \(someDouble)")
       case let someString as String:
          print("\(someString)")
       case let phy as Chemistry:   
          print("Topics '\(phy.physics)', \(phy.equations)")
       default:
          print("None")
    }
 }

AnyObject

要表示任何类类型的实例，使用’AnyObject’关键字。

class Subjects {
    var physics: String
    init(physics: String) {
       self.physics = physics
    }
 }
 
 class Chemistry: Subjects {
    var equations: String
    init(physics: String, equations: String) {
       self.equations = equations
       super.init(physics: physics)
    }
 }
 
 class Maths: Subjects {
    var formulae: String
    init(physics: String, formulae: String) {
       self.formulae = formulae
       super.init(physics: physics)
    }
 }
 
 let saprint: [AnyObject] = [Chemistry(physics: "固体物理", equations: "Hertz"),
    Maths(physics: "流体动力学", formulae: "Giga Hertz"),
    Chemistry(physics: "Thermo physics", equations: "Decibels"),
    Maths(physics: "Astro Physics", formulae: "MegaHertz"),
    Maths(physics: "Differential Equations", formulae: "Cosine Series")]
 
 let samplechem = Chemistry(physics: "固体物理", equations: "Hertz")
 print("Instance: \(samplechem.physics)")
 print("Instance: \(samplechem.equations)")
 
 let samplemaths = Maths(physics: "流体动力学", formulae: "Giga Hertz")
 print("Instance: \(samplemaths.physics)")
 print("Instance: \(samplemaths.formulae)")
 
 var chemCount = 0
 var mathsCount = 0
 
 for item in saprint {
    if let print = item as? Chemistry {
       print("Chemistry: '\(print.physics)', \(print.equations)")
    } else if let example = item as? Maths {
       print("Maths: '\(example.physics)', \(example.formulae)")
    }
 }
 
 var exampleany = [Any]()
 exampleany.append(12)
 exampleany.append(3.14159)
 exampleany.append("AnyObject")
 exampleany.append(Chemistry(physics: "固体物理", equations: "Hertz"))
 
 for print in exampleany {
    switch print {
       case let someInt as Int:
          print("v: \(someInt)")
       case let someDouble as Double where someDouble > 0:
          print("Pi: \(someDouble)")
       case let someString as String:
          print("\(someString)")
       case let phy as Chemistry:
          print("Topics '\(phy.physics)', \(phy.equations)")
       default:
          print("None")
    }
 }