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3. Swift - 类型转换( Type Casting)

Swift - 类型转换( Type Casting)

要验证实例的类型'Type Casting'在Swift 4语言中发挥作用。 它用于检查实例类型是属于特定的超类还是子类，还是在自己的层次结构中定义。

Swift 4类型转换提供两个运算符'is'来检查值的类型和'as'，并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遵循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，它还检查类及其子类的层次结构，以检查并转换这些实例，使其成为同一层次结构。

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: "Fluid Dynamics", formulae: "Giga Hertz")]
let samplechem = Chemistry(physics: "solid physics", equations: "Hertz")
print("Instance physics is: \(samplechem.physics)")
print("Instance equation is: \(samplechem.equations)")
let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
print("Instance physics is: \(samplemaths.physics)")
print("Instance formulae is: \(samplemaths.formulae)")

当我们使用游乐场运行上述程序时，我们得到以下结果 -

Instance physics is: solid physics
Instance equation is: Hertz
Instance physics is: Fluid Dynamics
Instance formulae is: Giga Hertz

类型检查

使用'is'运算符完成类型检查。 'is'类型检查运算符检查实例是否属于特定的子类类型，如果它属于该实例则返回'true'，否则它将返回'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: "solid physics", equations: "Hertz"),
   Maths(physics: "Fluid Dynamics", 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: "solid physics", equations: "Hertz")
print("Instance physics is: \(samplechem.physics)")
print("Instance equation is: \(samplechem.equations)")
let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
print("Instance physics is: \(samplemaths.physics)")
print("Instance formulae is: \(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 in chemistry contains \(chemCount) topics and maths contains \(mathsCount) topics")

当我们使用游乐场运行上述程序时，我们得到以下结果 -

Instance physics is: solid physics
Instance equation is: Hertz
Instance physics is: Fluid Dynamics
Instance formulae is: Giga Hertz
Subjects in chemistry contains 2 topics and maths contains 3 topics

Downcasting

向下转换子类类型可以使用两个运算符（作为？和！）。'作为？' 当值返回nil时返回可选值。 它用于检查成功的向下倾斜。

'如！' 当向下转换返回nil值时，返回force unwrapping，如可选链接中所讨论的那样。 它用于在发生转发故障时触发运行时错误

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: "solid physics", equations: "Hertz"),
   Maths(physics: "Fluid Dynamics", 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: "solid physics", equations: "Hertz")
print("Instance physics is: \(samplechem.physics)")
print("Instance equation is: \(samplechem.equations)")
let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
print("Instance physics is: \(samplemaths.physics)")
print("Instance formulae is: \(samplemaths.formulae)")
var chemCount = 0
var mathsCount = 0
for item in sa {
   if let print = item as? Chemistry {
      print("Chemistry topics are: '\(print.physics)', \(print.equations)")
   } else if let example = item as? Maths {
      print("Maths topics are: '\(example.physics)', \(example.formulae)")
   }
}

当我们使用游乐场运行上述程序时，我们得到以下结果 -

Instance physics is: solid physics
Instance equation is: Hertz
Instance physics is: Fluid Dynamics
Instance formulae is: Giga Hertz
Chemistry topics are: 'solid physics', Hertz
Maths topics are: 'Fluid Dynamics', Giga Hertz
Chemistry topics are: 'Thermo physics', Decibels
Maths topics are: 'Astro Physics', MegaHertz
Maths topics are: 'Differential Equations', Cosine Series

类型转换:任何和任何对象

关键字“Any”用于表示属于包括函数类型在内的任何类型的实例。

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: "solid physics", equations: "Hertz"),
   Maths(physics: "Fluid Dynamics", 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: "solid physics", equations: "Hertz")
print("Instance physics is: \(samplechem.physics)")
print("Instance equation is: \(samplechem.equations)")
let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
print("Instance physics is: \(samplemaths.physics)")
print("Instance formulae is: \(samplemaths.formulae)")
var chemCount = 0
var mathsCount = 0
for item in sa {
   if let print = item as? Chemistry {
      print("Chemistry topics are: '\(print.physics)', \(print.equations)")
   } else if let example = item as? Maths {
      print("Maths topics are: '\(example.physics)', \(example.formulae)")
   }
}
var exampleany = [Any]()
exampleany.append(12)
exampleany.append(3.14159)
exampleany.append("Example for Any")
exampleany.append(Chemistry(physics: "solid physics", equations: "Hertz"))
for print in exampleany {
   switch print {
      case let someInt as Int:
         print("Integer value is \(someInt)")
      case let someDouble as Double where someDouble > 0:
         print("Pi value is \(someDouble)")
      case let someString as String:
         print("\(someString)")
      case let phy as Chemistry:   
         print("Topics '\(phy.physics)', \(phy.equations)")
      default:
         print("None")
   }
}

当我们使用游乐场运行上述程序时，我们得到以下结果 -

Instance physics is: solid physics
Instance equation is: Hertz
Instance physics is: Fluid Dynamics
Instance formulae is: Giga Hertz
Chemistry topics are: 'solid physics', Hertz
Maths topics are: 'Fluid Dynamics', Giga Hertz
Chemistry topics are: 'Thermo physics', Decibels
Maths topics are: 'Astro Physics', MegaHertz
Maths topics are: 'Differential Equations', Cosine Series
Integer value is 12
Pi value is 3.14159
Example for Any
Topics 'solid physics', Hertz

AnyObject (AnyObject)

要表示任何类类型的实例，请使用“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: "solid physics", equations: "Hertz"),
   Maths(physics: "Fluid Dynamics", 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: "solid physics", equations: "Hertz")
print("Instance physics is: \(samplechem.physics)")
print("Instance equation is: \(samplechem.equations)")
let samplemaths = Maths(physics: "Fluid Dynamics", formulae: "Giga Hertz")
print("Instance physics is: \(samplemaths.physics)")
print("Instance formulae is: \(samplemaths.formulae)")
var chemCount = 0
var mathsCount = 0
for item in saprint {
   if let print = item as? Chemistry {
      print("Chemistry topics are: '\(print.physics)', \(print.equations)")
   } else if let example = item as? Maths {
      print("Maths topics are: '\(example.physics)', \(example.formulae)")
   }
}
var exampleany = [Any]()
exampleany.append(12)
exampleany.append(3.14159)
exampleany.append("Example for Any")
exampleany.append(Chemistry(physics: "solid physics", equations: "Hertz"))
for print in exampleany {
   switch print {
      case let someInt as Int:
         print("Integer value is \(someInt)")
      case let someDouble as Double where someDouble > 0:
         print("Pi value is \(someDouble)")
      case let someString as String:
         print("\(someString)")
      case let phy as Chemistry:
         print("Topics '\(phy.physics)', \(phy.equations)")
      default:
         print("None")
   }
}

当我们使用游乐场运行上述程序时，我们得到以下结果 -

Instance physics is: solid physics
Instance equation is: Hertz
Instance physics is: Fluid Dynamics
Instance formulae is: Giga Hertz
Chemistry topics are: 'solid physics', Hertz
Maths topics are: 'Fluid Dynamics', Giga Hertz
Chemistry topics are: 'Thermo physics', Decibels
Maths topics are: 'Astro Physics', MegaHertz
Maths topics are: 'Differential Equations', Cosine Series
Integer value is 12
Pi value is 3.14159
Example for Any
Topics 'solid physics', Hertz