无涯教程-Swift - 类型转换

为了验证的类型，"Type Casting"以Swift 4语言开始发挥作用，它用于检查类型是属于特定的超类还是子类，或者是否在其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"，并将类型值转换为其他类型，类型转换还检查类型是否遵循特定的协议一致性标准。

类层结构

类型转换用于检查的类型，以查明它是否属于特定的类类型。另外，它检查类及其子类的层次结构以检查和转换这些，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)")

运行上述程序时，我们得到以下输出-

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

类型检查

类型检查是使用" is"运算符完成的。 " is"类型检查运算符检查是否属于特定的子类类型，如果该属于该，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: "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")

运行上述程序时，我们得到以下输出-

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

向下转换

向下转换子类类型可以使用两个运算符(as和as!)完成，当值返回nil时，返回一个可选值，它用于检查成功的向下转换。

'如!'当向下转换返回nil值时，返回可选解链中讨论的强制展开。在下行失败的情况下，它用于触发运行时错误

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)")
   }
}

运行上述程序时，我们得到以下输出-

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

关键字" Any"用于表示一个，该属于包括函数类型在内的任何类型。

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")
   }
}

运行上述程序时，我们得到以下输出-

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

为了表示任何类类型的，使用了" 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")
   }
}

运行上述程序时，我们得到以下输出-

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

参考链接

https://www.learnfk.com/swift/swift-type-casting.html