Swift 在switch语句中测试类的类型

在Swift中，你可以在switch语句中使用is关键字来测试一个对象的类类型。此外，你还会有一些例子说明如何将一个对象的类型转换为预期的类型。

下面是一个测试原始数据类型的例子

在这个例子中，你将检查原始数据类型的类型，如String、Int、Double等。你可以使用switch语句来检查多个条件，如下面的例子。

例子

import Foundation
func checkType(_ value: Any) {
   switch value {
      case is String: print("The value is String.")
      case is Int: print("The value is Int.")
      case is Int32: print("The value is Int32.")
      case is Double: print("The value is Double.")
      case is Bool: print("The value is Boolean.")
      default: print("The value is of some other type.")
   }
}
checkType(16)
checkType(16.0)
checkType("16")
checkType(true)
checkType(Float(16.0))

输出

The value is Int.
The value is Double.
The value is String.
The value is Boolean.
The value is of some other type.

在这个例子中，值是一个 “Any “类型的实例，它将使用switch语句的关键字来测试值的原始类型。如果值是String的一个实例，第一个case block将执行。如果它是任何其他类型的实例，另一个案例块将被执行，如果它不是上述任何一种类型，默认块将被执行。

下面是一个测试自定义类型的例子

以类似的方式，你也可以检查自定义类型的类型。这意味着你可以检查由你创建的类的类型。

例子

import Foundation
class Car {  
   let numberOfWheels: Int
   let colorCode: String

   init(wheels: Int, color: String) {
      self.numberOfWheels = wheels
      self.colorCode = color
   }
}
class Person {
   let name: String
   let age: Int

   init(name: String, age: Int) {
      self.name = name
      self.age = age
   }
}
class Student {
   var school: String
   var grade: Int

   init(school: String, grade: Int) {
      self.school = school
      self.grade = grade
   }
}
func checkType(_ object: Any) {
   switch object {
      case is Car: print("The object is a Car.")
      case is Person: print("The object is a Person.")
      case is Student: print("The object is a Student.")
      default: print("The object is of some other type.")
   }
}
let car = Car(wheels: 4, color: "red")
let person = Person(name: "Amit Yadav", age: 23)
let student = Student(school: "St. Primary School", grade: 5)
checkType(car)
checkType(student)
checkType(person)

输出

The object is a Car.
The object is a Student.
The object is a Person.

如果你看到这两个例子，我们正在检查给定对象或值的类型。但是如果你想把给定的对象类型转换为预期的类型并在以后使用它，建议使用类型转换方法作为操作符。使用这种方法，你可以对对象进行类型转换以访问其属性。

下面是一个使用as操作符的例子

在Swift中，还有一个关键词 “as”，你可以用来检查对象的类型。这个操作符是用来将一个对象从一种类型类型化为另一种类型。它返回一个所需类型的投射实例。

如果条件匹配为真，或者对象被转换为给定类型，则返回一个实例，否则返回nil，”如果 “条件将不执行。

例子

import Foundation
class Car {    
   let numberOfWheels: Int
   let colorCode: String

   init(wheels: Int, color: String) {
      self.numberOfWheels = wheels
      self.colorCode = color
   }
}
class Person {
   let name: String
   let age: Int

   init(name: String, age: Int) {
      self.name = name
      self.age = age
   }
}
class Student {
   var school: String
   var grade: Int

   init(school: String, grade: Int) {
      self.school = school
      self.grade = grade
   }
}
func checkType(_ object: Any) {
   switch object {       
      case let carObject as Car:
         print("The object is a Car type with \(carObject.numberOfWheels) wheels.")   
      case let personObject as Person:
         print("The object is a Person type with the name \(personObject.name).")       
      case let studentObject as Student:
         print("The object is a Student type with school \(studentObject.school).")
      default: print("The object is of some other type.")
   }
}
let car = Car(wheels: 4, color: "red")
let person = Person(name: "Amit Yadav", age: 23)
let student = Student(school: "St. Primary School", grade: 5)
checkType(car)
checkType(student)
checkType(person)

输出

The object is a Car type with 4 wheels.
The object is a Student type with school St. Primary School.
The object is a Person type with the name Amit Yadav.