19.类型转换

类型转换在 Swift 中使用 is 和 as 操作符实现，也可以用它来检查一个类型是否遵循了某个协议。

class Person {
    var name:String
    init(name:String) {
        self.name = name
    }
}
class Student: Person {
    var id:Int = 10086
}
class Teacher: Person {
    var job = "Teach"
}

// 数组类型：[Person]
var persons = [
    Person.init(name: "人"),
    Student.init(name: "生"),
    Teacher.init(name: "师"),
    Student.init(name: "生")
]
var studentCount = 0
var teacherCount = 0
var otherCount = 0
for item in persons {
    if item is Student { // is 判断是否是这个类型
        studentCount += 1
        if let stu = item as? Student { // as? 向下转换类型
            print("stu.id = \(stu.id)")
        }
    }else if item is Teacher {
        teacherCount += 1
        if let tea = item as? Teacher {
            print("tea.id = \(tea.job)")
        }
    }else if item is Person{ // will always true
        otherCount += 1
    }
}
print("person = \(otherCount),student = \(studentCount),teacher = \(teacherCount)")

Any 和 AnyObject

• Any 可以表示任何类型

  var things: [Any] = []
  
  things.append(0)// 值类型
  things.append(0.0)
  things.append(42)
  things.append(3.14159)
  things.append("hello") 
  things.append((3.0, 5.0))
  things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman")) // 函数类型
  things.append({ (name: String) -> String in "Hello, \(name)" }) // 闭包类型
  

• AnyObject 可以表示任何类类型的实例。

可以在 switch 表达式的 case 中使用 is 和 as 操作符来找出只知道是 Any 或 AnyObject 类型的常量或变量的具体类型。

for thing in things {
    switch thing {
    case 0 as Int:
        print("zero as an Int")
    case 0 as Double:
        print("zero as a Double")
    case let someInt as Int:
        print("an integer value of \(someInt)")
    case let someDouble as Double where someDouble > 0:
        print("a positive double value of \(someDouble)")
    case is Double:
        print("some other double value that I don't want to print")
    case let someString as String:
        print("a string value of \"\(someString)\"")
    case let (x, y) as (Double, Double):
        print("an (x, y) point at \(x), \(y)")
    case let movie as Movie:
        print("a movie called \(movie.name), dir. \(movie.director)")
    case let stringConverter as (String) -> String:
        print(stringConverter("Michael"))
    default:
        print("something else")
    }
}

Any 类型可以表示所有类型的值，包括可选类型。Swift 会在你用 Any 类型来表示一个可选值的时候，给你一个警告。如果你确实想使用 Any 类型来承载可选值，你可以使用 as 操作符显式转换为 Any，如下所示：

let optionalNumber: Int? = 3
things.append(optionalNumber)        // 警告
things.append(optionalNumber as Any) // 没有警告