1.as
(1)从派生类转换成基类,(upcasts)向上转换,即:子类向父类转换.
定义一个父类People
class People : NSObject {
var name : String
init(_ name : String) {
self.name = name
}
}
定义子类:
class Teacher : People {
}
class Student : People {
}
//展示名字
func showName(_ people : People) {
print("The name is \(people.name)")
}
//实例化
let tea = Teacher("Mr.Li")
let stu = Student("Tom")
let people_1 = tea as People
let people_2 = stu as People
showName(people_1)
showName(people_2)
打印结果:
(2)消除二义性,数值类型转换.
let age = 22 as Int //22.5 as Int 编译器会自动报错,因为Double不能直接转成Int,不向低范围转换.
let height = 161 as CGFloat
let money = (50/2) as Double
print("age = \(age),height = \(height),money = \(money)")
打印结果:
(3)switch 语句中进行模式匹配.通过switch语法检测对象的类型，根据对象类型进行处理。
switch people_1 {
case _ as Student:
print("是Student类型，打印学生成绩单...")
case _ as Teacher:
print("是Teacher类型，打印老师工资单...")
default:
break
}
打印结果:
2. as!
向下转型（Downcasting）时使用。由于是强制类型转换，如果转换失败会报 runtime 运行错误。
let people_3 : People = Student("蒙奇_D_路飞")
let people_4 = people_3 as! Student
print("这个人的名字是:\(people_4.name)")
print("people3的类型是:\(people_3.self). people4的类型是:\(people_4.self)")
打印结果:
3. as?
跟as!转换一样,区别是as?转换不成功时会返回nil对象,不会报错. 转换成功时返回的是可选类型.
所以:如果可以确定100%能转换成功就使用as!, 否则使用as?
if let someone = people_3 as? Teacher {//if 条件语句里定义的变量,作用域只有在紧跟着它的那句语句里才有用.(swift4.0)
print("\(someone.name) is a student")
}else{
//这里 someone不能使用,使用时编译报错:没有定义.
print("someone is not a student")
}
打印结果:
这里people_3是Student类型,转Teacher类型失败,返回nil对象,所以打印走的是else语句.
