[移动开发]swift5.1学习-小知识

1.Any、AnyObject

Swift提供了2种特殊的类型:Any、AnyObject
Any:可以代表任意类型(枚举、结构体、类，也包括函数类型)
AnyObject:可以代表任意类类型(在协议后面写上: AnyObject代表只有类才能遵守这个协议)
在协议后面写上: class也代表只有类才能遵守这个协议

2.is、as?、as!、as

is用来判断是否为某种类型
as用来做强制类型转换

protocol Runnable { func run() } class Person {}
class Student : Person, Runnable {
    func run() {
        print("Student run")
    }
    func study() {
        print("Student study")
    }
}
var stu: Any = 10
print(stu is Int) // true
stu = "Jack"
print(stu is String) // true
stu = Student()
print(stu is Person) // true 
print(stu is Student) // true 
print(stu is Runnable) // true

var stu: Any = 10
(stu as? Student)?.study() // 没有调用study 
stu = Student()
(stu as? Student)?.study() // Student study 
(stu as! Student).study() // Student study 
(stu as? Runnable)?.run() // Student run

var data = [Any]() data.append(Int("123") as Any)
 var d = 10 as Double
print(d) // 10.0

3.X.self、X.Type、AnyClass

X是类名

X.self是一个元类型(metadata)的指针(8个字节，本质上是OC的isa指针)，metadata存放着类型相关信息 ，相当于OC的X.class
X.self属于X.Type类型

 class Person {}
class Student : Person {}
var perType: Person.Type = Person.self 
var stuType: Student.Type = Student.self 
perType = Student.self

var anyType: AnyObject.Type = Person.self 
anyType = Student.self
public typealias AnyClass = AnyObject.Type 
var anyType2: AnyClass = Person.self 
anyType2 = Student.self

ar per = Person()
var perType = type(of: per) // Person.self 
print(Person.self == type(of: per)) // true

4.Self与self

self代表当前实例，Self代表当前类型
Self一般用作返回值类型，限定返回值跟方法调用者必须是同一类型(也可以作为参数类型)

 class Person {
    var age = 1
    static var count = 2
    func run() {
        print(self.age) // 1
        print(Self.count) // 2
    }
}

protocol Runnable {
    func test() -> Self
}
class Person : Runnable {
    required init() {}
    func test() -> Self { type(of: self).init() } 
}
class Student : Person {}
var p = Person()
// Person
print(p.test())
var stu = Student()
// Student
print(stu.test())