更新：由于
SE-0068 – Expanding Swift Self to class members and value types,Swift 3允许使用其他类型的Self.
您可以从类函数返回“Self”：
extension NSObject {
class func makeOne() -> Self {
return self()
}
}
所以你可以这样做：
let set : NSCountedSet = NSCountedSet.makeOne()
但是,以下两个不编译：
extension NSObject {
class func makeTwo() -> (Self,Self) {
return (self(),self())
}
class func makeMany() -> [Self] {
return [self(),self(),self()]
}
}
错误是：
<REPL>:11:34: error: 'Self' is only available in a protocol or as the result of a class method; did you mean 'NSObject'?
class func makeTwo() -> (Self,Self) {
^~~~
NSObject
<REPL>:11:40: error: 'Self' is only available in a protocol or as the result of a class method; did you mean 'NSObject'?
class func makeTwo() -> (Self,Self) {
^~~~
NSObject
<REPL>:15:35: error: 'Self' is only available in a protocol or as the result of a class method; did you mean 'NSObject'?
class func makeMany() -> [Self] {
^~~~
NSObject
我怀疑,问题在于自我是模棱两可的;它意味着“这个类或子类,无论我们被称为什么时发生的事情”.换句话说,Self是多态的.但是,例如,您无法创建由两个不同类组成的数组.虽然该类可能允许某个初始化程序,但我们无法事先知道它的子类将会.
解决方案是使用类名本身.这是一个struct Thing的例子：
extension Thing {
static func makeTwo() -> (Thing,Thing) {
return (Thing(),Thing())
}
}
