Swift class introspection & generics

I am trying to dynamically create a class instance based type using generics, however I am encountering difficulty with class introspection.

Here are the questions:

• Is there a Swift-equivalent to Obj-C's self.class?
• Is there a way to instantiate a class using the AnyClass result from NSClassFromString?
• Is there a way to get AnyClass or otherwise type information strictly from a generic parameter T? (Similar to C#'s typeof(T) syntax)

Solution 1:

Well, for one, the Swift equivalent of [NSString class] is .self (see Metatype docs, though they're pretty thin).

In fact, NSString.class doesn't even work! You have to use NSString.self.

let s = NSString.self
var str = s()
str = "asdf"

Similarly, with a swift class I tried...

class MyClass {

}

let MyClassRef = MyClass.self

// ERROR :(
let my_obj = MyClassRef()

Hmm… the error says:

Playground execution failed: error: :16:1: error: constructing an object of class type 'X' with a metatype value requires an '@required' initializer

 Y().me()
 ^
 <REPL>:3:7: note: selected implicit initializer with type '()'
 class X {
       ^

It took me a while to figure out what this means… turns out it wants the class to have a @required init()

class X {
    func me() {
        println("asdf")
    }

    required init () {

    }
}

let Y = X.self

// prints "asdf"
Y().me()

Some of the docs refer to this as .Type, but MyClass.Type gives me an error in the playground.

Solution 2:

Here's how to use NSClassFromString. You have to know the superclass of what you're going to end up with. Here are a superclass-subclass pair that know how to describe themselves for println:

@objc(Zilk) class Zilk : NSObject {
    override var description : String {return "I am a Zilk"}
}

@objc(Zork) class Zork : Zilk {
    override var description : String {return "I am a Zork"}
}

Notice the use of the special @obj syntax to dictate the Objective-C munged name of these classes; that's crucial, because otherwise we don't know the munged string that designates each class.

Now we can use NSClassFromString to make the Zork class or the Zilk class, because we know we can type it as an NSObject and not crash later:

let aClass = NSClassFromString("Zork") as NSObject.Type
let anObject = aClass()
println(anObject) // "I am a Zork"

And it's reversible; println(NSStringFromClass(anObject.dynamicType)) also works.

Modern version:

    if let aClass = NSClassFromString("Zork") as? NSObject.Type {
        let anObject = aClass.init()
        print(anObject) // "I am a Zork"
        print(NSStringFromClass(type(of:anObject))) // Zork
    }