Swift function swizzling / runtime

I've succeed with method swizzling in Swift. This example shows how to hook description method on NSDictionary

My implementation:

extension NSDictionary {
     func myDescription() -> String!{
        println("Description hooked")
        return "Hooooked " + myDescription();
    }
}

Swizzling code:

func swizzleEmAll() {
        var dict:NSDictionary = ["SuperSecret": kSecValueRef]
        var method: Method = class_getInstanceMethod(object_getClass(dict), Selector.convertFromStringLiteral("description"))

        println(dict.description) // Check original description

        var swizzledMethod: Method = class_getInstanceMethod(object_getClass(dict), Selector.convertFromStringLiteral("myDescription"))
        method_exchangeImplementations(method, swizzledMethod)

        println(dict.description) //Check that swizzling works
    }

Edited: This code will work for any custom Swift class that inherits from NSObject (but will not work for classes that don't.) More examples - https://github.com/mbazaliy/MBSwizzler

You would likely be able to swizzle swift-generated classes that inherit from Objective-C classes with no problem, since they appear to use dynamic method dispatch all the time. You may be able to swizzle methods of swift-defined classes that exist in the Objective-C runtime by virtue of being passed across the bridge, but the Objective-C side methods are likely to just be proxies back across the bridge to the swift-side runtime, so it's not clear that it'd be particularly helpful to swizzle them.

"Pure" swift method calls do not appear to be dispatched dynamically via anything like objc_msgSend and it appears (from brief experimentation) that the type safety of swift is implemented at compile time, and that much of the actual type information is absent (i.e. gone) at runtime for non-class types (both of which likely contribute to the purported speed advantages of swift.)

For these reasons, I expect that meaningfully swizzling swift-only methods will be significantly harder than swizzling Objective-C methods, and will probably look a lot more like mach_override than Objective-C method swizzling.