Foo.swift

class Foo : NSObject {
    class func shared() -> Foo {
        return Foo()
    }
    func foo() -> String {
        return "hello"
    }
}

let WeakFoo = NSClassFromString("Foo");

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?


    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        let x = WeakFoo?.shared().foo()
        print("Hello \(x)")
        
        return true
    }