gistfile1.swift

class Foo: NSObject {
    var obj: NSObject? = nil
}

func createObjectOfType(type: NSObject.self) -> Foo {
    let obj = type()
    if obj.respondsToSelector("obj") {
        obj.setValue(NSObject(), forKey: "obj") // never gets called
    }
    //obj.setValue(NSObject(), forKey: "obj") // Exceptions if you don't call it with 'Foo.self'
}

let fooObject = createObjectOfType(Foo.self) as! Foo
println("\(foo.obj)") // nil