[swift] NSClassFromString 无法获得该类

extension NSObject {
    // create a static method to get a swift class for a string name
    class func swiftClassFromString(className: String) -> AnyClass! {
        // get the project name
        if  let appName: String? = NSBundle.mainBundle().objectForInfoDictionaryKey("CFBundleName") as! String? {
            // generate the full name of your class (take a look into your "YourProject-swift.h" file)
            let classStringName = appName! + "." + className
            // return the class!
            return NSClassFromString(classStringName)
        }
        return nil;
    }
}