1. A method for operating a user device, comprising: loading a definition into a client application for execution on the user device, loading one or more classes defined in a central repository database of a server into the client application; determining that one or more custom classes not defined in the central repository database are contained in the definition; validating the one or more custom classes by ensuring that they contain one or more preset functions; and loading the one or more custom classes into the client application, thereby overriding or extending at least one of the one or more classes defined in the central repository database.