Licensing a web app to other developers

If I were to develop a web app that some of my clients wanted to purchase, then that’s quite straightforward as I just sell them a license to install and use the fully built app (exe / cgi etc…)
But a situation has potentially arisen where a client wants to roll my app out multiple times with additional enhancements / features. I do not want the work of developing those new additions, nor do I have the resources to manage large scale role out / deployment in the field.

Instead, I might like to License my “base app” on to another Xojo dev to add the enhancements and manage the distribution and roll out.

So, my question is, can I provide a Xojo project in such a way that certain Methods and/or Modules are “locked” ( source code is not visible), but which would allow the dev to build further functionality, using my existing Methods and also add new Methods / Modules of their own designs as they see fit. Is there any kind of procedural model for this?

you can place the hidden methods in a external library. (dll on windows).
Your code is save an you can use all functions the same way.

You can encrypt your code so that it can still be used and extended, but not changed by other developers.