Allow for custom functions in layout expressions

[olemartinorg]

Background

More advanced logic for expressions (as detailed further in the layout expressions project) might require custom function app developers can implement themselves.

Considerations

• Should we require implementations on both the backend and frontend? If the only implementation for a resource-heavy expression exists on the backend, we need to consider caching on the frontend. It could also be possible to implement custom function in the frontend only (as we already have in existing dynamic rendering)
• Strict typing, argument types, return types
• JSON schema support

[nkylstad]

@olemartinorg can we move this issue to app-frontend or app-lib repo? Or do you already have a corresponding issue there?

[olemartinorg]

Woah, strange! I think I might have created this issue in the wrong repo back in 2022. :shrug: It might also be that since this issue is possibly both a frontend and a backend issue at the same time, I put it in the Studio repo.. :thinking:

I think the underlying problem area overlaps a bit with this issue:

• https://github.com/Altinn/app-frontend-react/issues/1177

Providing a platform for entirely custom functions is not a focus area I've been thinking much about since the initial project completion, and it's entirely possible to contribute custom functions to both our backend and frontend. Anyway, I'll move the issue somewhere more fitting.