I've been working on my first real SwiftUI app (in macOS), and I've come to a point where I need to really implement my model. I suppose this is another form of “should this be a struct or a class?” but I don't yet have the intuition to know with any certainty that there's no way it could be a struct (I try to make things structs where possible). But I'm running into a protocols-with-associated-types issue, in that I wanted to make my root data object an array of protocols, but they have to conform to Identifiable, and that's not allowed (protocols with associated types can only be used as constraints).
Document Data Model
The app document is a kind of project, analogous to Xcode, wherein there's a left-hand pane, a list of entries for files, and a right-hand detail pane (source view).
In my case they're called Layers, and the data they represent is rendered visually on the right (it's a simplified GIS program). A layer can be as simple as an image file dropped onto the project window from the Finder, or it can be generated or derived from another layer.
In the first case, the model stores, say, the url to the file.
In the second case, the model stores a reference to the source layer, and some parameters describing how the derived data was generated. It probably also references a cache of that generated data.
Questions
Is this already clearly not a struct? Am I missing a Swiftism that would let me build more of these models with structs? Is heavy use of type erasure (something I've only studied briefly) to be expected in a "proper" Swift app like this?
Or do I just use classes and call it a day. I understand those fairly well 