This works for copying any layer from any layout or creating a brand new one.
Now this comes with issues, including some I have yet to figure out.
The first issue is that layers are assigned an sid value which I can only imagine is used to manage layers during runtime. I can't tell which issues having two layers on two different layouts share the same sid would create, but I can guess it could mess with the existance of that layer entirely, or even overwriting it entirely which would be bad. A fix would be to just make sure to deep copy the layer and change the sid instead of copying directly.
Having 2 layers share the same sid on the same layout seems to erase one of them. Or it could also be due to both layers sharing the same name. Anyway, it's problematic in any case.
The second issue is that objects aren't copied over from that piece of code only. I gathered that some other piece of code is in charge of actually creating the objects a layer is holding. I'll have to keep investigating to be sure.
The third issue seems to be a bug that should be a problem in most use cases. When I tried creating a new layer entirely, I tried setting it to non transparent, and to change the background color to cyan, just for the sake of testing the thing. The new cyan layer did appear in front of my other layer containing random sprites, but when I tried moving one of these sprites to the new layer the sprite wouldn't appear. This is weird because the debugger says that the sprite did in fact change layers, and when the layer is transparent, it works totally fine and the sprite gets thrown in front of all of the other sprites.
Another issue that I think will never be resolved is the case of global layers. Given my initial intent, my goal was to create new empty layers named after global layers so C2 would do the job of copying all of the objects for me, but unfortunately the only reference for "global layers" I was able to find in the code was about making sure two objects from global layers didn't share the same UID, which makes me think that global layers are entirely managed in the editor and when it comes to previewing and exporting, C2 copies all of the stuff on the global layers to all the overriden ones. So global layers don't exist during runtime. Which is kinda lame, because my original idea wont work, but kinda makes sense as well.
My initial question was "Can I make it so I can add all of my UI stuff through events, including the layers?" and the answer is "Yes, in theory, but if I don't wrap my head around these issues, I'll have to make one global layer, shove all of my UI in it, and create all the new empty layers at runtime, and manually move all of the UI to the right layer after creating it. Which hopefully works and doesn't make C2 freak out for a frame"
Also, I guess deleting layers, reordering them, and duplicating them should be possible if done right. It even seems that creating new layouts entirely could be a thing in theory. Which would be absolutely amazing for lots of things.
Anyway, this is what I gathered, but I didn't write any of that code, just read it and I guess I'd need to know what Ashley has to say about this. If I'm mistaken about anything in here, please tell me. If you have any insight to give me, please do. The following is addressed to you.
Please note that this is not a feature request even though this would be amazing to have in the new C3 runtime, even though it would be super dangerous in the hands of new users. I'm not making a feature request for this because I don't see the point of making one yet.
Finally, please note that this is all a discussion about what can or cannot be done using C2 by potentially rewriting parts of the runtime through a plugin, and while I do realise that this may not be a good choice to step out of what is documented in the SDK (because most undocumented stuff is undocumented for a reason), I wouldn't want to talk about that because it would make no sense to warn anyone about this. I just wanna have fun with the engine without breaking half of it, so I wanna know if there are things I need to know before trying to do so. By that I mean potential issues I didn't see coming, incompatibilities with what the editor might do, stuff breaking when exporting etc.
Thanks for reading all of this. Anyone curious about this can discuss it in this topic.