Add Ufo::new_layer(name) method

[madig]

Should insert an empty new named layer and return a mutable reference to it. UX problem: what if your chosen layer name exists already? Return Result<Error, &mut Layer>?

Maybe even a combination method like get_or_insert_new(name)? Or something mirroring https://doc.rust-lang.org/std/option/enum.Option.html#method.get_or_insert. Use case is mutably grabbing e.g. the public.background layer or creating it if it doesn't exist yet. For when you just want to add stuff into it.

[cmyr]

When we create a new UFO we always add public.background, I think.

I guess I Would want to think about this from an actual user's perspective: what are they trying to achieve? I don't use layers in norad right now, so I'm not sure. The problem with get_or_insert is that you need to pass an owned key to the getter every time you want to get the layer, which might be annoying?

I think that a simple and reasonable approach might be to just expect the caller to check if a given layer exists, and create it if not; this will not be heavily used API, so I don't think we should get too fancy. I think that returning an error if the layer already exists is a simple and correct solution. 🤷

[madig]

We only create the default layer.

I just hit this again where I iterate over a data source and sometimes write a background layer for a glyph. The code now looks like this:

if let Some(parent_name) = layer.name.strip_prefix("mask.") {
    if let Some(drawn_instance) = drawn_instances.get_mut(parent_name) {
        match drawn_instance.find_layer_mut(|l| l.name == "public.background") {
            // If we already have a background layer...
            Some(background) => // ...,
            // Otherwise make a new one and insert the glyph.
            None => {
                // creation dance...
            }
        };
    }
} else {
    // ...
}