I recently wrote a small side project using vala + gtk4.
I tried blueprint, and while I liked the format, I struggled with a lack of documentation on how to do some more advanced things. Gtk's .ui files are ok, but you still end up writing a lot of code to hook everything up.
While I really like vala as a language, I think it'd be great to write all my logic and models in vala, then use this swift library for the UI.