The Swift manual is pretty good actually, documentation around Foundation is pretty complete although a bit spare but yeah....UIKit and other libraries used for creating iOS applications are really not very well documented. The last few years I've been copying code from WWDC demo's to learn about new stuff. I tried to learn how to capture RAW data when the API was just out, so no Stack Overflow answer out yet. It was hard as hell.
But anyway that's not a Swift problem. Swift itself is pretty easy to get into.
Ya it is UI bound (though you’d think that’s where a lot of the documentation would be!). I’d also say JSON handling and other data handling aspects are poorly documented, would you agree?
I agree Codable is one of the worst ways of dealing with JSON apart from manually parsing through dictionaries. I mean It Just Works on a very clean and consistent API, but if people started to mix snake_case with PascalCase, return "1" or any other garbage shit people write when they only thing they have to care about it JS clients then you're typing a lot of unreadable boilerplate.
Since we have custom attributes I will investigate soon if there's a nice framework around that can make it work a bit like the usual JSON libraries in for example C# work.
But anyway that's not a Swift problem. Swift itself is pretty easy to get into.