Explanations of design intent might be the most underrated kind of documentation. There's so little of that nowadays in general, in any technology. Personally, I blame shortened attention spans, the death of programming books, and the rise of Stack Overflow. I'm not familiar with the Apple ecosystem, is it really worse in this regard than others?
I also blame dogmatic misunderstandings that have crystalized around "Agile." Maybe back in the day a some cookbook process specified a lot of useless documentation be created, but many people seem to have thrown the baby out with the bathwater and declared all documentation useless or not worth the effort.
Also some programming books are to blame. IIRC, the book "Clean Code" completely rejects comments because they can get out of sync with the code and therefore be misleading, and "clean code" should be self-documenting. However, all code can ever tell you is what is, it can never tell you why or what was really intended. Also, when you really think about it, method and variable names are comments too, which can get just as out of sync with what's really going on than a comment block.
What it recommends to just delete are comments like
var speed # the speedIt's the easiest to keep in sync. It lives in the code it documents. It's in the same version control repo and the same version of the source building a binary can build the docs. If it ever does get out of sync with the code it's the most straightforward to fix because it's the same process for fixing an issue in the code itself.
Tests are not documentation. They only test a very limited "what" or "how" and give no explanation of "why". If it's closed source software and you're only shipping a binary, the tests do not go with the binaries so they're meaningless to third parties. Sample code isn't much better. Without the organizational pressure to keep it in sync with the code it's demoing and "why" explanations it doesn't provide a lot of useful insight for third parties.
If you didn't attend that WWDC session or pour over all the recordings you've missed that particular key concept. So you're then sort of feeling your way around some new technology and making it work but it's not as efficient or elegant as if you had the whole picture.
Because there's so little documentation time and Apple does not (as a rule) do engineering blogs or similar there's not a lot of of opacity into inner workings or designs. From the outside there's a lot more reading of tea leaves than you see for other platforms.
No blogs is fine, that's just the corporate culture, but no blogs and no good up to date architectural documentation is a huge problem. It leads to cargo-cult understanding or complete misunderstandings by outside developers. Hell, it leads to cargo-cult understanding by internal developers.
I think a great example of this is the difference in docs between OSS Python projects.
Most include the auto generated Sphinx (or whatever) docs, with usually just a README level veneer. I quickly end up needing to wander the code. I swear any OAuth2 client library is fated into falling into this.
The great ones include a “quick start”, overall philosophy/architecture, sections on each sub component, common examples of “advanced usage”, and then the generated API docs (and source code). They let you dig deeper as you learn more, and hit more complicated requirements in your project. Click, requests, Flask, etc. are great examples of this to me.
Sphinx and other tools can be leveraged for all of that. But it does have to be written and maintained.
And not having access to the source (internally or externally) kills me for app dependencies. Sometimes I need to look just to understand the doc or see if I’m making a mistake or there’s a bug. Not trying to get into OSS philosophy, just the practical part for me.
Even the official Python docs can stink in places. The subprocess module replaces X, Y, and Z to be “simpler”, but if you want to know the method args, go read the docs on the thing it “replaces.”
I love “native” apps on my mac, and pay for quite a few. But I’d never try to make one. Even XCode is befuddling to me. It’s not an IDE, it’s an opaque RAD that makes me nostalgic for early 90’s Delphi.
I do recall from my iOS development times that Apple's documentation and articles were really good.
The reason I think the quality and quantity dropped was the internal schedules barely (or don't) leave enough time for the engineering work so there's very little time available for high level documentation. Internally tons of "documentation" existed as Radar comments or exchanges on internal mailing lists. Maybe a group's wiki had some crystallized documentation or high level architectural descriptions but good luck accessing it from outside that org. My favorite was some discussion about overall design or architecture that got the "let's take this offline" where all the helpful details ended up shared in an in-person meeting.
The internal secrecy and rapid development pace made it really difficult to get good overviews of technologies internally. I really sympathize with outside developers trying to cobble together an understanding of something where the documentation sucks or is missing.
The DocC tooling isn't going to be anymore effective than doxygen or other auto generated documentation without good architectural documentation. A function definition is nice but knowing you need to decombulate a frobnob before snizzlizing or that decombulation can only safely be done on the main thread is often more important. I can read a function signature but I can't necessarily know all the keys that go into some configuration NSDict passed into it.
I hope the situation improves if the DocC tooling lowers the friction for writing documentation. It sucks having to mix a WWDC presentation, sample code, and iffy docs into a semblance of usable architectural documentation.