I've been (very slowly) dabbling around writing a Game Boy emulator in Swift and, surprisingly, some of the biggest difficulties I've had have been actually getting stuff on-screen, rather than the emulation itself. Turning an array of bytes into pixels on-screen, timers accurate enough to run my main loop at 59.97Hz, window behaviour etc.
I know all this stuff exists and isn't even that hard, but actually finding what I need has proven quite difficult. I found that Google searches for NSWhatever tended to return results for UIWhatever, and there are far fewer StackOverflow threads due to a lot of this stuff being from the early 2000s.
Apple's documentation for their newest stuff is infamously barebones, but I've also found it difficult for the very old stuff too -- much of it hasn't been updated for Swift, so I've been at the mercy of Xcode autocomplete to find the new names for various constants, etc. Good Mac development is starting to feel like a lost art.