I found myself wasting months to do something that should have been handled better in iOS.

- APIs return non-descriptive error codes like -16405. Some of them aren't documented. And even the ones that are are not found clearly. Different codes are documented in different files. If you don't know where to look, you won't find it. Java-style descriptive error names like InvalidFrameRateException are sorely missing. iOS should either adopt exceptions or at least string error codes like INVALID_FRAME_RATE, not -16405.

Sometimes, there's no error code at all. I'm trying to record a video, using a AVCaptureMovieFileOutput, which is supposed to save to a file, and it doesn't. There's no error code, no exception, no log message telling me what went wrong and how to fix it. I spent a day or two trying various possibilities but nothing worked.

- Common abstractions like a photo gallery class aren't missing. It took me a month to write my own, because there are a lot of cases to deal with: swiping left and right, pinching to zoom, double-tapping to zoom, keeping the photo centered while zooming, taking care not to zoom the whitespace, keeping the photo gallery in sync with the Photos app (the user might delete a photo either in your app or in the Photos app, and you need to sync in both directions), and so on. Even now my gallery class doesn't support swipe down to close, which the Photos app supports, as I was told yesterday. That's what happens when you make people reimplement things — you'll get an inconsistent UX.

Forget a photo gallery class. Even a zoomable photo view is missing. It took me days of messing with UIScrollView to figure this out, until I found the ray wenderlich tutorial.

- Some APIs come in both sync and async versions, while others, like permissions, come only in an async version, though a sync version would be easier. Even the async APIs are inconsistent -- some invoke your callback on the main thread (UIKit APIs), some invoke it on the same thread you invoked it on, and some in a random thread (AVCapture). I filed a radar asking for consistency (maybe all async APIs can take a queue to invoke the callback on) but Apple closed it as WontFix.

Unfortunately, Swift doesn't have async/await, so dealing with the async APIs is a pain, and for some reason, this is not even a priority in Swift 5. It's being put off for years.

- Permissions are a pain to deal with, especially the common case that your app can't run without some permissions, like Camera and Photos for a camera app. I had to deal with the following cases:

+ You should take care to request a permission only after the previous one has been approved or denied by the user. Otherwise, you'll get a prompt for Camera, and before you respond, it disappears and is replaced by a prompt for Photos. And before you can respond, it disappears and is replaced by Location. When you respond to it, the Camera prompt re-appears, but again disappears and is replaced by Photos. This is a bad UX, caused by iOS not queueing multiple permission requests internally and asking the user for one only after the previous request was responded to.

+ You should ask for location before camera, because a GPS fix can take time.

+ You should take care to obtain permission before accessing the camera, otherwise the camera will vend frames consisting of black pixels, which you might accidentally save to the Photos app.

+ iOS distinguishes between a permission being denied (by the user) and restricted (by an admin or parent). My app would crash in the latter case, because the camera device is not found. But not in the former, because the camera exists but vends black frames.

+ If the user changes permissions of your app while it's running, Apple says that iOS will kill your app and restart it so that you don't have to deal with permission changing dynamically. Except when it doesn't, which goes back to the denied vs restricted case above.

In summary, you waste months of your time working around insufficiently-documented iOS issues, or debugging things, or reimplementing common things in every app.

I conclude that iOS is a poorly-designed platform from the developer point of view. Maybe other platforms are worse, I don't know, but iOS certainly is far from what it could have been. Ideally, you should implement only what makes your app unique.