I see macCatalyst as a great way to upgrade my existing Mac app with features that are far easier to code for iPad than native Mac apps. The iPad SDK's are two decades newer than the Mac sdk's, and reflect a lot more convenient-intention and elbow-grease-polish at being convenient than macOS's native AppKit SDk.
Unfortunately, many of the things that are supposed to "just work", like multi-windowed apps, or compositional collection view layout, "just don't". they crash, or fail in so many ways that work fine when compiled and run an on actual iPad.
I've filed over a dozen bugs that weren't just "show-stoppers", they were "development stoppers", as in, until this bug is fixed, my whole app crashes at entry-level stuff, and thus I cannot continue development.
However, the bugs are getting fixed (and new ones are being created), so I still expect to be able to deliver the next major update of my app with macCatalyst, and ditch the AppKit code.
Also the insane lack of / splintering of documentation for supporting document-based apps is maddening. Apple's own app-templates can't get multi-window-document-based app support right. So I'm glad to hear they're gonna improve the docs, because they're awful.
What really frosts me is how little Apple's user experience evangelists have learned from the dramatic success of iPhone & iPad contrasted with Mac. They're telling us to do things in a more 'Mac'-like way, which I think the iPhone proved wasn't a good idea. I get that a mouse pointer isn't a finger, and the Mac can't do "direct interaction", and is stuck with "indirect", but so much of what I wrote for iPad just works better on Mac than a Mac-like interface did, that is if Apple can fix the bugs.
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I think that is exactly where this is all going. Catalyst is the slow turning of the ship toward apps that can run on Apple's own A-chips. Inevitably they will ditch Intel and go with their own silicon. But with the way Apple is run now I don't see them bothering to license any Rosetta-like translation software, especially in a world where cloud and subscription based software is prevalent.
TL;DR Catalyst is the beginning of the end for OS X. When Apple makes the jump to putting their own chips in Macs you'll see OS XI.
I have no idea whether Apple will go with arm-chips or not, but if that was their sole logic, I would think they'd just add a processor option for arm, instead of porting an entire UI framework and supporting application management. Back with the intel transition, we just checked a checkbox, and as long as we followed Apple's best practices for endian-independent code, we were fine. In fact, with most iOS apps running in the simulator on intel hardware, our iOS code is generally already processor agnostic. And most of the same processor-independent coding best practices are still applicable on Mac. (Many of them are built-in features of Swift!)
I think that Apple could really help support the mac app ecosystem by bringing 32bit OS X apps to MacOS!
Apple gave us a 10-year warning they were dropping 32-bit support. 10 years ago, I started my first job out of school helping a Mac app company begin upgrading to 64-bit processor support, and that company had been dragging it's feet.