The latest fad in tech punditry is to claim the barrier to have iOS apps on the Mac is the fact these two graphical frameworks are so different that it makes the work of developers too complex. This is false.
Let’s start with the elephant in the room: an app generally has to bring more to the table than its UI. But the underpinning coverage is total. There is less than 5% of classes and methods that are available on iOS and not on the Mac. On the other hand, there are more methods in the same frameworks on the mac side. So for everything but the UI, “porting” only means recompiling. If your MVC is well implemented, that part at least won’t cause any issue.
The UI part is a mixed bag, but the paradigms are the same nowadays. It wasn’t the case until fairly recently but table views are cell or view based, the delegates are as expected, etc, etc. As an experiment, on uncomplicated examples, I did search and replace only, no tweaking, and it worked.
Now, of course, the issue isn’t technical: the iOS mono-window and mono-view is wasted on the mac. A lot of applications take the “landscape ipad” paradigm to make it less obvious, including Apple : you have the sidebar and the main view, and it works just like the master/detail project template that comes with Xcode.
Porting a successful app from iOS to the Mac is indeed a bit of work. The Mac is window centric and iOS is view centric. Some things you cannot do on iOS are possible on the Mac, like covering parts of your UI, or dragging and dropping elements. It is definitely a very different way to think about the user experience, and the design choices are certainly less constrained and less obvious. But there is no real technical hurdle, unless the vast majority of your app logic lives in the view controllers rather than in a separate codebase. And then again, the Mac now has NSViewController that works exactly like, who would have guessed, UIViewController, and apps can run in full screen mode, so who knows?
The tools (Xcode, IB, etc) are the same. The non UI frameworks are the same. The UI frameworks are similar where it makes sense (putting stuff on screen) and dissimilar where you have to (input methods and window management). That’s it.
Now, you can definitely agree that the Mac app landscape is very different from the iOS one. People are used to having giant things that install other things everywhere, demos, sharewares, unrestricted access to the filesystem, the possibility to copy and paste anything from anywhere, or drag and drop anything from anywhere, and to put it somewhere else, where it will do something. They have multiple apps that come and go depending on modal dialog boxes that show up, and pieces of stuff like palettes that they can arrange any damn way they please, thank you very much. For all these reasons, designing a successful Mac app is challenging. Big screens, small screens, people who like lots of little windows, a few big windows, people who use spaces, people who use keyboard shortcuts more than the mouse, people who don’t know how menus work, people who have a gazillion of menu items, fonts that can be changed systemwide, colorschemes, those are all valid reasons to dread an attempt at making an app that will appeal to most people.
But you don’t get to play the technical hurdle card. All these interactions have been studied, refined, and solved over 30 years of graphical interfaces. You have to choose what will work best for your needs, and yes, this is hard. But it’s not about code.