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Computers & Software

Porting an iOS Project to the Mac

I just finished porting my electronic calipers mobile iOS app, EP Calipers, to the Mac. In doing so I decided to bite the bullet and change the programming language from the original Objective C (ObjC) to Apple’s new language, Swift. Here are some observations.

The Swift programming language

I’m comfortable now with Swift. Swift is an elegant language with a modern syntax. ObjC is a very weird looking language in comparison. You get used to ObjC, but, after writing Swift for a while, your old ObjC code looks awkward. Comparing the two languages is a little like comparing algebraic notation to reverse polish notation (i.e. like comparing (1 + 3) to (1 3 +)). I’ll just give a few examples of the differences. The chapter “A Swift Tour” in Apple’s The Swift Programming Language is good resource for getting up to speed in Swift quickly.

Here’s how an object variable is declared and initialized in ObjC:

Widget *widget = [[Widget alloc] init];

Note that in ObjC objects are declared as pointers, and both the memory allocation for the object and initialization are explicitly stated. ObjC uses messaging in the style of SmallTalk. The brackets enclose these messages. So in the commonly used object construction idiom shown, the Widget class is sent the allocation message, and then the init message. A pointer to a widget results.

The same declaration in Swift:

var widget = Widget()

With Swift the syntax is much cleaner. The keyword var indicates variable initiation. Pointer syntax is not used. The type of the variable doesn’t have to be given if it can be inferred from the initiation. Swift is not a duck-typed language, like, for example, Ruby. It is strongly statically typed. It’s just that if the compiler can figure out the typing, there’s no need for you to do the typing (Sorry for the puns — couldn’t resist). Note that the constructor is just the class name followed by parentheses. If there are parameters for the constructor, they are indicated with parameter names within the parentheses. Finally, note that no semicolon is needed at the end of the line.

Swift has a lot of other niceties. Simple data types like integer, float and double are full-fledged objects in Swift (Int, Float, Double). Unlike ObjC, where only pointers can be nil, all classes in Swift, even classes like Int, can potentially be equal to nil, if the variable is defined as an Optional with the following syntax:

var count: Int? // count can be equal to nil

In order to use an Optional variable, you need to “unwrap” it, either forcibly with an exclamation point:

let y = count! + 1 // will give runtime error if count == nil

or, more safely:

if let tmp = count { // no runtime error if count == nil
     y = tmp + 1
 }

In that statement, the if statement evaluates to false if count is nil. This if statement also demonstrates more of Swift’s nice features. There are no parentheses around the conditional part of the if statement, and the options following the if statement must be enclosed in braces, even if they are only a single line long. This is the kind of syntax rule that would have prevented the Apple’s gotoFail bug and one wonders if that very bug may have led to incorporation of this rule into Swift.

Because Swift has to coexist with the ObjC API, there are conventions for using ObjC classes in Swift. Some ObjC classes, like NSString, have been converted to Swift classes (String class). Most retain their ObjC names (e.g. NSView) but their constructors and methods are changed to Swift syntax. Many methods are converted to properties. For example:

ObjC

NSView *view = [[NSView alloc] initWithFrame:rect];
 [view setEnabled:true];

Swift

let view = NSView(frame: rect)
 view.enabled = true

Properties are declared as variables inside the class. You can add setters and getters for computed properties. When properties are assigned Swift calls the getting and setting code automatically.

There are other improvements in Swift compared to ObjC, too numerous to mention. For example, no header files: wonderful! Swift is easy to learn, easy to write, and lets you do everything that you could do in ObjC, in a quicker and more legible fashion. Well named language, in my opinion.

Mac Cocoa

The other hurdle I had in porting my app was translating the app’s API. Apple iOS is not the same as Apple Cocoa. Many of the foundational classes, like NSString (just String in Swift) are the same, but the user interface in iOS uses the Cocoa Touch API (UIKit), whereas Cocoa uses a different API. The iOS classes are prefixed with UI (e.g. UIView), whereas the Cocoa classes use the NS prefix (NSView).

The naming and functionality of the classes between to two systems is very similar. Of course Cocoa has to deal with mouse and touchpad events, whereas iOS needs to interpret touches as well as deal with devices that rotate. Nevertheless much of the iOS code could be ported to Cocoa just by switching from the UI classes to their NS equivalents (of course while also switching from ObjC to Swift syntax). As expected, the most difficult part of porting was in the area of user input — converting touch gestures to mouse clicks and mouse movement. It is also important to realize that the origin point of the iOS graphics system is at the upper left corner of the screen, whereas the origin in Mac windows is at the lower left corner of the screen. This fact necessitated reversing the sign of y coordinates in the graphical parts of the app.

Although there’s no doubt the UI is different between the two platforms, there does seem to be some unnecessary duplication of classes. Why is there a NSColor class in Cocoa and a UIColor class in iOS, for example? Perhaps if Apple named the classes the same and just imported different libraries for the two platforms, the same code could compile on the two different platforms. Apple has elected to support different software libraries for computers and mobile devices. Microsoft is going in the other direction, using the same OS for both types of devices. I think Apple could get pretty close to having the same code run on both types of devices, at least on the source code (not the binary) level, with a little more effort put into their APIs. I suspect that at some point in the future the two operating systems will come together, despite Tim Cook’s denials.

IKImageView

I used IKImageView, an Apple-supplied utility class, for the image display in my app. In my app, a transparent view (a subclass of NSView) on which the calipers are drawn is overlaid on top of an ECG image (in a IKImageView). It is necessary for the overlying calipers view to know the zoomfactor of the image of the ECG so that the calibration can be adjusted to match the ECG image. In addition in the iOS version of the app I had to worry about device rotation and adjusting the views afterwards to match the new size of the image. On a Mac, there is no device rotation, but I wanted the user to be able to rotate the image if needed, since sometimes ECG images are upside down or tilted. It’s also nice to have a function to fit the image completely in the app window. But because of the way IKImageView works, it was impossible to implement rotation and zoom to fit window functionality and still have the calipers be drawn correctly to scale. With image rotation, IKImageView resizes the image, but reports no change in image size or image zoom. The same problem occurs with the IKImageView zoomToFit method. I’m not sure what is going on behind the scenes, as IKImageView is an opaque class, but this resizing without a change in the zoom factor would break my app. So zoomToFit was out. I was able to allow image rotation, but only when the calipers are not calibrated. This make sense anyway, since in most circumstances, rotating an image will mess up the calibration (unless you rotate by 360°, which seems like an edge case). Other than these problems with image sizing, the IKImageView class was a good fit for my app. It provides a number of useful if sketchily documented methods for manipulating images that are better than those provided by the standard NSImageView class.

Saving and printing

As mentioned, my app includes two superimposed views, and I had trouble figuring out how to save the resulting composite image. IKImageView can give you the full image, but then it would be necessary to redraw the calipers proportionally to the full image, instead of to the part of the image contained in the app window. I came close to implementing this functionality, but eventually decided it wasn’t worth the effort. Similarly printing is not easy in an NSView based app (as opposed to a document based app), since the First Responder can end up being either view or the enclosing view of the window controller. I wished there was a Cocoa method to save the contents of a view and its subviews. Well there is, sort of: the screencapture system call. It’s not perfect; screencapture includes the window border decoration. But it was the easiest solution to saving the composite image in the app window. The user then has the ability to further edit the image with external programs, or print it via the Preview app.

Sandboxing

Mac apps need to be “sandboxed,” meaning if the app needs access to user files, or the network, or the printer, or other capabilities you have to specifically request these permissions, or, as Apple terms it, entitlements. Since the app needed access to open user image files, I just added that specific permission.

Submission to the App Store

Submitting a Mac app to the App Store is similar to submission of an iOS app — meaning if you are not doing it every day, it can be confusing. The first problem I had was the bundle ID of my app was the same as the bundle ID of the iOS version of the app. Bundle IDs need to be unique across both the Mac and iOS versions of your apps. Then there was the usual Apple app signing process which involves certificates, provisioning profiles, identifies, teams, etc., etc. I did encounter one puzzling glitch which involved a dialog appearing asking to use a key from my keychain, and the dialog then not working when clicking the correct button. I had to manually go into the keychain program to allow access to this key. So, in summary it was the usually overly complicated Apple App Store submission process, but in the end it worked.

And so…

Because the Apple API is so similar between Cocoa and iOS, porting my app to the Mac was easier, even with the language change from ObjC to Swift, than porting between different mobile platforms. I have ported apps between iOS and Android, and it is a tougher process. As for Swift, I’m happy to say goodbye to ObjC. Don’t let the door hit you on your way out!

By mannd

I am a retired cardiac electrophysiologist who has worked both in private practice in Louisville, Kentucky and as a Professor of Medicine at the University of Colorado in Denver. I am interested not only in medicine, but also in computer programming, music, science fiction, fantasy, 30s pulp literature, and a whole lot more.

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