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Coupling in a Cocoa[ Touch] App

This is one of my occasional “problem looking for a solution” posts. It’d be great to discuss this over on App.net or G+ or somewhere. I don’t think, at the outset of writing this post, that the last sentence is going to solve the problems identified in the body.

I’ve built applications in a few different technologies, and I think that the Cocoa and Cocoa Touch apps I’ve seen have been the most tightly coupled to their host frameworks with the possible exception of Delphi. I include both my code and that I’ve seen of other people—I’m mainly talking about my work of course. I’ve got a few ideas on why that might be.

The coupling issue in detail

Ignore Foundation for a moment. A small amount of Foundation is framework-ish: the run loop and associated event sources and sinks. The rest is really a library of data types and abstracted operating system facilities.

But now look at AppKit or UIKit. Actually, imagine not looking at them. Consider removing all of the code in your application that directly uses an AppKit or UIKit class. Those custom views, gone. View controller subclasses? Boom. Managed documents? Buh-bye.

OK, now let’s try the same with Core Data. Imagine it disappeared from the next version of the OS. OK, so your managed objects disappear, but how much of the rest of your app uses them directly? Now how about AVFoundation, or GameKit, and so on?

I recently took a look at some of my code that I’d written very recently, and found that less code survived these excisions than I might like. To be sure, it’s a greater amount than from an application I wrote a couple of years ago, but all the same my Objective-C code is tightly coupled to the Apple frameworks. The logic and features of the application do not stand on their own but are interwoven with the things that make it a Mac, or iPhone app.

Example symptoms

I think Colin Campbell said it best:

iOS architecture, where MVC stands for Massive View Controller

A more verbose explanation is that these coupled applications violate the Single Responsibility Principle. There are multiple things each class is doing, and this means multiple reasons that I might need to change any of these classes. Multiple reasons to change means more likelihood that I’ll introduce a bug into this class, and in turn more different aspects of the app that could break as a result.

Particularly problematic is that changes in the way the app needs to interact with the frameworks could upset the application behaviour: i.e. I could break what the app does by changing how it presents a UI or how it stores objects. Such modifications could come about when new framework classes are introduced, or new features added to existing classes.

What’s interesting is that I’ve worked on applications using other frameworks, and done a better job: I can point at an Eclipse RCP app where the framework dependencies all lie at the plugin interface boundaries. Is there something specific to Cocoa or Cocoa Touch that leads to applications being more tightly coupled? Let’s look at some possibilities.

Sample code

People always malign sample code. When you’re just setting out, it assumes you know too much. When you’re expert, it takes too many shortcuts and doesn’t display proper [choose whichever rule of code organisation is currently hot]. Sample code is like the avocado of the developer documentation world: it’s only ripe for a few brief minutes between being completely inedible and soft brown mush.

Yes, sample code is often poorly-decoupled, with all of the application logic going in the app delegate or the view controller. But I don’t think I can blame my own class design on that. I remember a time when I did defend a big-ass app delegate by saying it’s how Apple do it in their code, but that was nearly a decade ago. I don’t think I’ve looked to the sample code as a way to design an app for years.

But sample code is often poorly-decoupled regardless of its source. In researching this post, I had a look at the sample code in the Eclipse RCP book; the one I used to learn the framework for the aforementioned loosely-coupled app. Nope, that code is still all “put the business logic in the view manager” in the same way Apple’s and Microsoft’s is.

Design of the frameworks and history

I wonder whether there’s anything specific about the way that Apple’s frameworks are designed that lead to tight coupling. One thing that’s obvious about how many of the frameworks were designed is the “in the 1990s” aspect of the issue.

Documentation on object-oriented programming from that time tells us that subclassing was the hotness. NSTableView was designed such that each of its parts would be “fully subclassable”, according to the release notes. Enterprise Objects Framework (and as a result, Core Data) was designed such that even your data has to be in a subclass of a framework class.

Fast forward to now and we know that subclassing is extremely tight coupling. Trying to create a subclass of a type you do control is tricky enough, but when it’s someone else’s you have no idea when they’re going to add methods that clash with yours or change behaviour you’re relying on. But those problems, real though they are, are not what I’m worried about here: subclassing is vendor lock-in. Every time you make it harder to extract your code from the vendor’s framework, you make it harder to leave that framework. More on that story later.

So subclassing couples your code to the superclass, and makes it share the responsibility of the superclass and whatever it is you want it to do. That becomes worse when the superclass itself has multiple responsibilities:

  • views are responsible for drawing, geometry and event handling.
  • documents are responsible for loading and saving, managing windows, handling user interaction for load, save and print requests, managing the in-memory data representing the document, serialising document access, synchronising user interface access with background work and printing.
  • pretty much everything in an AppKit app is responsible in some way for scripting support which is a cross-cutting concern.

You can see Apple’s frameworks extricating themselves from the need to subclass everything, in some places. The delegate pattern, which was largely used either to supply data or let the application respond to events from long-running tasks, is now also used as a decorator to provide custom layout decisions, as with the 10.4 additions to the NSTableViewDelegate, UITableViewDelegate and more obviously the UICollectionViewDelegateFlowLayout. Delegate classes are still coupled to the protocol interface, but are removed from the class hierarchy giving us more options to adapt our application code onto the interface.

Similarly, the classes that offer an API based on supplying a completion handler require that the calling code be coupled to the framework interface, but allow anything to happen on completion. As previously mentioned you sometimes get other problems related to the callback design, but at least now the only coupling to the framework is at the entry point.

No practice at decoupling

This is basically the main point, isn’t it? A lack of discipline comes from a lack of practice. But why should I (and, I weakly argue, other programmers whose code I’ve read) be out of practice?

No push to decouple

Here’s what I think the reason could be. Remind me of the last application you saw that wasn’t either a UIKit app or an AppKit app. Back in the dim and distant past there might be more variety: the code in your WebObjects Objective-C server might also be used in an AppKit+EOF client app. But most of us are writing iOS software that will only run on iOS, so the effort in decoupling an iOS app from UIKit has intellectual benefits but no measurable tangible benefits.

The previous statement probably shouldn’t be taken as absolute. Yes, there are other ways to do persistence than Core Data. Nonetheless, it’s common to find iOS apps with managed object contexts passed to every view controller: imagine converting that ball of mud to use BNRPersistence.

What about portability? There are two options for writing cross-platform Objective-C software: frameworks that are API compatible with UIKit or AppKit, or not. While it is possible to build an application that has an Objective-C core and uses, say, Qt or Win32 to provide a UI, in practice I’ve never seen that.

There also aren’t any “alternative” Objective-C application frameworks on the platforms ObjC programmers do support. In the same way that you might want to use the same Java code in a SWT app, a Swing app, and a Spring MVC server; you’re not going to want to port your AppKit app to SomeoneElsesKit. You’ll probably not want to move away from UIKit to $something_else in the near future in the same way it might be appropriate to move from Struts to Spring or from Eclipse RCP to NetBeans Platform.

Of course, this is an argument with some circular features. Because it’s hard to move the code I write away from UIKit, if something else does come along the likelihood is I’d be disinclined to take advantage of it. I should probably change that: vendor lock-in in a rapidly changing industry like software is no laughing matter. Think of the poor people who are stuck with their AWT applications, or who decided to build TNT applications on NeWS because it was so much more powerful than X11.

Conclusion

This is the last sentence, and it solves nothing; I did tell you that would happen.