Chuvmey is a Klingon word meaning "leftovers" - it was the only way I could think of to keep the MVC abbreviation while impressing upon you, my gentle reader, the idea that what is often considered the Controller layer actually becomes a "Stuff" layer. Before explaining this idea, I'll point out that my thought processes were set in motion by listening to the latest Mac Developer Roundtable (iTunes link) podcast on code re-use.
My thesis is that the View layer contains Controller-ey stuff, and so does the Model layer, so the bit in between becomes full of multiple things; the traditional OpenStep-style "glue" or "shuttle" code which is what the NeXT documentation meant by Controller, dynamic aspects of the model which could be part of the Model layer, view customisation which could really be part of the View layer, and anything which either doesn't or we don't notice could fit elsewhere. Let me explain.
The traditional source for the MVC paradigm is Smalltalk, and indeed How to use Model-View-Controller is a somewhat legendary paper in the use of MVC in the Smalltalk environment. What we notice here is that the Controller is defined as:
The controller interprets the mouse and keyboard inputs from the user, commanding the model and/or the view to change as appropriate.
We can throw this view out straight away when talking about Cocoa, as keyboard and mouse events are handled by NSResponder, which is the superclass of NSView. That's right, the Smalltalk Controller and View are really wrapped together in the AppKit, both being part of the View. Many NSView subclasses handle events in some reasonable manner, allowing delegates to decorate this at key points in the interaction; some of the handlers are fairly complex like NSText. Often those decorators are written as Controller code (though not always; the Core Animation -animator proxies are really controller decorators, but all of the custom animations are implemented in NSView subclasses). Then there's the target-action mechanism for triggering events; those events typically exist in the Controller. But should they?
Going back to that Smalltalk paper, let's look at the Model:
The model manages the behavior and data of the application domain, responds to requests for information about its state (usually from the view), and responds to instructions to change state (usually from the controller).
If the behaviour - i.e. the use cases - are implemented in the Model, well where does that leave the Controller? Incidentally, I agree with and try to use this behavior-and-data definition of the Model, unlike paradigms such as Presentation-Abstraction-Control where the Abstraction layer really only deals with entities, with the dynamic behaviour being in services encapsulated in the Control layer. All of the user interaction is in the View, and all of the user workflow is in the Model. So what's left?
There are basically two things left for our application to do, but they're both implementations of the same pattern - Adaptor. On the one hand, there's preparing the Model objects to be suitable for presentation by the View. In Cocoa Bindings, Apple even use the class names - NSObjectController and so on - as a hint as to which layer this belongs in. I include in this "presentation adaptor" part of the Controller all those traditional data preparation schemes such as UITableView data sources. The other is adapting the actions etc. of the View onto the Model - i.e. isolating the Model from the AppKit, UIKit, WebObjects or whatever environment it happens to be running in. Even if you're only writing Mac applications, that can be a useful isolation; let's say I'm writing a Recipe application (for whatever reason - I'm not, BTW, for any managers who read this drivel). Views such as NSButton or NSTextField are suitable for any old Cocoa application, and Models such as GLRecipe are suitable for any old Recipe application. But as soon as they need to know about each other, the classes are restricted to the intersection of that set - Cocoa Recipe applications. The question of whether I write a WebObjects Recipes app in the future depends on business drivers, so I could presumably come up with some likelihood that I'm going to need to cross that bridge (actually, the bridge has been deprecated, chortle). But other environments for the Model to exist in don't need to be new products - the unit test framework counts. And isn't AppleScript really a View which drives the Model through some form of Adaptor? What about Automator…?
So let me finish by re-capping on what I think the Controller layer is. It's definitely an adaptor between Views and Models. But depending on who you ask and what software you're looking at, it could also be a decorator for some custom view behaviour, and maybe a service for managing the dynamic state of some model entities. To what extent that matters depends on whether it gets in the way of effectively writing the software you need to write.