In a typical model/view/controller implementation (setting aside, of course, the debate over what MVC really is), we implement a model (which maps to a document, file, or database), a controller (which manages the views), and a collection of views, generally contained within a window.

Now this model maps nicely into the iPhone API and the Android API: the basic controller class for the iPhone is UIViewController; for Android it is android.app.Activity. And of course the view hierarchy represents the UIViews and android.view.View objects, both built-in objects and custom objects. The controller object also makes the ideal target for iPhone delegates and data objects and for the various Android interfaces which do essentially the same thing.

On the desktop, most applications are built with either a single model and a single controller. Rarely do we have multiple controllers opened onto the same model, and when this happens, more often than not the multiple controllers are multiple instances of the same class declaration: multiple text editors opened onto the same text file, for example.

Mobile devices are different. On a mobile device you can have a top level screen with one controller drill down to a separate screen (with a separate controller) which displays an individual item, which drills down to a third screen showing information about that individual item, which drills down into a fourth showing the setting for a single field.

In this example, we have four (increasingly fine grained) views into the same model: in the first screen we have a top level view; in the next, a view of an item, and so forth. The original method I was using in order to code for this particular model was to create multiple model-like objects and pass them down to individual view controllers:

Now this model tends to be somewhat informal: my top-level model is (for example) an ArrayList of Items. When it’s time to edit an item, we pass the Item as the sub-model object into the sub-control display. And when it’s time to edit some piece of the Item, we pull that piece out and pass that to the next sub-control display.

The whole process becomes messy for a variety of reasons. First, it creates a strict dependency in the flow of views; even if we use interfaces and protocols in order to isolate a control (and its respective views), we still have a lot of work to do if we want to rearrange views. Second, it creates the problem that we somehow need to pass information about which sub-item was updated back to the previous view controller. This becomes problematic since the events for UIViewController and Activity don’t map well to the notion of “saving” or “marking dirty”: there is no single event we can easily grab that consistently means “You’re going away and the view above you is becoming active.”

Android presents a few twists on this as well. First, when your Android Activity goes into the background it can be deleted from memory in a low-memory situation. That is, you can find yourself in the situation where your Activity goes away and has to be rebuilt from scratch. This implies that you must strictly separate the sub-model from the controller (as opposed to the iPhone, where you can informally combine the model and view controller into the same UIViewController class). Second, well-factored activities can be set up to be brought up by external applications: it is possible your Item editor activity can be brought up by a separate application if it knows how to launch you with the correct Intent.

It strikes me that the proper way to handle this is to have a single unified model, and instead of passing sub-models, we instead pass a small record indicating what part of the model we are working on:

The idea is that the Δ object represents a small record which indicates to the sub-controller which part of the model it is working on. This has the advantage that updates are communicated to each of the controllers which are working on this model as the object updates. This also works well on Android, as individual control/view components can disappear and re-register themselves; the delta record is passed in the Intent which created that component, so when the Activity is reconstructed the Intent can be used to put the view controls back into the correct state.

I have a simple nutrition journal application I’ve been building for the iPhone which I was going to rewrite from the ground up using this model. In future posts I’ll update the post to indicate if this works well or not.