Override Interface Builder instantiation of objects?

Question

I'm developing for the iPhone, and I have a class DataManager, that is used to maintain my application data. When the application launches/exits, the data is read from/written to disk to create an instance of this class, using the NSKeyedArchiver (and Unarchiver) classes, since the DataManager adheres to the NSCoding protocol.

One problem I'm having is that I need the DataManager to be accessible by many of my other IB classes, so it is defined as an object in IB, and those classes have an outlet to it. The DataManager is being created using the standard init: method (or maybe initWithCoder:?), but since IB doesn't have the proper file (or NSData from the file) to instantiate the object, it has no initial contents.

So, is there any way to tell IB not to instantiate the class automatically? This will instead be performed using my application delegate, something like:

AppDelegate.h

IBOutlet DataContext *context;

AppDelegate.m

context = [NSKeyedUnarchiver unarchiveObjectWithData:dataLoadedFromFile];

As you can see, this presents a problem. Wouldn't the context be instantiated twice, once by InterfaceBuilder, then a second time by my application delegate?

I would like to prevent maintaining the context as an ivar in the delegate, since that seems to stray from the MVC paradigm, and leans toward the singleton pattern instead. (The controller should not be responsible for the data in my mind. It can maintain a reference to it obviously, but should not be responsible for offering it to other classes.)

Answer 1

When the application launches/exits, the data is read from/written to disk to create an instance of this class, using the NSKeyedArchiver (and Unarchiver) classes, since the DataManager adheres to the NSCoding protocol.

One problem I'm having is that I need the DataManager to be accessible by many of my other IB classes, so it is defined as an object in IB … As you can see, this presents a problem. Wouldn't the context be instantiated twice, once by InterfaceBuilder, then a second time by my application delegate?

Yup.

First, you should think about whether this is a controller or a model object. It sounds to me like it's a controller.

If it is, then you should move the model to a separate object or objects, and make those NSCoding-compliant, and make the data manager load and save those objects. A bonus of this solution is that you could tell the data manager to save the objects and purge them when you get a low-memory warning, not just at quit time.