Question

I’m coding my first app in XCode 4.2 supporting iOS 4.x through the current releases. To support iOS 4.0, I’m not using Storyboard feature and also using unsafe_unretained for weak references. I have AppDelegate files (.h and .m) along with several view controllers with UITabBarController. In my first view controller, in the -viewDidLoad method, I initialize two NSDictionaries and also start a timer with 1 sec interval. In the selector method, I have to pickup a random number between 0 and 7 to pick a corresponding value in both the dictionaries. The dictionaries are used only in the first view controller and not anywhere.

My first question is

1. where do I load those two dictionaries – in the AppDelegate -didFinishLaunchingWithOptions: method or in the first view controller’s -viewDidLoad method?
2. I also wanted to support iPad. If that’s the case, do I create a common class library to support iPhone/iPod/iPad?. If that is the recommended way, can I move the common functionality to the AppDelegate .m file instead?

Please advise.

Answer 1

You can move your common data and business logic into a separate set of model classes outside of the UI layers and appdelegates. This is one of the main benefits of the MVC pattern – by separating and making a clear distinction, it’s easy to have separate view layers (one for phone and one for iPad).

That means all the data (dictionaries), logic with your random numbers and timers would be encapsulated and shared. That also allows you to cleanly unit test the majority model and logic programmatically. It also means you can make substantial changes to your algorithms and minimize the code churn.

When the timer goes off it can either post a notification or you can have a delegate pattern where it does a call back.

Related Post: Delegates vs. events in Cocoa Touch

If you do a shared model, one option is to use a singleton pattern where you access the model like:

[MyModel sharedInstance];