iPhone MVC. Problems with the model.

Question

I'm new to programming, iphone application programming in specific. After reading a bunch about MVC I decided to give it a try in a small application. As to my understanding, MVC works like this:

Model: data, manipulating data, retrieving data. ViewController: formats data from the model (NSDate to specific style), etc. View: the actual gui.

If this is indeed a correct formulation of basic MVC theory, my confusion lies in how data is passed between the model, VC, and view. Example: if I make calls to twitter and get the data in the model, how do I (correctly) pass this information to the VC for further work. I know that between the VC and View one mostly uses IBOutlets. The Model is my real problem.

In my last app I made an NSString variable in the app delegate so I could access that data from any class. However, I read that this is not the best approach when the app becomes complex because the delegate is in charge of starting, ending the app, not holding data.

I've read of delegation methods, singleton's, NSNotification (which I've used to call methods in other classes). The problem is that I don't really understand how to use these techniques to pass data from the model to other views.

Please let me know if my question is unclear.

Answer 1

Control over the application resides in the controller, so it is the object that will retrieve or save persisted data, update views with that data, and handle various events. Consider it the glue between the model and the view!

For example, if you were to click on a button to open a new modal view, you'd handle that event in your view controller. In the method that responds to the clicked button, you will create or access the new view controller and present it using presentModalViewController:animated:. If that new view and controller needs data that your current controller has access to, you could set a property in the new controller to refer to the object.

Answer 2

If you think about reusability, the main components that can be reused again are your model objects and view objects. They can be moved to different apps and still used properly. Your view controller is what is really specific to your app and where most of the app logic lies.

So in your example, you could have a twitter object that stores information and tweets from a user perhaps. You would create that class with all its functions separately within its own .h and .m file. Then in your view controller, instantiate the twitter class with data that is retrieved and begin using it from within the view controller.

Your view controller is actually retrieving the data but your model object is the one maintaining the data. In this way, you can pass on the model data with your twitter object to other view controllers.