This is based on the stanford assignment 2B but its the basic layout for the design using the MVC. MVC is a template for design that you can use in iphone dev.
The model is where you define a new class of object and the methods it will perform. In this case, simply the contraints of min and max sides as well as the angles, name and description methods of a polygon.
Then the controller files define the class for all the interface controls and the methods they use.
Basically these controls will communicate with the model objects to update them of changes from the user actions in the view.
The model will then check itself and notify the controller of its methods and the controller will turn around and update the view objects in the xib.
EXAMPLE WITH AN ANIMAL CATALOG
The model will have the definition of an animal. If its a mammal or reptile, what color it is, does it have hair, how many legs or extremities.
In the view you will display an animal with data received from the controller. Also the controller will present the view to the interface for him to select different animals from the screen.
The controller will translate user selections to go fetch the animal properties according to user selection and update the view the user sees.