What are the Responsibilities of a Model Class in MVCS?
Model classes encapsulate and provide an API for the application data model. A Model class is the gatekeeper for your application's data. Other actors in the application make requests for data through the API provided by the Model. As data is updated through the Model, the Model is equipped to broadcast events to the framework informing other actors of changes to the data model so they may adjust their state accordingly.
In addition to controlling access to the data model, the Model is routinely used to perform operations on the data to keep the data in a valid state. This includes performing calculations on the data, or other areas of domain specific logic. This responsibility of the Model is extremely important. The Model is the tier of any given application with the highest potential for portability. By placing domain logic on the Model, future implementations of the model will not have to repeat this same logic as they would if it was placed in the View or Controller tiers.
As an example, your Model class might perform a sales tax calculation on the shopping cart data that it is string. A Command will access this method, and the final calculation will be dispatched as an event that a Mediator is listening for. The mediator will then update its view component with the updated value. In the first iteration of the application it was a typical Flex application. This calculation could have easily been performed on a Mediator, or even the view itself. The second iteration of the application is a mobile Flash application that requires an entirely new view form factor. Since this logic is contained in the Model, it can be reused for both form factors with entirely different views.