" The controller accesses the model, possibly updating it in a way appropriate to the user's action (e. g . , controller updates user's shopping cart) A view uses the model to generate an appropriate user interface (e. g. , view produces a screen listing the shopping cart contents). The view gets its own data from the model (or direct from the data access layer - sometimes there is no domain logic to do). The model should have no direct knowledge of the view. Model Railroad Track Planning However, the observer pattern can be used to provide some indirection between model and view, allowing the model to notify interested parties of a change. A view object can register itself with the model and listen for changes but the model itself remains view-agnostic. "

" (2) prescriptive, i. e. , the model prescribes the original. If the deviation from model to original cannot be tolerated then the original has to be changed. Design models are in a prescriptive reference mode to their originals, i. e. , the implementation. Recreation Model Railroad (3) idealizing, i. e. "