2.4.4 Lazy Model


A Lazy Model is an intermediate solution between Active and Passive Model that retains centrality of the notification bookkeeping, but delegates notification triggering. It is a good strategy to retain full control on the notification flow as in the Passive model, but at the same time keep the listeners’ list centralized so that they can all be notified of the changes, no matter which Controller performs them.

With the resulting design, the Controller is free to perform multiple changes on the Model object, and trigger the notification when done.


Like an Active solution, listeners register on the Model and await for notification; Differently from it, methods that modify the Model do not call Model.notify_listeners(). Instead, the Controller issues the call.

Practical example

A trivial example of Controller code using a Lazy Model notification strategy would behave as follows

class Controller():
    def perform_operation(self):
        model = self.model
        # None of these setters trigger notification.

        # Views are notified of the changes here.