Past exam of the computer science course of the University of Oxford / 2015 / Object oriented programming / 2 / d / Solution

In the Observer pattern, an observable subject keeps registered observers behind a shared object interface. After a relevant update, the subject calls that object interface on each observer. Observers can register and unregister independently, and the subject needs no knowledge of their concrete software classes.

For this model-view-controller application, DataList is the subject, while graph views are observers. After a successful insertion, a notification tells each registered view to redraw from the updated model. An invalid insertion leaves the model unchanged and needs no change notification. Calling observers only after restoring the representation invariant prevents views from seeing a half-completed insertion. Hence