Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Imperative programming (resit) / 8 / c

A widget or model implementing Publisher is the subject. It calls publish(event) when an observable change occurs. An interested Reactor subscribes with listenTo(subject), and adds a partial function to reactions that handles the event types it understands. deafTo(subject) ends that subscription when it is no longer needed.

The connection is therefore made through publisher and reactor interfaces rather than through a hard-coded list of particular application classes. A reactor may observe several publishers and inspect an event's source to distinguish them. The implementation's registration machinery delivers published events to interested reactions; registering a reaction alone is not a substitute for subscribing to the source.^[ref]

For the converter, the three text fields are publishers and the frame is a reactor. Register the three fields once, then handle their edit-completion events to update the stored numeric state and refresh the appropriate displayed values.