 Solution

The Observer pattern lets interested observers subscribe through a common interface to a subject's events or state changes. The subject notifies registered observers, which perform their own reactions; subscriptions can later be removed. A document model can thereby update multiple editor views without containing explicit knowledge of each view class.

It avoids tightly coupling the subject to every concrete consumer. Adding another view does not require adding a new view-specific call inside core model logic. Notifications should expose a coherent updated state, and unused observers should unsubscribe. The pattern does not itself prevent feedback cycles: observers that update each other's sources can still cause repeated or reentrant notifications. The supplied buttons and containing frame use this subscription-and-reaction design through Scala Swing.