= 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.
\b[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>.
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