= Solution
In <Scala Swing>, a `Publisher` is the observable subject and a `Reactor` supplies an observer's reactions. A reactor calls `listenTo(pounds, euros, rate)` to subscribe to the relevant publishing components. It adds a <partial function> from events to actions with `reactions += { case ... => ... }`; unmatched event types need no action. When a publisher calls `publish(e)`, subscribed reactions that handle that event are notified. Calling `deafTo` removes subscriptions that are no longer wanted.<Scala Swing event-system documentation>[README, About scala-swing: Publisher/Reactor, listenTo and reactions; archived Reactor API: deafTo]{ref}
The publisher does not need to know the concrete frame or its currency-conversion code. A `TextField` supplies the relevant editing events, and the frame's reaction distinguishes the source field using `EditDone(source)`. \b[Both registration and a matching reaction are needed]: installing a case handler without subscribing does not establish the notification path. The observer logic should update the conversion model once and then refresh the other fields from that coherent state.
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