= Solution
A <Scala Swing> `Publisher` is the subject-side event source. Calling `publish(e)` notifies its subscribed reactors. A `Reactor` is the observer-side participant: `listenTo(p1,p2)` registers it with the specified sources, `deafTo` ends those subscriptions, and `reactions` contains the event-handling cases.
``
listenTo(pounds, euros, rate)
reactions += {
case EditDone(source) => handleCommittedEdit(source)
}
``
Here the containing frame supplies the reactor behavior, and each text field supplies publication. Defining `reactions` without `listenTo` is insufficient to establish the subscription. The pattern-matching cases can handle only events of interest while other events are ignored. This is the <Observer pattern> expressed through event publication rather than a separate hand-written `update` method for every subject.<Scala Swing event-system documentation>[Archived Reactor API: listenTo, deafTo and reactions; official library README: Publisher/Reactor event-system overview]{ref}
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