swing.Publisher calls PartialFunction listeners for undefined values

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Publisher has a list of listeners of the Reaction type (ie PartialFunction[Event, Unit]), but publishes events to all listeners, even if the Reaction is not defined for that value:

https://github.com/scala/scala/blob/6ff3c3fd/src/swing/scala/swing/Publisher.scala#L47

A Coursera thread with people affected by this issue:

https://class.coursera.org/reactive-001/forum/thread?thread_id=1315

will pull request shortly...

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Imported From: https://issues.scala-lang.org/browse/SI-8019?orig=1
Reporter: @rtyley
Affected Versions: 2.10.3

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@rtyley said:
Pull Request submitted with scala/scala#3205

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@Sciss said:
This is a possibly expensive fix. If you look at https://github.com/rtyley/scala/blob/4ef949da930c2797e46fb206ea541da54811d9ba/src/swing/scala/swing/Reactions.scala#L23 you see that Reactions.Impl's apply again needs to call isDefined for each registered function, so now with standard reactions isDefined will be called twice. That's probably why publish took the shortcut.

The better solution IMO would be to make subscribe package private.

An immediate improvement could be to check the listener for Reactions in publish, and then shortcutting to apply.