Jean-Francois,
The problem with holding listeners weakly is that people commonly create listener objects on the fly and then retain no other references to them once they've added them. So these listeners are thrown away on the next GC cycle, and they stop working.
One solution to this is to have a tagging interface (we call ours WeakListener) that tells the event source that it should hold the listener weakly. All listeners that don't implement this interface are held strongly. This way, you as the implementer of the listener are telling the event source whether to hold the listener weakly (meaning you better have a strong reference to it) or strongly (meaning you don't have to). As you say, it's pretty easy to write a listeners collection that honors this WeakListener interface and throws away the expired listeners when you ask for the current list of listeners. (We don't use an iterator, but rather a method that returns an array of listeners, for thread safety reasons.)
Luke
Jean-Francois Poilpret wrote:
...
To Howard:
During the night (French people say that the "night gives good advice";-)),
I thought of a quite easy workaround, that can be done at the event
supplying service level: just use a weak reference to the listeners (instead
of a strong one), I think that should do the trick.
Maybe it would even be possible to write a general "WeakEventListeners"
class to manage all that for the event supplier:
- when addListener, is called generate a weak reference to the added
listener and store it in the list
- create a specific Iterator for WeakEventListeners that would skip the refs
that have become weak-reachable (and remove these from its list at the same
time).
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