Re: [platform-ui-dev] Design Question About Listeners & Event Dispatchin

[John Arthorne <John_Arthorne@xxxxxxxxxx>]

You should absolutely be guarding against
this.  A common case is that a listener receives an event, and then
decides it no longer needs to listen and removes itself.  The JFace
ListenerList class handles this case, but is still not completely thread-safe.
There is a thread-safe ListenerList in core, but it's not API (it probably
should be): org.eclipse.core.internal.runtime.ListenerList.
 Here are some discussions from the last time this topic came up:

https://bugs.eclipse.org/bugs/show_bug.cgi?id=55265
https://bugs.eclipse.org/bugs/show_bug.cgi?id=63271

Douglas Pollock <douglas.pollock@xxxxxxxx>
Sent by: platform-ui-dev-bounces@xxxxxxxxxxx

06/05/2005 02:43 PM

Please respond to
dpollock and "Eclipse Platform UI component developers list."

To	platform-ui-dev@xxxxxxxxxxx
cc
Subject	[platform-ui-dev] Design Question About Listeners & Event        Dispatching

My question is whether it should be expected that
the list of listeners can be 
modified while executing a listener.  Should we always design for
this case?  
In other words, should it be possible for a listener to add or remove a
listener while handling an event?


There is a lot of existing code of the following form, or similar but using
iterators.  In the case of iterators, a ConcurrentModificationException
will 
be thrown.  In the case of indexed access, subtle bugs can exist where
listeners are skipped or handled twice.

        if (activityListeners != null) {
            for (int i = 0; i < activityListeners.size();
i++) {
                ((IActivityListener)
activityListeners.get(i))
                    
   .activityChanged(activityEvent);
                
    }
                
}

If we write defensively, then we need to do a copy of some sort to avoid
corrupting the collection.

                
int commandListenersSize = commandListeners.size();
        if ((commandListeners != null)
                && (commandListenersSize
> 0)) {
            final ICommandListener[] listeners
= (ICommandListener[]) 
commandListeners
                    .toArray(new
ICommandListener[commandListenersSize]);
            for (int i = 0; i < commandListenersSize;
i++) {
                final ICommandListener
listener = listeners[i];
                listener.commandChanged(commandEvent);
            }
        }


Should we expend the effort to hunt down the places where we don't defend
against this case?  Or fix them on a case-by-case basis?



cheers,
d.
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