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

[Susan M Franklin <susan_franklin@xxxxxxxxxx>]

I just hit this issue as well while
working on
https://bugs.eclipse.org/bugs/show_bug.cgi?id=91343#c13

My plan was to use an iterator so that
the more specific exception would be thrown.

My opinion is that it until the runtime
implementation is API, it should be fixed on a case by case basis.  It's
too bad this got deferred at the last minute for 3.0.  Here we are
again near the end of a release...is it too late to get the runtime one
made API?  

susan

Douglas Pollock <douglas.pollock@xxxxxxxx> Sent by: platform-ui-dev-bounces@xxxxxxxxxxx 05/06/2005 11:43 AM Please respond to dpollock,"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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