Hi Martin

While I agree your proposed solution would work, I still don’t understand why 
JavaFX should keep on supporting duplicates in listener collections. Can anyone 
come up with just 1 example of an application that might be depending on having 
two listeners on the same Observable? E.g. this kind of code:

myObservable.addListener(myChangeListener); //add it
myObservable.addListener(myChangeListener); //add it again

In what kind of situation would this sort of code make any sense?

If we all feel confident that the presence of duplicates listeners is always an 
error, I warmly recommend changing the API to be duplicate free.



On 22 Jan 2014, at 11:07, Martin Sladecek <martin.slade...@oracle.com> wrote:

> Hi all,
> I would like to start discussion about an addition to API in Observable, 
> ObservableValue and all Observable collections.
> There were multiple requests for a way how to avoid duplicates in listeners 
> lists. The way RT-25613 solves this is that it introduces public boolean 
> hasListener(ListenerType listener) which would return true if the provided 
> listener is already registered.
> This has one significant drawback that all of Observable* are actually 
> interfaces. Means we can only add hasListener as a defender method. The 
> problem is with the default implementation. We cannot return anything 
> meaningful, so we have to throw an UnsupportedOperationException. The problem 
> is that this might blow up unexpectedly when some "older" Observable 
> implementation is used. Also, it might be easy to miss when implementing the 
> interface, since the IDE might not force you to implement it.
> So as an alternative solution, I propose adding something like:
> ensureListener(ListenerType listener)
> which would make sure the listener is on the list and if a listener is 
> already present, the number of times listener is registered on the Observable 
> will NOT grow after this call.
> The default implementation (for Observable) would look like this:
> public default void ensureListener(InvalidationListener listener) {
>    removeListener(listener);
>    addListener(listener);
> }
> subclasses might do something more effective. The same would apply to 
> ObservableValue and ChangeListener and Observable[List|Set|Map] and 
> [List|Set|Map]ChangeListener.
> What do you think?
> JIRA link: https://javafx-jira.kenai.com/browse/RT-25613
> -Martin

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