[8u] API Request: RT-25613, ObservableValue should have a hasListener(listener) method

Randahl Fink Isaksen randahl at rockit.dk
Wed Jan 22 02:23:30 PST 2014

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.sladecek at 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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