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

[Tomas Mikula]

There is no official API. You are free to copy-paste the code from the link
I posted to your project.


On Wed, Jan 22, 2014 at 1:59 PM, Randahl Fink Isaksen <randahl at rockit.dk>wrote:

> Hi Tomas
>
> Great idea! I can see how the InvalidationSubscriber idea would work – but
> it is just a concept right? There is no official API for this, right?
>
> Randahl
>
>
>
>
>
> On 22 Jan 2014, at 13:34, Tomas Mikula <tomas.mikula at gmail.com> wrote:
>
> Hi Randahl,
>
> I'm curious about an example where you would take advantage of the
> behavior where multiple addListener(listener) calls add the listener just
> once.
>
> Anyway, here [1] are helper classes InvalidationSubscriber and
> ChangeSubscriber that allow you to do that:
>
> InvalidationSubscriber subscriber = new InvalidationSubscriber(observable,
> listener);
>
> subscriber.subscribe(); // registers the listener
> subscriber.subscribe(); // no-op
>
> Cheers,
> Tomas
>
> [1] https://gist.github.com/TomasMikula/8557825
>
> On Wed, Jan 22, 2014 at 11:23 AM, Randahl Fink Isaksen <randahl at rockit.dk>wrote:
>
>> 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.
>>
>> Yours
>>
>> Randahl
>>
>>
>>
>>
>> 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
>>
>>
>
>