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

Martin Sladecek martin.sladecek at oracle.com
Wed Jan 22 02:50:47 PST 2014

On 01/22/2014 11:27 AM, Tom Schindl wrote:
> On 22.01.14 11:07, Martin Sladecek 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.
> Well this would destroy the order! I expect listeners to be called in
> the correct order not? Why doing a remove and not simply check if the
> listener has already been added?
> Tom

Because there's no way to do it in the interface, hence the problem with 
hasListener default implementation.

Yes, the order would be broken, but it's actually not guaranteed. 
Although FX internally uses a List, 3rd party implementations of 
Observable or ObservableValue might use a Set for listeners for example. 
This was the idea, but funnily enough, the current Observable javadoc is 
quite strict on the duplicates which would rule out the Set:

      * Adds an {@link InvalidationListener} which will be notified 
whenever the
      * {@code Observable} becomes invalid. If the same
      * listener is added more than once, then it will be notified more than
      * once. That is, no check is made to ensure uniqueness.

Personally, I would rather make this requirement less strict and allow 
both List backed and Set backed implementations.


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