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) {

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


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