public void addListener( @Nonnull EntityCapabilitiesListener listener ) {
        allUserCapabilitiesListeners.add( listener );
    }
Registers an event listener that will be invoked when the detected entity capabilities for any entity
have changed.

If the listener already is registered, the invocation of this method will be a no-op.

@param listener The event listener to be invoked when entity capabilities have changed.
xmppserver/src/main/java/org/jivesoftware/openfire/entitycaps/EntityCapabilitiesManager.java