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https://issues.apache.org/jira/browse/LANG-580?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12797257#action_12797257
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Oliver Heger commented on LANG-580:
-----------------------------------
I like the idea to add support for event listeners to [lang] because it can be
tricky to implement this in a thread-safe and performant way.
That said I am not sure how much benefit the proposed solution would actually
offer. IMO support for event listeners has two aspects: storing the registered
listeners (i.e. the implementation of the {{addListener()}} and
{{removeListener()}} methods, and firing events (i.e. {{fireXXX()}} methods).
The {{EventSupport}} interface and the {{AbstractEventSupport}} class mainly
focus on the former.
The JDK contains at least two classes I am aware of that are well suited for
managing event listeners:
* {{javax.swing.event.EventListenerList}} is used by Swing and is a bit more
complicated because it can deal with event listeners of multiple types.
* {{java.util.concurrent.CopyOnWriteArrayList}} is from its semantics a proper
choice for storing event listeners.
So {{AbstractEventSupport}} would probably be a thin wrapper over one of these
classes. Would this really provide benefit for users? If we could add some
support for the {{fire()}} methods, the answer would be a clear yes, but I
don't know how this can be achieved (it would probably be easier if Java had
support for closures).
> Add Event Support Utilities
> ---------------------------
>
> Key: LANG-580
> URL: https://issues.apache.org/jira/browse/LANG-580
> Project: Commons Lang
> Issue Type: New Feature
> Components: General
> Affects Versions: 3.0
> Environment: Java SE 5.0+
> Reporter: Michael Wooten
> Priority: Minor
> Fix For: 3.0
>
> Original Estimate: 96h
> Remaining Estimate: 96h
>
> I would like to propose some support be added to Lang for basic event
> handling. This would be based on the way that PropertyChangeSupport can be
> used to add and remove listeners and post events.
> Add interface EventSupport<L extends EventListener>
> addListener(L listener)
> The signature for the method that can add a listener of some subtype of
> EventListener
> removeListener(L listener)
> The signature for the method that can remove a listener of some subtype of
> EventListener
> Add class AbstractEventSupport implements EventSupport<L>, Iterable<L>
> AbstractEventSupport(Object eventSource)
> Constructs a new AbstractEventSupport object and associates it with the
> object that will be used as the source of all events (much like
> PropertyChangeSupport).
> addListener(L)
> An implementation that adds a listener to an internal collection.
> removeListener(L)
> An implementation that removes a listener from an internal collection.
> iterator()
> Returns an iterator over the attached listeners.
> getSource()
> Returns a reference to the source object of all events.
> The best way to describe this would be to demonstrate an example of how it
> can be used.
> public class ButtonPressedEventSupport extends
> AbstractEventSupport<ButtonPressedListener> {
> public ButtonPressedEventSupport(Object source) { super(source); }
> public void fireButtonPressed(Button button) {
> ButtonPressedEvent bpe = new ButtonPressedEvent(getSource(), button);
> for (ButtonPressedListener listener : this)
> {
> listener.buttonPressed(bpe);
> }
> }
> }
> public class MyWindow implements EventSupport<ButtonPressedListener> {
> private final ButtonPressedEventSupport buttonPressedEventSupport;
> public MyWindow { buttonPressedEventSupport = new
> ButtonPressedEventSupport(this); }
> public void addListener(ButtonPressedListener listener) {
> buttonPressedEventSupport.addListener(listener); }
> public void removeListener(ButtonPressedListener listener) {
> buttonPressedEventSupport.removeListener(listener); }
> ...
> private void onDetectButtonPressed(Button button) {
> buttonPressedEventSupport.fireButtonPressed(button);
> }
> }
> I haven't compiled the above code. It's just an example of how these classes
> could be used so that you're not constantly rewriting the code and interfaces
> for adding and removing listeners, and it provides a fairly easy method of
> creating methods to fire events.
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