Sunday, July 15, 2012

jcomponent / listener / event, another analysis

http://docs.oracle.com/javase/tutorial/uiswing/events/eventsandcomponents.html shows which (type of) listener can attach to which
(type of) jcomponent. It leaves out the 3rd vital element -- the (type of) event. The relationship of the 3 entities is central to
Swing.

An AWTEvent is an object containing data about some "happening". It also "contians" a source jcomponent via getSource(). Another
important content is the type of event -- mouse move, resizing, checkbox click...via getID(). An event usually comes from a
jcomponent -- both user actions and code actions generate events.

When generated, the event doesn't "know" the listeners -- similar to how a MOM sender generates an "unaddressed" message. Instead,
the "source" jcomponent keeps the (list of) listeners.

I sometimes prefer to discuss in terms of event-queue task (qtask) rather than events. A qtask is an event + a single listener
object. Now I feel this is educational but imprecise.

Recall a listener object is an initially stateless functor object + optional objects passed in to ctor + optional objects in the
enclosing class or enclosing method. The last group are more tricky.

Here's my hypothesis. When an event occurs,
Event object constructed
Event object euqueued
EDT picks up the event and invokes Component.processEvent(theEvent)
Inside processEvent(), each listener is invoked sequentially.

http://tech.stolsvik.com/2009/03/awt-swing-event-pumping-and-targeting.html described some of the details in a
MouseEvent.mouseClicked() example.

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