Tuesday, March 13, 2012

each task instance in EventQueue has ptr to 1 listener

EventObject.java doesn't contain a "handle" [2] on the listener INSTANCE, but look at an event-queue-task ("qtask") in the Swing EventQueue.

EDT runs all callback methods (such as actionPerformed) sequentially [1]. So if a jtable and a jtree both listen to an event, EDT must execute the 2 callbacks sequentially.

Imagine 2 qtask INSTANCES are in EventQueue. It's instructional to imagine a qtask having 3 handles
- on that one listener object, sometimes a jcomponent. See http://docs.oracle.com/javase/tutorial/uiswing/events/actionlistener.html
- on the event object, which provides a handle
- on the source object, perhaps a button or table cell

Note in our simple example one source object (button) generates one event (click) at a time. Given the source maintains 2 pointers to 2 listeners, that one event object must be "delivered to" 2 listeners -- 1:2 mapping.

Now the English word "deliver-to" is nice but rather vague or (worse) misleading and can permanent damage some learner's brain. Listener is unlike some jms client process capable of processing event. Instead, Listener is typically
- a stateful object (perhaps a jcomponent) holding a handle on some RESOURCE -- DB, bunch of jcomponents..
- exposing a callback like actionPerformed()

So delivery-to means "EDT executing that non-static callback method on the listener instance, passing in the event object". This is a crucial and non-trivial data-flow worth a hard look and analysis
* Business logic is in ... the method source code;
* Data is passed via .... the event object and the event source
* Resources needed to handle the message are in ..... the listener object's non-static fields

[1] even though a callback method may dispatch the task to a worker thread or back to EDT again - by invokeLater() i.e. packing up the task into a qtask and "enqueue" to the end of the event queue.

[2] basically a reference or pointer, but it's good to use a generic term here.

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