Wednesday, March 14, 2012

## participants in a swing event ecosystem

Some interviewer once asked me to comment on the multi-threading issues in swing. I said that's easier to me than the chaos and entanglement passing objects between code modules. Half of that confusion relates to events.
Just to give you a taste. A lot of event listeners are inner classes so a callback method therein has handles on
* all (even private) fields of the enclosing class
* final fields of the objects pass to the listener ctor, which are implicitly saved as hidden fields of the listener INSTANCE,
* fields of the event source object (typically a jcomponent)
* fields of the event object -- PropertyChangeEvent for example

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In a typical [1] swing event data flow, there are 3+1 roles
- jcomponents
- listeners -- some can be jcomponents
- event objects
- event tasks
[1] in real life, the set-up is often complicated by thread, inner classes etc, so it's important to look at simple yet realistic examples.
Now, many of these roles are simple only conceptually -- actual object-graph is a bit complicated.
Jcomponent is the most Concrete thing among these mysterious creatures. Jcomponents can be an event source, and can optionally implement an listener interface such as ActionListener.java.
Event tasks, as described in another blog post, exists inside the JVM only, and not in java api. Each task points-to exactly one listener instance and one event object.
The listener role is tricky. In my experience, half the listeners are jcomponents, and half are non-jcomponents.
How about Action instance? I feel it is a glue-class for listener and jcomponent. See http://bigblog.tanbin.com/2009/10/abstractaction-brief-intro.html

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New York (Time Square), NY, United States
http://www.linkedin.com/in/tanbin