Wednesday, June 17, 2009

state of data model vs state of a visual component

Say myTable has a table model encapsulating a "vector3" holding 3 beans for 3 data rows. The actual visual image on screen is a native/physical/screen object outside JVM. Since objects all occupy memory and have individual states, the visual object and vector3 both hold data for the 3 beans.

Q: are there 2 copies of each bean?
%%A: I think so. The visual object is tightly guarded by kernel[1] since it's a shared hardware resource. i guess No application is allowed to directly modify visual objects. Java Application can freely modify a java vector, but vector3 object has to be distinct from the visual object. Therefore, for each attribute[1] of the beans in vector3, there's a copy of it in the visual object.

Q: Similarly, when a user types a letter "K" into a cell in myTable, it's probably saved in the visual object's memory (otherwise it will be lost in a clock cycle), but is there another distinct java object inside vector3, to be updated to "K"? The keyboard input generates an event encapsulating the text. Probably a copy of the text. But how about the bean in vector3? Is it directly modified by the keyboard input?

%%A: I would say no. It's extremely rare to have a variable directly updatable by hardware. In C It's known as __volatile__ variable. I think it's more common to have a java thread [4] updating vector3. Code running in that thread is regular java code. In contrast The visual object is tightly guarded and owned by the OS[3], not directly exposed to applications.

In a jtable, I confirmed that a special thread[4] updates vector3 (real data structure of the table model), then fires an event to the listeners.

Q: But what if a MOM thread also updates vector3?
%%A: veterans told me we need locks as "guard" over vector3. When users are editing a jtable, MOM thread should not touch it.

[1] Each non-primitive attribute has to be represented or serialized as primitive int/float/char, for it to be usable in the visual.
[3] Remember OS/kernel was created as a chokepoint access layer over shared hardware
[4] my test shows it's EDT

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