Question

I want to do animation in Java. I have looked at: http://docs.oracle.com/javase/tutorial/uiswing/misc/timer.html

What I’m surprised by is that in the timer action handler:

• we do NOT do actually drawing function calls; we just update the state — so it appears the timer action is used for “simulation”
• we do a repaint request
• Swing repaints its request at its convenience

This seems somewhat odd to me because:

• Swing can collate multiple repaint requests into one
• if I’m counting fps in the timer, I can be over counting (since multiple repaints becomes one)

Question: am I mis reading the sample code, or is this the “correct” way to do animation in Java?

Thanks!

Answer 1

It’s fairly normal to do the state updates and rendering separately.

Java/Swing is pretty normal in this regard.

Reasons:

• Lots of things can update the state (timer events, user input etc.) so it’s useful to make changes very lightweight and decouple them from screen refreshes so that you can do them at any time
• You often want screen refreshes to run at a different rate from the model state updates. Typically you want a fixed interval for animation / state updates but want the screen refreshes to happen as fast as they can. This is especially true if you are doing interpolation in your animations.
• Rendering often needs to take place in a specific context (in this case, the Swing UI thread). You don’t necessaily want to limit state changes to also only happen in this context
• It’s good practice to separate view (screen presentation) from the data model. Apart from being a good conceptual design distinction, this allows you do things like test the data model independently from the display, run a version of the model on a server without a GUI etc.