Most ActionScript / Flash applications have a main event loop, triggered by Event.ENTER_FRAME. This is where the animation moves along to the next frame, or the sprites of the game are re-drawn in their new places.

In the Flex framework, you are expected to call invalidateDisplayList on the framework to say you need an update, and actually do the update when the framework calls your updateDisplayList method. This is the invalidation / validation pattern.

I went searching in the Flex code to understand how this invalidation / validation step ties in with Flash’s event model. I ignored properties and sizing, and edited the code down to the bare essentials.

Here is what happens when you change the label of a button: