Framerates in Flash/Flex apps

In MXML-based apps, you set the target f开发者_高级运维ramerate for the app and I believe this is a core part of Flash as well. Two questions...

1. In many games you want the game to run as fast as it can for graphical smoothness, with some upper cap like 50-100Hz. How can you have variable framerates in Flash, or is it really not how things work?
2. What happens when an app cannot run at the target framerate? Do updates 'stack up' leading to other problems, or does Flash discard them?

I've a simple case where I'm moving a sprite from left to right at 100px/s, at 20fps it looks un-smooth. It's not jerky or anything, but you can clearly see the stepped motion, the artwork is black/white which accentuates it I think. I reckon a higher FPS is needed ideally, but on slower systems it might be too much and I don't want to run into nasty issues where I try to drive it too fast.

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If the ActionScript takes too much time and the player can't maintain the frame rate that was specified, the frame rate just drops. There's nothing to stack up, you just get frames generated less often. For this reason when frame rate is important it's critical to make sure the AS code is not taking more time than you have available given the desired frame rate. Also make sure all calculates for movement are based on time and not frames.

As far as animation as 20fps, yeah, it will not look smooth. Bump up the frame rate. :-)

http://www.morearty.com/blog/2006/07/17/flex-tip-a-higher-frame-rate-even-makes-text-entry-look-better/

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If Flash is unable to execute at the desired framerate, it will begin dropping frames. You can read the details of this here.

If you are dealing with framerates in Flash a lot, it can be helpful to understand the 'elastic racetrack' model that Flash uses. You can see details about it here, but the basic idea is that the amount of time spend executing code or rendering a frame can vary on a frame by frame basis.