Determinate progress bar

[Alessandro Sangiuliano]

Hello, in my free time I do some progresses in the rick theme development

https://github.com/AlessandroSangiuliano/rik.theme

I'm working on the indeterminate progress bar, drawing one for the theme.

The real problem is the determinate progress bar, during the loading, 
the bar 'jumps'. I'd like to have an effect like OS X or Windows bars, 
their bars doesn't jump but fill the container in a fluid way as effect.
I think this effect is obtained using a kind of animation.
Do I should use NSAnimation or what?

Any kind advice is welcome, considering I have no experience on any kind 
of animations (OpenGL etc etc)

If you have any kind of examples, they are welcome too.

Regards,
Alex - Slex