I had already done some keyframe animation code in java for Swing 'n Strike. I decided to migrate some of the code. The keyframe animation classes consisted mainly of two classes, a Keyframe class and a KeframesCollection class. Each instance of a Keyframe contains information about the position, transparency, frame number/time of the keyframe, what kind of interpolation to use with the next keyframe (linear,easing in/out, etc). The interpolation method I had in java, used to give back an instance of a Keyframe containing the interpolated values. But with Objective-C (actually, because of C), now I could keep a reference to a primitive value, e.g. any int, any unsigned char, any float, or any point... and the interpolate method would directly manipulate that reference. Awesome!
It's quite powerful and easy to use. Just create a keyframes collection. Give an address to what it will be modifying, give it some keyframes, and in the update method, we play the keyframe animation.