CAAnimation that calls a method in periodic animation-progress intervals?

Question

Say I want to animate a ball rolling 1000 pixels to the right, specifying a timing function in the process – something like this:

UIView *ball = [[UIView alloc] initWithFrame:CGRectMake(0,0,30,30)];
CABasicAnimation* anim =
        [CABasicAnimation animationWithKeyPath:@"transform.translation.x"];
anim.toValue = [NSNumber numberWithFloat:ball.frame.origin.x + 1000.0];
                                            // move 1000 pixels to the right
anim.duration = 10.0;
anim.timingFunction = [CAMediaTimingFunction functionWithControlPoints:
                      0.1 :0.0 :0.3 :1.0]; // accelerate fast, decelerate slowly
[ball.layer addAnimation:anim forKey:@"myMoveRightAnim"];

What I ultimately want is to have a method, say -(void)animationProgressCallback:(float)progress, be called during the animation, in regular intervals of the animation's progress in terms of the absolute "distance" between start and end values, i.e. ignoring the timing function.

I'll try to explain with the above example with the ball rolling 1000px to the right (charted by the y axis, in our case 100%=1000px):

I want my callback method to be invoked whenever the ball has progressed 250 pixels. Because of the timing function, the first 250 pixels might be reached in ti₀=2 seconds, half the total distance reached just ti₁= 0.7 seconds later (fast acceleration kicks in), the 750px mark another ti₂= 1.1 seconds later, and needing the remaining ti₃= 5.2 seconds to reach the 100% (1000px) mark.

What would be great, but isn't provided:

If the animation called a delegate method in animation-progress intervals as described, I wouldn't need to ask this question… ;-)

Ideas how to solve the problem:

One solution I can think of is to calculate the bezier curve's values, map that to the ti_k values (we know the total animation duration), and when the animation is started, we sequentially perform our animationProgresssCallback: selector with those delays manually.

Obviously, this is insane (calculating bezier curves manually??) and, more importantly, unreliable (we can't rely on the animation thread and the main thread to be in sync – or can we?).

Any ideas??

Looking forward to your ideas!

Answer 1

Ideas how to solve the problem:

One solution I can think of is to calculate the bezier curve's values, map that to the tik values (we know the total animation duration), and when the animation is started, we sequentially perform our animationProgresssCallback: selector with those delays manually.

Obviously, this is insane (calculating bezier curves manually??) and, more importantly, unreliable (we can't rely on the animation thread and the main thread to be in sync – or can we?).

Actually this is reliable. CoreAnimation is time based, so you could use the delegate to be notified when the animation really starts. And about calculating the bezier path... well look it this way: It could be worse if you would want to implement a surface in OpenGLES you would have to calculate a Cubic Bezier!. lol. Your case is only one dimension, is not that hard if you know the maths.