Slow down or pause NSAnimationContext - cocoa

Question

I have an NSAnimationContext (just a scrolling view) that I would like to slow down and/or pause whenever the cursor enters the view. I have already implemented the detection for when this happens - now I just need to figure out how to slow down the animation that is already in process. I have figured out how to do this with CALayers - but I need to use the animator proxy unable to use several AppKit views within this animation so Core Animation will not work. Does anyone know how to do this? Is there a way to keep track of NSAnimationContexts and then change them later on?
Here is a subsection of my code, The first block is cyclicly called. Everytime one agnation completes the next will begin.
[NSAnimationContext runAnimationGroup:^(NSAnimationContext *context){
context.duration = pixels/speed;
[[currentTweetView animator] setFrame:endRect];
} completionHandler:^{
[currentTweetView removeFromSuperview];
currentTweetView = nil;
[self nextAnimationWithAnimationIndex:currentIndex];
}];
Here is the code in the mouseEntered: method. Whenever this is called, neither completionHandler is ever called and the app freezes.
[NSAnimationContext runAnimationGroup:^(NSAnimationContext *context){
[[[self.subviews objectAtIndex:0] animator] setFrame:finalRect];
context.duration = 100.0;
} completionHandler:^{
NSLog(#"done");
}];
Also, is there any way to end an NSAnimationContext early and not call the completion handler?

Answer 1

I think if you just set the property via the animator proxy again, under a different NSAnimationContext, it will replace the animation that was in progress. This would be analogous to retargeting the animation (e.g. to a new destination).