iPhone animation based on input values (touches) not time

Question

For an animation effect perfectly suited to an animation group approach as shown in Brad Larson\'s answer here, I need the animation to proceed according to inputs. Specific

Answer 1

You may be able to use the hierarchical nature of timelines in layer trees to achieve what you’re looking for. Objects implementing CAMediaTiming, which include both CAAnimation and CALayer, inherit a timespace from their parent, which they can modify (via scaling, shifting and repeating) and propagate to their children. By setting the speed property of a layer to 0.0 and adjusting the timeOffset property manually, you can decouple that layer (and all of its sublayers) from the usual notion of time.

What you’d do in your case is define the animations for all your menu items to animate their position along your desired CGPath from time t₀ to t₁, with the appropriate timeOffset on each animation to keep your items appropriately spaced. Note that a beginTime of 0.0 on an animation is usually interpreted as starting from the time the animation was added to its layer, so if you want t₀ to be 0.0, you'll probably have to set it to a tiny epsilon > 0.0.

CAKeyframeAnimation *animation = [CAKeyframeAnimation animationWithKeyPath:@"position"];
animation.beginTime = 1e-100;
animation.duration = 1.0;
animation.fillMode = kCAFillModeBoth;
animation.removedOnCompletion = NO;
animation.path = path;
animation.calculationMode = kCAAnimationPaced;
animation.timeOffset = timeOffset;

You’d then set the speed property to 0.0 on a parent layer (containing only these menu items) and update its timeOffset to values between t₀ and t₁ in response to your touch events.

This approach has two potential caveats. Because you’ve taken over the nature of time on this layer subtree, you probably won’t be able to animate other properties at the same time. Additionally, if you want coasting behavior for a fast flick, you’ll probably need to animate time forward on your own.