iOS - How to make an animation track touches

Question

What is the best way to implement a smooth reversing animation which tracks touches? I am referring to those animations in which, for example, if the user executes a swipe

Answer 1

What I don't grasp - and I'm comfortable using CAAnimations and gestures - is how something can be both animated and interactive.

It is because, having set up an animation, you can set that animation to any "frame" you wish. Thus you can track the animation in correspondence to the movement of a gesture.

The way this works is that an animation is a feature of the render tree, belonging to a CALayer. CALayer implements the CAMediaTiming protocol. The timeOffset of a CALayer thus determines what "frame" of an animation that layer displays. If a complex animation involves many different layers, no problem; just set the timeOffset of their mutual superlayer, to control the frame of the entire animation.

That in fact is exactly how the new iOS 7 interactive custom transition feature works (to which you rightly refer in your question). For instance, in this example code:

https://github.com/mattneub/Programming-iOS-Book-Examples/blob/master/iOS7bookExamples/bk2ch06p296customAnimation2/ch19p620customAnimation1/AppDelegate.m

... I keep updating the UIPercentDrivenInteractiveTransition to tell it how far through the gesture the user is, and the animation therefore tracks the gesture. Now ask yourself: how the heck is this possible???

Well, the UIPercentDrivenInteractiveTransition, in turn, behind the scenes, keeps adjusting the layer's timeOffset to portray the animation at the corresponding frame. (You can actually add logging code to my example to see that this is true.)

Moreover, when I end the gesture at an incomplete point, the animation either hurries to its end or runs backwards to its beginning - again, this is because of the CAMediaTiming protocol, which lets you change the speed of the animation, including a negative value to run it backwards.