I was looking at WWDC Video Advanced Techniques with UIKit Dynamics. I tried to go through the explanation how to create a customized transition using UIDynamics. When the UIGravityBehavior will be created, they also use a method I can't access (second line):
[self.gravityBehavior = [UIGravityBehavior alloc] initWithItems:@[myView]];
[self.gravityBehavior setXComponent:0.0 yComponent:3.0];
I also found a Github project where you can see the runtime Headers of UIGravityBehavior. There the method is available. I don't understand why the method is called inside the video. Is this just an explanation what happens when the app is running or do I have to implement a specific delegate to get access to it?