iOS 7 introduced UI Dynamics, which provides physics-based animation for UI elements. The overall coordination of UI Dynamics is performed by a UIKit.UIDynamicAnimator, which holds references to a number of UIKit.UIDynamicBehaviors, which define behaviors such as gravity, pushing, and collision detection. Each UIKit.UIDynamicBehavior holds references to some amount of UIKit.IUIDynamicItems, which represent the objects to which the physics model is applied.

Two classes that implement UIKit.IUIDynamicItem are UIKit.UIView and UIKit.UICollectionViewLayoutAttributes.