# UIDynamicBehavior An object that confers a behavioral configuration on one or more dynamic items, for their participation in 2D animation. ``` @MainActor class UIDynamicBehavior ``` ## Overview A *dynamic item* is any iOS or custom object that conforms to the [`UIDynamicItem`](/documentation/UIKit/UIDynamicItem) protocol. The [`UIView`](/documentation/UIKit/UIView) and [`UICollectionViewLayoutAttributes`](/documentation/UIKit/UICollectionViewLayoutAttributes) classes implement this protocol starting in iOS 7.0. You can implement this protocol to use a dynamic animator with custom objects for such purposes as reacting to rotation or position changes computed by an animator—an instance of the [`UIDynamicAnimator`](/documentation/UIKit/UIDynamicAnimator) class. This parent class, [`UIDynamicBehavior`](/documentation/UIKit/UIDynamicBehavior), is inherited by the primitive dynamic behavior classes [`UIAttachmentBehavior`](/documentation/UIKit/UIAttachmentBehavior), [`UICollisionBehavior`](/documentation/UIKit/UICollisionBehavior), [`UIGravityBehavior`](/documentation/UIKit/UIGravityBehavior), [`UIDynamicItemBehavior`](/documentation/UIKit/UIDynamicItemBehavior), [`UIPushBehavior`](/documentation/UIKit/UIPushBehavior), and [`UISnapBehavior`](/documentation/UIKit/UISnapBehavior). You can subclass [`UIDynamicBehavior`](/documentation/UIKit/UIDynamicBehavior). By using the [`addChildBehavior(_:)`](/documentation/UIKit/UIDynamicBehavior/addChildBehavior(_:)) method in an instance of this class or in a custom subclass, you can create composite behaviors of your own design. When you subclass [`UIDynamicBehavior`](/documentation/UIKit/UIDynamicBehavior), you typically need to provide one or more initializers, along with other housekeeping methods such as those implemented in the iOS primitive dynamic behaviors. To perform per-step logic in a dynamic animation, provide a block object using the [`action`](/documentation/UIKit/UIDynamicBehavior/action) property. To access the dynamic animator that a dynamic behavior is associated with, use the [`dynamicAnimator`](/documentation/UIKit/UIDynamicBehavior/dynamicAnimator) property. To respond to a dynamic behavior being added to or removed from a dynamic animator, implement the [`willMove(to:)`](/documentation/UIKit/UIDynamicBehavior/willMove(to:)) method. ## Topics ### Configuring a dynamic behavior [`action`](/documentation/UIKit/UIDynamicBehavior/action) The block you want to execute during dynamic animation. [`- addChildBehavior:`](/documentation/UIKit/UIDynamicBehavior/addChildBehavior(_:)) Adds a dynamic behavior, as a child, to a custom dynamic behavior. [`childBehaviors`](/documentation/UIKit/UIDynamicBehavior/childBehaviors) Returns the array of dynamic behaviors that are children of a custom dynamic behavior. [`- removeChildBehavior:`](/documentation/UIKit/UIDynamicBehavior/removeChildBehavior(_:)) Removes a child dynamic behavior from a custom dynamic behavior. ### Responding to changes in the behavior tree [`dynamicAnimator`](/documentation/UIKit/UIDynamicBehavior/dynamicAnimator) The dynamic animator that the dynamic behavior is associated with. [`- willMoveToAnimator:`](/documentation/UIKit/UIDynamicBehavior/willMove(to:)) Called when the dynamic behavior is added to, or removed from, a dynamic animator. ## Relationships ### Conforms To [`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol) [`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible) [`Equatable`](/documentation/Swift/Equatable) [`CVarArg`](/documentation/Swift/CVarArg) [`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible) [`Sendable`](/documentation/Swift/Sendable) [`Hashable`](/documentation/Swift/Hashable) ### Inherited By [`UIAttachmentBehavior`](/documentation/UIKit/UIAttachmentBehavior) [`UISnapBehavior`](/documentation/UIKit/UISnapBehavior) [`UIGravityBehavior`](/documentation/UIKit/UIGravityBehavior) [`UIPushBehavior`](/documentation/UIKit/UIPushBehavior) [`UIFieldBehavior`](/documentation/UIKit/UIFieldBehavior) [`UIDynamicItemBehavior`](/documentation/UIKit/UIDynamicItemBehavior) [`UICollisionBehavior`](/documentation/UIKit/UICollisionBehavior) ### Inherits From [`NSObject-swift.class`](/documentation/ObjectiveC/NSObject-swift.class) --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)