# UICollisionBehavior An object that confers to a specified array of dynamic items the ability to engage in collisions with each other and with the behavior’s specified boundaries. ``` @MainActor class UICollisionBehavior ``` ## Overview A collision behavior also specifies some characteristics of its items’ collisions, with other characteristics optionally specified by a [`UIDynamicItemBehavior`](/documentation/UIKit/UIDynamicItemBehavior) object. 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 use a custom object as a dynamic item for such purposes as reacting to rotation or position changes computed by a dynamic animator—an instance of the [`UIDynamicAnimator`](/documentation/UIKit/UIDynamicAnimator) class. To use a collision behavior with a dynamic item, perform these two steps: 1. Associate the item with the behavior using the [`addItem(_:)`](/documentation/UIKit/UICollisionBehavior/addItem(_:)) method, or initialize a new collision behavior with an array of items using the [`init(items:)`](/documentation/UIKit/UICollisionBehavior/init(items:)) method. 2. Enable the behavior by adding it to an animator using the [`addBehavior(_:)`](/documentation/UIKit/UIDynamicAnimator/addBehavior(_:)) method The coordinate system that pertains to a collision behavior, and the types of dynamic items you can use with the behavior, depend on how you initialized the associated animator. For details, read the Overview of [`UIDynamicAnimator`](/documentation/UIKit/UIDynamicAnimator). You can add multiple collision behaviors to a dynamic animator. A dynamic item can be part of any number of collision behaviors, provided those behaviors belong to the same animator. For example, you can specify a collision behavior for a set of say, blue, items and another for, say, pink items. When you add both behaviors to a dynamic animator, blue items can collide with each other and pink items can collide with each other, but a blue item and a pink item would not collide—they would ignore each other. By default, a collision behavior’s items can collide with each other *and* with any boundaries you’ve specified for the behavior. If you want to specify that a behavior’s items collide only with each other, or only with boundaries, explicitly set the [`collisionMode`](/documentation/UIKit/UICollisionBehavior/collisionMode) property. You can define a collision boundary with a bezier path (see the [`addBoundary(withIdentifier:for:)`](/documentation/UIKit/UICollisionBehavior/addBoundary(withIdentifier:for:)) method) or with a line segment (see the [`addBoundary(withIdentifier:from:to:)`](/documentation/UIKit/UICollisionBehavior/addBoundary(withIdentifier:from:to:)) method). When you use a collision behavior with a dynamic animator you’ve initialized with a reference view or a collection view layout, you can also specify a collision boundary according to the bounds of the dynamic animator’s coordinate system (see the [`setTranslatesReferenceBoundsIntoBoundary(with:)`](/documentation/UIKit/UICollisionBehavior/setTranslatesReferenceBoundsIntoBoundary(with:)) method). > Important: > When setting the initial position for a dynamic item, you must ensure that its bounds do not intersect any collision boundaries. The animation behavior for such a misplaced item is undefined. To respond to collisions, implement a delegate object that adopts the [`UICollisionBehaviorDelegate`](/documentation/UIKit/UICollisionBehaviorDelegate) protocol. Add the delegate to the behavior using the [`collisionDelegate`](/documentation/UIKit/UICollisionBehavior/collisionDelegate) property. You can include a collision behavior in a custom, composite behavior by starting with a [`UIDynamicBehavior`](/documentation/UIKit/UIDynamicBehavior) object and adding a collision behavior with the [`addChildBehavior(_:)`](/documentation/UIKit/UIDynamicBehavior/addChildBehavior(_:)) method. If you want to influence a collision behavior at each step of a dynamic animation, implement the inherited [`action`](/documentation/UIKit/UIDynamicBehavior/action) method. ## Topics ### Initializing and managing a collision behavior [`- addItem:`](/documentation/UIKit/UICollisionBehavior/addItem(_:)) Adds a dynamic item to the collision behavior’s item array. [`- initWithItems:`](/documentation/UIKit/UICollisionBehavior/init(items:)) Initializes a collision behavior with an array of dynamic items. [`- removeItem:`](/documentation/UIKit/UICollisionBehavior/removeItem(_:)) Removes a specific dynamic item from the collision behavior. [`items`](/documentation/UIKit/UICollisionBehavior/items) Returns the set of dynamic items you’ve added to the collision behavior. ### Customizing the collision behavior [`collisionDelegate`](/documentation/UIKit/UICollisionBehavior/collisionDelegate) The delegate object that you want to respond to collisions for the collision behavior. [`UICollisionBehaviorDelegate`](/documentation/UIKit/UICollisionBehaviorDelegate) To respond to UIKit dynamic item collisions, configure a custom class to adopt the [`UICollisionBehaviorDelegate`](/documentation/UIKit/UICollisionBehaviorDelegate) protocol. Then, in a collision behavior (an instance of the [`UICollisionBehavior`](/documentation/UIKit/UICollisionBehavior) class), set the delegate to be an instance of your custom class. ### Configuring a collision behavior [`- addBoundaryWithIdentifier:forPath:`](/documentation/UIKit/UICollisionBehavior/addBoundary(withIdentifier:for:)) Adds a collision boundary, specified as a Bezier path, to the collision behavior. [`- addBoundaryWithIdentifier:fromPoint:toPoint:`](/documentation/UIKit/UICollisionBehavior/addBoundary(withIdentifier:from:to:)) Adds a collision boundary, specified as a line segment, to the collision behavior. [`boundaryIdentifiers`](/documentation/UIKit/UICollisionBehavior/boundaryIdentifiers) The set of boundary identifiers that you’ve added to the collision behavior. [`- boundaryWithIdentifier:`](/documentation/UIKit/UICollisionBehavior/boundary(withIdentifier:)) Returns a specified Bezier-path boundary. [`collisionMode`](/documentation/UIKit/UICollisionBehavior/collisionMode) The type of edges that participate in collisions for the collision behavior. [`- removeAllBoundaries`](/documentation/UIKit/UICollisionBehavior/removeAllBoundaries()) Removes all previously-specified collision boundaries from the collision behavior. [`- removeBoundaryWithIdentifier:`](/documentation/UIKit/UICollisionBehavior/removeBoundary(withIdentifier:)) Removes a specific collision boundary from the collision behavior. [`- setTranslatesReferenceBoundsIntoBoundaryWithInsets:`](/documentation/UIKit/UICollisionBehavior/setTranslatesReferenceBoundsIntoBoundary(with:)) Specifies a collision boundary based on the bounds of the animation reference system, with optional insets. [`translatesReferenceBoundsIntoBoundary`](/documentation/UIKit/UICollisionBehavior/translatesReferenceBoundsIntoBoundary) Specifies whether a boundary based on the reference system is active. ### Constants [`Mode`](/documentation/UIKit/UICollisionBehavior/Mode) The types of edges that participate in collisions for a collision behavior. ## Relationships ### Conforms To [`Hashable`](/documentation/Swift/Hashable) [`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible) [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol) [`Sendable`](/documentation/Swift/Sendable) [`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible) [`Equatable`](/documentation/Swift/Equatable) [`CVarArg`](/documentation/Swift/CVarArg) ### Inherits From [`UIDynamicBehavior`](/documentation/UIKit/UIDynamicBehavior) --- Copyright © 2026 Apple Inc. 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