# UIPushBehavior A behavior that applies a continuous or instantaneous force to one or more dynamic items, causing those items to change position accordingly. ``` @MainActor class UIPushBehavior ``` ## 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 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. The default magnitude of a push behavior’s force vector is `nil`, equivalent to no force. A continuous force vector with a magnitude of `1.0`, applied to a 100 point x 100 point view whose density value is `1.0`, results in view acceleration of 100 points / second² in the direction of the vector; this value is also known as the UIKit Newton. You express a push behavior’s force vector in terms of magnitude ([`magnitude`](/documentation/UIKit/UIPushBehavior/magnitude)) and radian angle ([`angle`](/documentation/UIKit/UIPushBehavior/angle)). Instead of using radian angle, you can equivalently express direction using *x* and *y* components by using the [`pushDirection`](/documentation/UIKit/UIPushBehavior/pushDirection) property. Whichever approach you use, the alternate, equivalent values update automatically. For each dynamic item that you associate with a push, the force is applied at the item center or at a specified offset from the center in item-relative coordinates. To use a push behavior with a dynamic item, perform these two steps: 1. Associate the item with the behavior using the [`addItem(_:)`](/documentation/UIKit/UIPushBehavior/addItem(_:)) method, or initialize a new push behavior with an array of items using the [`init(items:mode:)`](/documentation/UIKit/UIPushBehavior/init(items:mode:)) method 2. Enable the behavior by adding it to an animator using the [`addBehavior(_:)`](/documentation/UIKit/UIDynamicAnimator/addBehavior(_:)) method After enabling a push behavior, you can activate it and deactivate it using the [`active`](/documentation/UIKit/UIPushBehavior/active) property. The coordinate system that pertains to a push 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 include a push behavior in a custom, composite behavior by starting with a [`UIDynamicBehavior`](/documentation/UIKit/UIDynamicBehavior) object and adding a push behavior with the [`addChildBehavior(_:)`](/documentation/UIKit/UIDynamicBehavior/addChildBehavior(_:)) method. If you want to influence a push behavior at each step of a dynamic animation, implement the inherited [`action`](/documentation/UIKit/UIDynamicBehavior/action) method. ## Topics ### Initializing and managing a push behavior [`active`](/documentation/UIKit/UIPushBehavior/active) The state of the push behavior’s force: either active or inactive. [`- addItem:`](/documentation/UIKit/UIPushBehavior/addItem(_:)) Adds a dynamic item to the behavior’s dynamic item array. [`- initWithItems:mode:`](/documentation/UIKit/UIPushBehavior/init(items:mode:)) Initializes a push behavior with an array of dynamic items. [`- removeItem:`](/documentation/UIKit/UIPushBehavior/removeItem(_:)) Removes a specific dynamic item from the behavior. [`items`](/documentation/UIKit/UIPushBehavior/items) Returns the set of dynamic items you’ve added to the push behavior. ### Configuring a push behavior [`- setAngle:magnitude:`](/documentation/UIKit/UIPushBehavior/setAngle(_:magnitude:)) Sets the angle and magnitude of the force vector for the behavior. [`angle`](/documentation/UIKit/UIPushBehavior/angle) The angle, in radians, of the force vector for the behavior. [`magnitude`](/documentation/UIKit/UIPushBehavior/magnitude) The magnitude of the force vector for the push behavior. [`mode`](/documentation/UIKit/UIPushBehavior/mode-swift.property) Returns the force mode for the push behavior. [`- setTargetOffsetFromCenter:forItem:`](/documentation/UIKit/UIPushBehavior/setTargetOffsetFromCenter(_:for:)) Sets the offset, from the center of a dynamic item, at which to apply the push behavior’s force vector. [`- targetOffsetFromCenterForItem:`](/documentation/UIKit/UIPushBehavior/targetOffsetFromCenter(for:)) Returns the offset, from the center of a dynamic item, at which the push behavior’s force vector is applied. [`pushDirection`](/documentation/UIKit/UIPushBehavior/pushDirection) The direction of the force vector for the behavior, expressed as *x* and *y* components and using standard UIKit geometry. ### Constants [`Mode`](/documentation/UIKit/UIPushBehavior/Mode-swift.enum) The type of force for the push behavior. ## Relationships ### Conforms To [`CVarArg`](/documentation/Swift/CVarArg) [`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible) [`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible) [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol) [`Sendable`](/documentation/Swift/Sendable) [`Hashable`](/documentation/Swift/Hashable) [`Equatable`](/documentation/Swift/Equatable) ### Inherits From [`UIDynamicBehavior`](/documentation/UIKit/UIDynamicBehavior) --- Copyright © 2026 Apple Inc. 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