# ForceEffectProtocol A protocol that defines a custom force effect. ``` protocol ForceEffectProtocol ``` ## Overview A custom force effect is a function of a set of input rigid body attributes and returns force-like vector quantities. You can declare this custom force effect’s input types ([`parameterTypes`](/documentation/RealityKit/ForceEffectProtocol/parameterTypes)) and output types ([`forceMode`](/documentation/RealityKit/ForceEffectProtocol/forceMode)) by conforming to `ForceEffectProtocol`. For example, you can declare a custom force effect that depends on rigid bodies’ position and mass, and computes acceleration for each rigid body. ```swift struct MyCustomForce : ForceEffectProtocol { var parameterTypes: PhysicsBodyParameterTypes { [.position, .mass] } var forceMode: ForceMode = .acceleration func update(parameters: inout ForceEffectParameters) { } } ``` Register your custom force effect to enable the physics system to compute forces affected by rigid bodies. ```swift MyCustomForce.register() ``` At each physics update, the [`update(parameters:)`](/documentation/RealityKit/ForceEffectProtocol/update(parameters:)) method receives the declared inputs from [`ForceEffectParameters`](/documentation/RealityKit/ForceEffectParameters). You can set the output forces for each rigid body with [`setForce(_:index:)`](/documentation/RealityKit/ForceEffectParameters/setForce(_:index:)). Sometimes, you may need to access properties from both your custom force effect and the current context while computing forces in the update function. The [`register(_:)`](/documentation/RealityKit/ForceEffectProtocol/register(_:)-1zt9t) method accepts an optional closure that allows you to capture the necessary properties. If you provide this closure to the register method, the update method is not required. ```swift struct MyCustomForceClosure: ForceEffectProtocol { var forceMode: RealityFoundation.ForceMode = .force var parameterTypes: PhysicsBodyParameterTypes { [.position, .mass] } let customProperty: Double = 1 } let contextualProperty: Double = 1 MyCustomForceClosure.register { event in // Access the effect property via `event.effect.customProperty`. // Access the input rigid body attributes via `event.parameters`. // Access the contextual property directly by `contextualProperty`. } ``` ## Topics ### Updating effects [`func update(parameters: inout ForceEffectParameters)`](/documentation/RealityKit/ForceEffectProtocol/update(parameters:)) Defines how the custom force effect computes forces at each physics simulation step. [`static func register(((inout ForceEffectEvent) -> Void)?)`](/documentation/RealityKit/ForceEffectProtocol/register(_:)-1zt9t) Registers the custom effect. [`struct PhysicsBodyParameterTypes`](/documentation/RealityKit/PhysicsBodyParameterTypes) Defines which rigid body inputs are required by a force effect’s update handler. [`struct ForceEffectParameters`](/documentation/RealityKit/ForceEffectParameters) The force effect input data to the effect’s update handler or closure. [`struct ForceEffectEvent`](/documentation/RealityKit/ForceEffectEvent) A struct that defines the arguments to the custom force effect update closure. [`struct UnsafeForceEffectBuffer`](/documentation/RealityKit/UnsafeForceEffectBuffer) Provides access to physics body parameters from the effect’s update function or event handler. ### Instance Properties [`var forceMode: ForceMode`](/documentation/RealityKit/ForceEffectProtocol/forceMode) The mode that controls how the physics system interprets the outputs from a user’s custom force computation. [`var parameterTypes: PhysicsBodyParameterTypes`](/documentation/RealityKit/ForceEffectProtocol/parameterTypes) The input types to user’s custom force computation. ### Type Methods [`static register(_:)`](/documentation/RealityKit/ForceEffectProtocol/register(_:)) Registers the custom effect. ## Relationships ### Conforming Types [`VortexForceEffect`](/documentation/RealityKit/VortexForceEffect) [`RadialForceEffect`](/documentation/RealityKit/RadialForceEffect) [`TurbulenceForceEffect`](/documentation/RealityKit/TurbulenceForceEffect) [`ConstantRadialForceEffect`](/documentation/RealityKit/ConstantRadialForceEffect) [`ConstantForceEffect`](/documentation/RealityKit/ConstantForceEffect) [`DragForceEffect`](/documentation/RealityKit/DragForceEffect) --- 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)