# customField(evaluationBlock:) Creates a field node that calculates and applies a custom force to the physics body. ``` class func customField(evaluationBlock block: @escaping SKFieldForceEvaluator) -> SKFieldNode ``` ## Parameters `block` A custom block to be executed when a physics body is affected by the field. Your block should calculate and return the force to be applied to the body. ## Return Value A new custom field node. ## Discussion The value returned by the custom block is a vector for an impulse force which is applied to the physics body being evaluated for that frame. Only the `x` and `y` components of the return value are used by SpriteKit, the `z` component is ignored. The values passed into the block by the `position` and `velocity` arguments measured in meters: if you need to convert them into points — as used by SpriteKit — multiply the values by 150. The following code shows how to create a custom field to emulate drag. The block returns the negative of the square root of the velocity of the physics body. This decelerates a physics body passing through the [`SKFieldNode`](/documentation/SpriteKit/SKFieldNode) object’s region. Listing 1. Creating a custom drag field ```swift let simpleDrag = SKFieldNode.customField { (position: vector_float3, velocity: vector_float3, mass: Float, charge: Float, deltaTime: TimeInterval) in return vector_float3(-sqrt(abs(velocity.x)) * sign(velocity.x), -sqrt(abs(velocity.y)) * sign(velocity.y), 0) } ``` --- 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)