---
title: "customField(evaluationBlock:)"
framework: spritekit
role: symbol
role_heading: Type Method
path: "spritekit/skfieldnode/customfield(evaluationblock:)"
---

# customField(evaluationBlock:)

Creates a field node that calculates and applies a custom force to the physics body.

## Declaration

```swift
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

Return Value A new custom field node.

## Discussion

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 object’s region. Listing 1. Creating a custom drag field 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) }

## See Also

### Creating Field Nodes

- [dragField()](spritekit/skfieldnode/dragfield().md)
- [electricField()](spritekit/skfieldnode/electricfield().md)
- [linearGravityField(withVector:)](spritekit/skfieldnode/lineargravityfield(withvector:).md)
- [magneticField()](spritekit/skfieldnode/magneticfield().md)
- [noiseField(withSmoothness:animationSpeed:)](spritekit/skfieldnode/noisefield(withsmoothness:animationspeed:).md)
- [radialGravityField()](spritekit/skfieldnode/radialgravityfield().md)
- [springField()](spritekit/skfieldnode/springfield().md)
- [turbulenceField(withSmoothness:animationSpeed:)](spritekit/skfieldnode/turbulencefield(withsmoothness:animationspeed:).md)
- [velocityField(with:)](spritekit/skfieldnode/velocityfield(with:).md)
- [velocityField(withVector:)](spritekit/skfieldnode/velocityfield(withvector:).md)
- [vortexField()](spritekit/skfieldnode/vortexfield().md)
- [SKFieldForceEvaluator](spritekit/skfieldforceevaluator.md)