Make particle system only be affected by certain categories of physics fields in SceneKit? - swift

Question

In SceneKit, it is possible to make a SCNPhysicsBody be unaffected by a SCNPhysicsField by giving them categoryBitMasks that when compared using bitwise AND, result in a value of zero.
Bodies who's category bit mask produce a non zero value when compared with that of the field are still affected by it.
Is it possible to do the same for the particles in a SCNParticleSystem, so that the particles will be unaffected by a SCNPhysicsField (edit: and still have the particles affected by other physics fields), but certain physics bodies or even other particle systems are affected by it?
To clarify:
What I want: A SCNParticleSystem that is affected by a SCNPhysicsField (specifically a linear gravity field). This can be done by setting the particle system's affectedByPhysicsFields property to true. However, this will make the particle system affected by all physics fields in the scene. I have another field (also a linear gravity field) that I want to only affect a SCNPhysicsBody which I don't want to be affected by the particle systems's field.
As it stands both the particle system and the physics body will be affected by both fields. Unless I give them categoryBitMasks to tell them which fields to interact with. Except particle systems don't have categoryBitMasks, and if a physics field's categoryBitMask is anything other than the default, it will be ignored by any particles.
So I can set it so that one of the fields only affects the body by giving them the same category. But I can't do the same for the particles. Is there any way to accomplish this? (Using the world gravity as one of the fields doesn't work because it doesn't have a categoryBitMask either.)
I've actually figured out a workaround (for now) by just using the particle system's acceleration property instead of a physics field. But I curious to see if it actually is possible to do it using a physics field.
edit: As requested here is some sample code. This code results in a sphere affected by two physics fields, and a particle system affected by one physics field. The comments explain the problem.
let PhysicsCategorySphere = 1 << 0
let PhysicsCategoryParticles = 1 << 1
scene.physicsWorld.gravity = SCNVector3(x: 0, y: 0, z: 0)
let sphere = SCNNode(geometry: SCNSphere(radius: 1))
sphere.physicsBody = SCNPhysicsBody.dynamicBody()
sphere.physicsBody?.categoryBitMask = PhysicsCategorySphere
//only physics fields with categoryBitMasks that match the sphere's can affect it
scene.rootNode.addChildNode(sphere)
let field = SCNPhysicsField.linearGravityField()
field.strength = 9.8
field.direction = SCNVector3(x: 0, y: 1, z: 0)
field.categoryBitMask = PhysicsCategorySphere
//only things with categories that match the field's will be affected by it
let fieldNode = SCNNode()
fieldNode.physicsField = field
scene.rootNode.addChildNode(fieldNode)
let particleSystem = SCNParticleSystem(named: "particles", inDirectory: "")
particleSystem.affectedByPhysicsFields = true
//there is no way that I know of to give the particle system a physics category
let particleNode = SCNNode()
particleNode.addParticleSystem(particleSystem)
scene.rootNode.addChildNode(particleNode)
let particleField = SCNPhysicsField.linearGravityField()
particleField.strength = 20
particleField.direction = SCNVector3(x: 0, y: 0, z: 1)
//particleField.categoryBitMask = PhysicsCategoryParticles
//if the field is given a category, it will no longer affect the particles
//the default is to match all categories, thus the particle field will also affect the sphere
let particleFieldNode = SCNNode()
particleFieldNode.physicsField = particleField
scene.rootNode.addChildNode(particleFieldNode)

Answer 1

Ok, I don't know how I overlooked this, but the answer is right in the documentation for SCNPhysicsField's categoryBitMask property.
To determine whether a field affects the particles spawned by an
SCNParticleSystem object, SceneKit performs the same check using the
categoryBitMask property of the node containing the particle system.