choose what objects radial gravity fields affect swift

Question

I'm working on an Xcode project with swift and sprite kit, and I'm wondering how I can choose which objects are affected by radial gravity fields. I currently have two stars and some planets, and each star has a radial gravity field following it. The problem is that the stars are drawn to their own gravity fields. How can I make it so that each gravity field only affects one star and all the planets, but not the star it's following. I know that it has something to do with categoryBitMask and/or fieldBitMask, but I don't know exactly how. Thanks in advance. Below is the code for a star and a gravity field. I don't want that gravity field to affect that star.

class star: SKSpriteNode {

required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")

 }

init(imageNamed: String){
    let imageTexture = SKTexture(imageNamed: imageNamed)

    super.init(texture: imageTexture, color: nil, size: imageTexture.size())

    let radius = self.size.width / 2

    self.physicsBody = SKPhysicsBody(circleOfRadius: radius )
    physicsBody?.dynamic = false

}
}

    let star1 = star(imageNamed: "star")
    let gravityField1 = SKFieldNode.radialGravityField()

    star1.position = CGPoint(x: self.size.width / 4, y: self.size.height / 2)
    star1.physicsBody?.friction = 0
    star1.physicsBody?.linearDamping = 0
    star1.physicsBody?.angularDamping = 0
    star1.physicsBody?.angularVelocity = 0.2
    star1.zPosition = 1
    star1.name = "star"
    addChild(star1)


    gravityField1.enabled = true;
    gravityField1.position = CGPoint(x: self.size.width / 4, y: self.size.height / 2)
    gravityField1.strength = Float(pow(radius1, 2)) * pow(10, -3)

    addChild(gravityField1)

Answer 1

Here's an example of how to set the category and field bit masks:

Set the category bit masks for the gravity fields

gravityField1.categoryBitMask = gravityField1Category
gravityField2.categoryBitMask = gravityField2Category

Set bit masks such that each star is affected by the other star's gravity field but not its own

star1.physicsBody?.fieldBitMask = gravityField2Category
star2.physicsBody?.fieldBitMask = gravityField1Category

Set bit masks so that the planets are affected by both gravity fields

planet.physicsBody?.fieldBitMask = gravityField1Category | gravityField2Category