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I have a game in which two balls should be able to pass through a third object, which functions as a finish line. The two balls are SKShapeNodes, the finish 'line' is an SKSpriteNode. All of them have SKPhysicBodys attached to them with categories set as follows:

struct PhysicsCategory {

    static let ball: UInt32 = 0x1 << 1

    static let line: UInt32 = 0x1 << 2

}





ball.physicsBody = SKPhysicsBody(circleOfRadius: ballSize)

ball.physicsBody?.categoryBitMask = PhysicsCategory.ball

ball.physicsBody?.contactTestBitMask = PhysicsCategory.line

ball.physicsBody!.collisionBitMask = 1



line.physicsBody = SKPhysicsBody(rectangleOf: line.size)

line.physicsBody?.isDynamic = false

line.physicsBody?.categoryBitMask = PhysicsCategory.line

line.physicsBody?.contactTestBitMask = PhysicsCategory.ball

line.physicsBody!.collisionBitMask = 1

Since I want the balls to be able to cross the finish line without collision, while still being able to get notified about this, I have set the collisionBitMask for both the balls and line to the same value.

This gives me the result I'm looking for. However, this seems to have as a side-effect that the two balls are able to pass through each other as well, which shouldn't happen.

Also, if I set the value of collisionBitMask to anything other than 1, the balls and line bump of each other. I have read through Apple's documentation on this, but I think I misunderstood / missed something here.

My guess is that since both balls see each other as having the same collisionBitMask. the overlap happening is to be expected. What I don't get is how I can avoid this, without having to add a separate category per ball.

Can anyone point me in the right direction? How can I make the balls pass through the finish line, while not being able to overlap each other?

I tend to get this mixed up from time to time, but hopefully I am recalling this correctly.

If all you care about is getting the collision call back on the line that is primarily controlled by contactTestBitMask to ball as you have. That is what sends the contact to the delegate method. collisionBitMask controls what actually controls physic calculations ie what will bounce off each other.

By default collisionBitMask will collide with everything so you want to specify only the things you want it to calculate physics on. You have everything set to 1 which isn't anything specific in your scene. This is why everything passes through each other. I believe 2 would be ball and 4 is line with what you provided (if you print out PhysicsCategory.ball and PhysicsCategory.line that is what it logs out). Typically is is best practice to use the enums or structs you create instead of specifying a number. With that said I tend to use 0 for nothing.

To fix this you want to tell your balls that you want them to collide with PhysicsCategory.ball and you want your line to collide with nothing so you want to set it to 0.

struct PhysicsCategory {

    static let ball: UInt32 = 0x1 << 1

    static let line: UInt32 = 0x1 << 2

}





ball.physicsBody = SKPhysicsBody(circleOfRadius: ballSize)

ball.physicsBody?.categoryBitMask = PhysicsCategory.ball

ball.physicsBody?.contactTestBitMask = PhysicsCategory.line

ball.physicsBody!.collisionBitMask = PhysicsCategory.ball //changed to ball



line.physicsBody = SKPhysicsBody(rectangleOf: line.size)

line.physicsBody?.isDynamic = false

line.physicsBody?.categoryBitMask = PhysicsCategory.line

line.physicsBody?.contactTestBitMask = PhysicsCategory.ball

line.physicsBody!.collisionBitMask = 0 //you want no physics calculations