didBeginContact not called : this instance is unique to me - swift

Question

So, I am still experimenting with Sprite Kit for my first time ever, and I would like to test for collision. So, I searched around a bit in Apple's documentation, around Stack Overflow, online tutorials, and other forums. However, I was unable to find something a tip or code that makes what I am doing work. So, here are the relevant pieces of code:
This is the code for an obstacle:
func createObstacle(){
var ball = SKShapeNode(circleOfRadius: 20)
var width = UInt32(self.frame.width)
var random_number = arc4random_uniform(width)
ball.position = CGPointMake(CGFloat(random_number), frame.height+20)
ball.strokeColor = SKColor.blackColor()
ball.glowWidth = 1.0
ball.fillColor = SKColor.darkGrayColor()
ball.physicsBody = SKPhysicsBody(circleOfRadius: 20)
ball.physicsBody!.affectedByGravity = true
ball.physicsBody?.categoryBitMask = 6
ball.physicsBody?.dynamic = true
self.addChild(ball)
}
This is relevant code for the thing that it would collide with:
let circle = SKShapeNode(circleOfRadius: 20)
circle.physicsBody = SKPhysicsBody(circleOfRadius: 20)
circle.fillColor = SKColor.blueColor()
circle.strokeColor = SKColor.blueColor()
circle.glowWidth = 1.0
circle.physicsBody?.categoryBitMask = 4
circle.physicsBody?.dynamic = true
circle.physicsBody?.affectedByGravity = false
And this is the code for contact:
func didBeginContact(contact: SKPhysicsContact) {
var firstBody: SKPhysicsBody!
var secondBody: SKPhysicsBody!
if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
firstBody = contact.bodyA
secondBody = contact.bodyB
}
else {
firstBody = contact.bodyB
secondBody = contact.bodyA
}
if ((firstBody.categoryBitMask == 4 && secondBody.categoryBitMask == 6) || (firstBody.categoryBitMask == 6 && secondBody.categoryBitMask == 4)){
println("HI")
}else{println("NO")}
}
Sadly, nothing is being printed at all, so something's wrong. Any idea why this doesn't work?

Answer 1

Your class should have delegate SKPhysicsContactDelegate.
class GameScene: SKScene, SKPhysicsContactDelegate {
In didMoveToView write this:
physicsWorld.contactDelegate = self
EDIT
Define CategoryBitMask like this
struct PhysicsCategory {
static let circleCategory : UInt32 = 0b1 // 1
static let ballCategory : UInt32 = 0b10 // 2
}
Give CategoryBitMask to circle and ball
circle.physicsBody?.categoryBitMask = PhysicsCategory.circleCategory
ball.physicsBody?.categoryBitMask = PhysicsCategory.ballCategory
Then check contact like this:
(func didBeginContact(contact: SKPhysicsContact) {
if ((contact.bodyA.categoryBitMask == 0b1 && contact.bodyB.categoryBitMask == 0b10 ) || ( contact.bodyA.categoryBitMask == 0b1 && contact.BodyB.categoryBitMask == 0b1 ))
println("Contact")
}
}
Sorry for typos didnt used editor