didBeginContact not being invoked - ios

Question

I have 2 SKSpriteNodes whose contact needs to be detected. I've tried various methods and looked up a lot of stuff but can't seem to get an answer. Below is my code. controlCircle is a class level variable because it needs to be used in other methods. The objects are generated fine.
class GameScene: SKScene, SKPhysicsContactDelegate {
var controlCircle = SKSpriteNode()
var mainCategory : UInt32 = 1 << 0
var dropCategory: UInt32 = 1 << 1
override func didMoveToView(view: SKView) {
self.physicsWorld.contactDelegate = self
self.physicsWorld.gravity = CGVectorMake(0,0)
//CREATING FIRST OBJECT AND ADDING PHYSICS BODY
var mainRadius = 20.0;
var controlCircle = SKSpriteNode(color: UIColor.clearColor(), size:CGSizeMake(mainRadius * 2, mainRadius * 2))
var circleBody = SKPhysicsBody(circleOfRadius: mainRadius)
circleBody.dynamic = false
circleBody.usesPreciseCollisionDetection = true
controlCircle.physicsBody = circleBody
var bodyPath = CGPathCreateWithEllipseInRect(CGRectMake((controlCircle.size.width/2), controlCircle.size.height/2, controlCircle.size.width, controlCircle.size.width),
nil)
var circleShape = SKShapeNode()
circleShape.fillColor = UIColor.brownColor()
circleShape.lineWidth = 0
circleShape.path = bodyPath
controlCircle.addChild(circleShape)
controlCircle.position = CGPointMake(self.frame.width/2, self.frame.height/2)
self.addChild(controlCircle)
controlCircle.physicsBody.categoryBitMask = mainCategory
controlCircle.physicsBody.contactTestBitMask = dropCategory
//CREATING SECOND OBJECT AND ADDING PHYSICS BODY
var radius = 10.0;
var drop = SKSpriteNode(color: UIColor.clearColor(), size:CGSizeMake(radius * 2, radius * 2))
var dropBody = SKPhysicsBody(circleOfRadius: radius)
dropBody.dynamic = false
dropBody.usesPreciseCollisionDetection = true
drop.physicsBody = dropBody
var dropPath = CGPathCreateWithEllipseInRect(CGRectMake((drop.size.width/2), drop.size.height/2, drop.size.width, drop.size.width),
nil)
var dropShape = SKShapeNode()
dropShape.fillColor = UIColor.blackColor()
dropShape.lineWidth = 0
drop.name = "dropMask"
dropShape.path = dropPath
drop.addChild(dropShape)
drop.position = CGPointMake(CGFloat(xValue), self.frame.height-5)
self.addChild(drop)
drop.physicsBody.categoryBitMask = dropCategory
drop.physicsBody.contactTestBitMask = mainCategory
}
func didBeginContact(contact: SKPhysicsContact) -> Void{
NSLog("Hello")
}
}

Answer 1

Good news everyone,
SKPhysicsBody documentation:
The dynamic property controls whether a volume-based body is affected by gravity, friction, collisions with other objects, and forces or impulses you directly apply to the object.
So here is an idea to accomplish stationary objects when not dragging, yet receive collision events. On touchesBegan on a SKSpriteNode, set its physicsBody.dynamic = true, on touchUp physicsBody.dynamic = false
Or, you could just remove gravity from your system and have all nodes with physicsBody.dynamic = true