title: SpriteKit's didBeginContact can't be called
body: <p>I tested with a simple scene. </p>
  
  <p>Func <code>didBeginContact</code> can't be called.</p>
  
  <p>I have try to use <code>collisionBitMask</code> and <code>s1.physicsBody = SKPhysicsBody(circleOfRadius: 100)</code>. The problem was persistent.</p>
  
  <p>What can I do to fix it? </p>
  
  <pre><code>import SpriteKit
  
  class Test2Scene: SKScene, SKPhysicsContactDelegate {
      override func didMoveToView(view: SKView) {
          physicsWorld.contactDelegate = self
          physicsWorld.speed = 0
  
  
          let s1 = SKSpriteNode(imageNamed: kImagePlayer)
          s1.position = CGPointMake(100, 100)
          s1.physicsBody = SKPhysicsBody(rectangleOfSize: s1.size)
          s1.physicsBody?.categoryBitMask = 1
          s1.physicsBody?.contactTestBitMask = 2
          //s1.physicsBody?.collisionBitMask = 2
          self.addChild(s1)
  
  
          let s2 = SKSpriteNode(imageNamed: kImagePlayer)
          s2.position = CGPointMake(100, 500);
          s2.runAction(SKAction.moveToY(0, duration: 1))
          s2.physicsBody = SKPhysicsBody(rectangleOfSize: s2.size)
          s2.physicsBody?.categoryBitMask = 2
          //s2.physicsBody?.collisionBitMask = 1
          self.addChild(s2);
  
          print("view did load")
      }
  
      func didBeginContact(contact: SKPhysicsContact) {
          print("aaa")
      }
      func didEndContact(contact: SKPhysicsContact) {
          print("bbb")
      }
  }
  </code></pre>