title: Cannot assign a value of type "Game Scene" to a value of type "SKPhysicsContactDelegate"
body: <p>I am trying to build flappy bird on my own. Everything works fine but now I have added the CategoryBitMasks and all that stuff and every time I try adding this piece of code: </p>
  
  <pre><code>self.physicsworld.contactDelegate = self
  </code></pre>
  
  <p>I get the error message :</p>
  
  <blockquote>
    <p>Cannot assign a value of type 'GameScene' to a value of type 'SKPhysicsContactDelegate'.</p>
  </blockquote>
  
  <p>Here is the code that I have written so far:</p>
  
  <pre><code>import SpriteKit
  
  class GameScene: SKScene {
  
      //---------------
      //Bird
      //---------------
      var bird = SKSpriteNode()
  
      //---------------
      //Pipe's
      //---------------
      var pipeUp:SKSpriteNode = SKSpriteNode()
      var pipeDown:SKSpriteNode = SKSpriteNode()
      var pipeDownAnimation:NSMutableArray = NSMutableArray()
      var pipeUpAnimation:NSMutableArray = NSMutableArray()
  
      //---------------
      //Pipe spawningtimes
      //---------------
      var lastYieldTimeInterval:NSTimeInterval = NSTimeInterval()
      var lastUpdateTimeInterval:NSTimeInterval = NSTimeInterval()
  
      //---------------
      //Category bitmasks
      //---------------
      let birdCategory:UInt32 = 0x1 &lt;&lt; 2
      let pipeUpCategory:UInt32 = 0x1 &lt;&lt; 1
      let pipeDownCategory:UInt32 = 0x1 &lt;&lt; 0
  
      //didMoveToView Funtion
      override func didMoveToView(view: SKView) {
  
          setUpWorld()
  
      }
  
      override func touchesBegan(touches: Set&lt;NSObject&gt;, withEvent event: UIEvent) {
  
          if bird.position.y &gt; frame.size.height - bird.size.height/2 {
  
          }
          else {
              bird.physicsBody!.velocity = CGVectorMake(0, 0.0)
              bird.physicsBody!.applyImpulse(CGVectorMake(0, 17))
          }
      }
  
      func setUpWorld () {
  
          //---------------
          //Physics World
          //---------------
          physicsWorld.gravity = CGVectorMake(0, -5.0)
  
          //---------------
          //setting up the background
          //---------------
          var background = SKSpriteNode(imageNamed: "background")
          background.position = CGPointMake(frame.size.width/2, frame.size.height/2)
          background.size = CGSizeMake(frame.size.width, frame.size.height)
          background.zPosition = -1.0
  
          addChild(background)
  
          //---------------
          //setting up the hills
          //---------------
          var hills = SKSpriteNode(imageNamed: "hills")
          hills.position = CGPointMake(frame.size.width/2, 300)
          hills.size = CGSizeMake(frame.size.width, 300)
          hills.zPosition = 0
  
          addChild(hills)
  
          //---------------
          //setting up the ground
          //---------------
          var ground = SKSpriteNode(imageNamed: "ground")
          ground.size = CGSizeMake(frame.size.width, 150)
          ground.position = CGPointMake(frame.size.width/2, 75)
          ground.zPosition = 2.0
  
          ground.physicsBody = SKPhysicsBody(rectangleOfSize: CGSizeMake(ground.size.width, ground.size.height))
          ground.physicsBody!.dynamic = false
          ground.physicsBody!.affectedByGravity = false;
  
          addChild(ground)
          //---------------
          //setting up the bird
          //---------------
          bird = SKSpriteNode(imageNamed: "bird")
          bird.position = CGPointMake(100, frame.size.height/2)
          bird.setScale(0.7)
          bird.zPosition = 1.0
  
          bird.physicsBody = SKPhysicsBody(rectangleOfSize: bird.size)
          bird.physicsBody!.dynamic = true
          bird.physicsBody!.categoryBitMask = birdCategory
          bird.physicsBody!.contactTestBitMask = pipeUpCategory
          bird.physicsBody!.contactTestBitMask = pipeDownCategory
          bird.physicsBody!.collisionBitMask = pipeUpCategory &amp; pipeDownCategory
          bird.physicsBody!.usesPreciseCollisionDetection = true
  
          addChild(bird)
      }
  
      func addPipes () {
  
          //---------------
          //Random Pipe Positions
          //---------------
          var randomPositionUp = CGPoint()
          var randomPositionDown = CGPoint()
  
          var random = arc4random() % 10
  
          if random == 0 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 170)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height - 40)
          }
          else if random == 1 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 160)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height - 30)
          }
          else if random == 2 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 150)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height - 20)
          }
          else if random == 3 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 140)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height - 10)
          }
          else if random == 4 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 130)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height)
          }
          else if random == 5 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 120)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height + 10)
          }
          else if random == 6 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 110)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height + 20)
          }
          else if random == 7 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 100)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height + 30)
          }
          else if random == 8 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 90)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height + 40)
          }
          else if random == 9 {
              randomPositionUp = CGPointMake(frame.size.width + pipeUp.size.width * 2, frame.size.height/2 - 80)
              randomPositionDown = CGPointMake(randomPositionUp.x, frame.size.height + 50)
          }
  
          //---------------
          //Pipe UP
          //---------------
          pipeUp = SKSpriteNode(imageNamed: "PipeUp")
          pipeUp.position = randomPositionUp
          pipeUp.setScale(0.5)
          pipeUp.zPosition = 1.0
  
          pipeUp.physicsBody = SKPhysicsBody(rectangleOfSize: CGSizeMake(pipeUp.size.width, pipeUp.size.height))
          pipeUp.physicsBody!.dynamic = false
          pipeUp.physicsBody!.affectedByGravity = false
          pipeUp.physicsBody!.categoryBitMask = pipeUpCategory
          pipeUp.physicsBody!.contactTestBitMask = birdCategory
          pipeUp.physicsBody!.collisionBitMask = birdCategory
          pipeUp.physicsBody!.usesPreciseCollisionDetection = true
  
          addChild(pipeUp)
  
          pipeUpAnimation.addObject(SKAction.moveToX(-pipeUp.size.width * 2, duration: 7))
          pipeUpAnimation.addObject(SKAction.removeFromParent())
          pipeUp.runAction(SKAction.sequence(pipeUpAnimation as [AnyObject]))
  
          //---------------
          //Pipe Down
          //---------------
          pipeDown = SKSpriteNode(imageNamed: "PipeDown")
          pipeDown.position = randomPositionDown
          pipeDown.setScale(0.5)
          pipeDown.zPosition = 1.0
  
          pipeDown.physicsBody = SKPhysicsBody(rectangleOfSize: CGSizeMake(pipeDown.size.width, pipeDown.size.height))
          pipeDown.physicsBody!.dynamic = false
          pipeDown.physicsBody!.affectedByGravity = false
          pipeDown.physicsBody!.categoryBitMask = pipeDownCategory
          pipeDown.physicsBody!.contactTestBitMask = birdCategory
          pipeDown.physicsBody!.collisionBitMask = birdCategory
          pipeDown.physicsBody!.usesPreciseCollisionDetection = true
  
          addChild(pipeDown)
  
          pipeDownAnimation.addObject(SKAction.moveToX(-pipeUp.size.width * 2, duration: 7))
          pipeDownAnimation.addObject(SKAction.removeFromParent())
          pipeDown.runAction(SKAction.sequence(pipeDownAnimation as [AnyObject]))
      }
  
      func updateWithTimeSinceLastUpdate (timeSinceLastUpdate: CFTimeInterval) {
  
          lastYieldTimeInterval += timeSinceLastUpdate
  
          if lastYieldTimeInterval &gt; 2 {
              lastYieldTimeInterval = 0
              addPipes()
          }
  
      }
  
      override func update(currentTime: CFTimeInterval) {
  
          var timeSinceLastUpdate = currentTime - lastUpdateTimeInterval
          lastUpdateTimeInterval = currentTime
  
          if timeSinceLastUpdate &gt; 1 {
  
              timeSinceLastUpdate = 1/60
              lastUpdateTimeInterval = currentTime
  
          }
          updateWithTimeSinceLastUpdate(timeSinceLastUpdate)
      }
  
      func didBeginContact(contact: SKPhysicsContact) {
  
          var firstBody:SKPhysicsBody
          var secondBody:SKPhysicsBody
  
          if contact.bodyA.categoryBitMask &lt; contact.bodyB.categoryBitMask {
              firstBody = contact.bodyA
              secondBody = contact.bodyB
          }
          else {
              firstBody = contact.bodyB
              secondBody = contact.bodyA
          }
  
          if firstBody.categoryBitMask &amp; birdCategory != 0 &amp;&amp; secondBody.categoryBitMask &amp; pipeDownCategory != 0 {
              birdDidCollidWithPipeDown(firstBody.node as! SKSpriteNode, pipeDown: secondBody.node as! SKSpriteNode)
          }
      }
  
      func birdDidCollidWithPipeDown(bird: SKSpriteNode, pipeDown: SKSpriteNode) {
  
          bird.removeFromParent()
  
      }
  }
  </code></pre>