title: SpriteKit colisions not detected
body: <p>I have 2 SKSpriteNodes in my scene and I would like to detect collisions between them. However this does not work.
  Here is the relevant code</p>
  
  <p>Node1:</p>
  
  <pre><code> PSCarNode *newCar = [PSCarNode new];
  newCar.size = CGSizeMake(CAR_WIDTH, CAR_HEIGHT);
  CGFloat carXval =  - CAR_WIDTH; // start off-screen
  newCar.position = CGPointMake(carXval, Yval);
  newCar.zPosition = 5;
  newCar.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(CAR_WIDTH, CAR_HEIGHT)];
  newCar.physicsBody.dynamic = YES;
  newCar.physicsBody.categoryBitMask = carCategory;
  newCar.physicsBody.contactTestBitMask = frogCategory;
  newCar.physicsBody.collisionBitMask  = 0;
  newCar.physicsBody.usesPreciseCollisionDetection = YES;
  [self addChild:newCar];
  </code></pre>
  
  <p>Node 2:</p>
  
  <pre><code>    player = [[PSPLAYERNode alloc] init];
  [player setPosition: CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame))];
  [player setSize:CGSizeMake(PLAYER_SIZE, PLAYER_SIZE)];
  player.zPosition = 1;
  player.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:(3/4)*PLAYER_SIZE];
  player.physicsBody.dynamic = YES;
  player.physicsBody.collisionBitMask = 1;
  player.physicsBody.categoryBitMask = playerCategory;
  player.physicsBody.contactTestBitMask = carCategory;
  [self addChild:player];
  </code></pre>
  
  <p>init:</p>
  
  <pre><code>    - (instancetype)initWithSize:(CGSize)size
  {
      self = [super initWithSize:size];
      if (self) {
          [self createScene];
          self.physicsWorld.gravity = CGVectorMake(0., 0.); // no gravity
          self.physicsWorld.contactDelegate = self;
          animationInProgress = NO;
      }
      return self;
  }
  </code></pre>
  
  <p>and delegate:</p>
  
  <pre><code>    - (void)didBeginContact:(SKPhysicsContact *)contact
  {
      NSLog(@"player hit");
  }
  </code></pre>
  
  <p>Why are there no collisions detected. I can clearly see the car node move over the player node, but nothing is shown in the log.</p>