title: How to implement mouse joint in Sprite Kit?
body: <p>I have working implementation of drag and drop feature written in Cocos2d + Box2d for iOS. I need to port it to Sprite Kit. The logic is pretty basic:</p>
  
  <ul>
  <li>when user touching the screen, find sprite under the finger</li>
  <li>create mouse joint between found sprite and the scene's physics body, set joint's target to position of touch</li>
  <li>when touches moved, update joint's target to new position</li>
  <li>when touches ended, remove the joint</li>
  </ul>
  
  <p>Everything is working quite well. I like the way physics is simulated when touches ends - the dragged shape has velocity dependent on dragging speed and direction, so when I left my finger from the screen while moving the sprite, it will continue moving in the same direction, but now affected by gravity and damping.</p>
  
  <p>Unfortunately, I'm not able to reproduce the same effect using Sprite Kit. There is no such joint like mouse joint, but I have tried using other joint types. I almost succeeded with SKPhysicsJointFixed - I'm creating new sprite and joint between it and dragged sprite, when touches moving I'm moving the newly created sprite. Unfortunately it doesn't work like mouse joint in Cocos2d + Box2d - while dragging the sprite, its velocity always equals zero. So, every time I left my finger from the screen, the dragged sprite stops immediately and start falling affected by gravity. No matter how fast I move the finger when dragging, after releasing dragged shape, it behaves exactly the same way.</p>
  
  <p>My question is how to implement mouse joint in Sprite Kit, or how to implement drag and drop feature that works like described above?</p>
  
  <p><strong>Update:</strong>
  This is a box2d mouse joint example that could give you more clear view on what I am trying to implement using Sprite Kit:
  <a href="http://blog.allanbishop.com/wp-content/uploads/2010/09/Box2DJointTutorial1.swf" rel="nofollow">http://blog.allanbishop.com/wp-content/uploads/2010/09/Box2DJointTutorial1.swf</a></p>