title: How to copy SKSpriteNode with SKPhysicsBody?
body: <p>I am curious about a situation that I came across today when trying to unarchive and copy a <code>SKSpriteNode</code> from one <code>SKScene</code> to another. In the output from the playground below you can see that both <code>linearDamping</code> and <code>angularDamping</code> or not being maintained after the copy (they seem to be dropping back to default values)</p>
  
  <pre><code>// PLAYGROUND_SW1.2 - SKSpriteNode Copy
  import UIKit
  import SpriteKit
  
  // ORIGINAL
  let spriteNode = SKSpriteNode()
  spriteNode.name = "HAPPY_NODE"
  let size = CGSize(width: 55.0, height: 150.0)
  let physics = SKPhysicsBody(rectangleOfSize: size)
  physics.linearDamping = 0.123
  physics.angularDamping = 0.456
  spriteNode.physicsBody = physics
  
  // COPY
  let spriteCopy = spriteNode.copy() as! SKSpriteNode
  
  // ORIGINAL
  spriteNode.name
  spriteNode.physicsBody?.linearDamping
  spriteNode.physicsBody?.angularDamping
  spriteNode.physicsBody?.area
  
  // COPY
  spriteCopy.name
  spriteCopy.physicsBody?.linearDamping
  spriteCopy.physicsBody?.angularDamping
  spriteCopy.physicsBody?.area
  </code></pre>
  
  <p>PLAYGROUND OUTPUT
  <img src="https://i.stack.imgur.com/GUe3U.png" alt="enter image description here"></p>
  
  <p>I am not sure that I am copying this correctly, both <code>SKSpriteNode</code> and <code>SKPhysicsBody</code> conform to <code>NSCopying</code> If you look at the output above the <code>area</code> property is maintained after the copy and to my knowledge this is based on the <code>size</code> specified when the <code>SKPhysicsBody</code> was created. </p>
  
  <p>Can anyone cast some light on this and maybe provide me with a pointer as to how I should be deep copying an <code>SKSpriteNode</code>?</p>