title: SKPhysicsBody optimization
body: <p>I have a 2D sidescrolling game. Right now, in order to jump, the player must be touching the ground. Therefor, I have a boolean, isOnGround, that is set to YES when the player collides with a tile object, and no when the player jumps. This generates a LOT of calls to didBeginContact method, slowing down the game.</p>
  
  <p>Firstly, how can I optimise this by using one big physics body for the tiles on the floor (for example clustering multiple adjacent tiles into one single physics body)?</p>
  
  <p>Secondly, is this even efficient? Is there a better way to detect if the play is on the ground? My current method opens up a lot of bugs, for example wall jumping. If a player collides with a wall, isOnGround becomes YES and allows the player to jump.</p>