title: Multithreading the pathfinding in a tower defense game
body: <p>I'm making a tower defense game and I'm stuck at the multithreading of the pathfinding. If i don't make the pathfinding an NSInvocationOperation then there is an increasing pause in the rendering the more fiends and towers i have.</p>
  
  <p>However i cannot solve the "locking" issue. I get frequent crashes about enumerating my fiend NSMutableArray while mutating it. If i instead try to lock the enumeration in the main thread while pathfinding is being done, the game still locks up and i have lost the whole point of the multithreading..</p>
  
  <p>How do i solve this kind of issue?</p>