title: collision detection function in objective-c/core graphics
body: <p>Is there a built-in function to detect collision between
  two circles?</p>
  
  <p>I have used <code>CGRectIntersectsRect(rect1,rect2)</code> to
  find the intersection between two rectangles.But if I want to know in which axis x or y they intersect how will i find it?</p>
  
  <p>Is there a built-in function or do you have any other ideas?</p>