title: Difference between NSOperation and NSInvocationOperation?
body: <p>1) what is happening when we add <code>NSOperation</code> or <code>NSInvocationOperation</code> in <code>NSoperationQueue</code>?</p>
  
  <p>2) What is the difference between <code>NSoperation</code> and <code>NSInvocationOperation</code>?</p>
  
  <p>3) which one is the best ?</p>