title: How to remove/cancel NSInvocationOperation from NSOperationQueue?
body: <p>Both of the following questions are being asked in context to maintain NSOperationQueue and NSInvocationOperation. </p>
  
  <p>As I have used this concept to download multiple videos, how do I remove/release added NSInvocationOperation from NSOperationQueue after completion of downloading a video? </p>
  
  <p>And also, what should I do if in case I want to stop to download specific video while the process of downloading in progress?</p>