title: What is the difference between NSInvocationOperation and NSBlockOperation
body: <p>There are three operation classes in Foundation Framework(<code>NSOperation</code>, <code>NSInvocationOperation</code> and <code>NSBlockOperation</code>).<br/></p>
  
  <p>I already read the <a href="https://developer.apple.com/library/mac/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationObjects/OperationObjects.html#//apple_ref/doc/uid/TP40008091-CH101-SW1" rel="noreferrer">concurrency programming guide</a> but I did't understand exactly what is the difference between these three classes. Please help me. </p>