title: back button on navigation toolbar not clickable until it finish complete processing of current view
body: <p>i have a view which has uinavigationcontroller with back button. On viewWillAppear method i am 
  calling a web service and bind the data in table view, it take some time to bind the data. In meantime user click back button it doesn't move to the previous view till it finish complete binding of table. how can i move to previous view till processing current view.</p>