title: Issue about viewDidAppear create table every time and then crashed
body: <p>I have two views with two tab means per view per tab and I'm taking data from server. So I have to use viewDidAppear:(BOOL)animated.</p>
  
  <p>In this method I add table, progress view and label and I have used "[self performSelectorInBackground:@selector(getDataFromServer:) withObject:string];" .</p>
  
  <p>So when I select this view, it start to loading data in background and at time when I move to another view and come back to first, it override another table, label and progress view. How many times I select this view while loading from server, it will add that times table, label and progressview and then crashed.</p>
  
  <p>What should I use with this view did appear from prevent this bugy things.</p>