title: How to do some stuff in viewDidAppear only once?
body: <p>I want to check the pasteboard and show an alert if it contains specific values when the view appears. I can place the code into <code>viewDidLoad</code> to ensure it's only invoked once, but the problem is that the alert view shows too quickly. I know I can set a timer to defer the alert's appearance, but it's not a good work-around I think.</p>
  
  <p>I checked the question <a href="https://stackoverflow.com/questions/22214843/ios-7-difference-between-viewdidload-and-viewdidappear">iOS 7 - Difference between viewDidLoad and viewDidAppear</a> and found that there is one step for checking whether the view exists. So I wonder if there's any api for doing this?</p>
  
  <p>Update: The "only once" means <strong>the lifetime of the view controller instance</strong>.</p>