title: Implementing the UIActivityItemSource protocol
body: <p>I have these protocol methods,</p>
  
  <p><code>activityViewControllerPlaceholderItem:</code> and <code>activityViewController:itemForActivityType:</code></p>
  
  <p>but they never get called. How do I tell the <code>UIActivityViewController</code> to call them?</p>
  
  <p>I also have an <code>UIActivityItemProvider</code> subclass, however I'm confused as to who calls these 2 methods. I'd really appreciate some sample code, as I can't find anything on the web. :)</p>