title: Download mutiple files in background thread
body: <p>I am making an iPad app where you can download files (like PDF, doc, etc) and view them offline.</p>
  
  <p>I already have the view part and you can download a file to the document directory.
  As it is now you need to wait for the download to be finished to move on.
  This can be solved by putting it in a thread, but what happens when the user downloads multiple files or even download the same file multiple times?</p>
  
  <p>My idea is to make a download queue, with a view for the progress.</p>
  
  <p>Workflow:</p>
  
  <ul>
  <li><p>The user opens a document and press download, the user gets a message that the download is started and can be viewed in the offline documents view.</p></li>
  <li><p>The user downloads 3 more documents.</p></li>
  <li><p>When the user goes to the offline document view the user sees a table view with 4 filled cells. 2 documents are done loading and 2 other are still downloading because there is a download/status bar shown in the table view cell.</p></li>
  <li><p>The downloaded documents can be viewed or deleted.</p></li>
  <li><p>The downloads in progress can not be watched (yet) but can be cancelled.</p></li>
  </ul>
  
  <p>I want to make a threaded download class where you can add urls to be downloaded. the class has methods to cancel and delete document-downloads, but also has methods to return the progress. 
  If possible the class can handle simultaneous downloads.</p>
  
  <p>The problem is, I don't know where to start?</p>