title: Keep track of download progress UI independantly
body: <p>I'm working on a magazine app(without newsstand kit) for iOS. There are 3 magazines and each magazine has 3 issues.
  My main view controller represent Magazine shelf, and magazine button will navigate to the their issue shelf(view controller) when user interacts.</p>
  
  <p>My problem is when downloading the issues, users may want to go back to the Magazine shelf. So in this case I lose my <code>issueCover</code> view class(where cover image, download button, progress bar lies in) and lost connection between <code>issueCover</code> view and Download Client class. </p>
  
  <p>I was trying to get progress from Download Client Class using <code>KVO</code> State Observing but since users can navigate, my <code>KVO</code> sending progress to the empty view and then leads to crash.</p>
  
  <p><a href="http://ios-blog.co.uk/tutorials/how-to-make-a-magazine-app-in-ios-part-ii/" rel="nofollow">What I do is the almost same with this tutorial app except magazine issue cover can be nil</a></p>
  
  <p>How can I keep track of this download progress float numbers even if view has disappeared and reappear?</p>