title: How can I make a certain UI only show up on some view controllers?
body: <p>I have a UIProgressView that I wish to display on two view controllers, A and B. I am currently using a UITabBarController with 4 view controllers, such that I can navigate to any of the four from the navigation tab. The issue is that I want this progress view to only be displayed on 2 of the 4 view controllers.</p>
  <p>My current approach is to build the same progress view separately on A and B. But the issue is that when I navigate from A to B or vice verse, the progress view does not increase smoothly, but updates in a very 'jumpy' manner where it goes to 0 before updating to the current progress.</p>
  <p>In the tab bar controller I do</p>
  <pre class="lang-swift prettyprint-override"><code>self.updateProgressView(prog)
  </code></pre>
  <p>inside of a completion handler as I need to wait for data before I can compute prog.</p>
  <p>In my VCs I do</p>
  <pre class="lang-swift prettyprint-override"><code>VC.updateProgressView = {prog in 
     progressBar.setProgress(prog)
  }
  </code></pre>