title: NSNotification and control flow
body: <p>I want to do the following:</p>
  
  <pre><code>1. display loader
  2. download database update and reload data
  3. hide loader
  </code></pre>
  
  <p>My idea is to send notifications at the begining and at the end of Database's update method to the main view controller, so that the view controller displays and hides the loader.</p>
  
  <p>I am worried about the control flow. Do I have any guarantee that the view controller will receive the notification and display the loader before the Database's update method proceeds?</p>