title: How to update widget UI on data change iOS 8 Today Extension
body: <p>I need to confirm these things</p>
  
  <ol>
  <li>Does widgetPerformUpdateWithCompletionHandler called by notification center it self or we manually call it in some way?</li>
  <li>If we don't have widgetPerformUpdateWithCompletionHandler control then how can we update the data on data change (i am using a service to fetch json feeds from URL)</li>
  </ol>
  
  <p>Here is what i have noticed.</p>
  
  <ol>
  <li>widgetPerformUpdateWithCompletionHandler is called just once when the widget is loaded.</li>
  <li>viewWillAppear/DidAppear called on switching between today and notification.</li>
  </ol>
  
  <p>Though i have read tutorials and other stuffs about it but this is the specific point which i am unable to understand. I haven't pasted any code here as it's a more of a conceptual thing.</p>
  
  <p>Kindly share knowledge about NCWidgetProviding delegates.</p>
  
  <p><strong><em>Updated:</em></strong>
  To enable the widget to update its view when it’s off-screen make the following changes to the <code>widgetPerformUpdateWithCompletionHandler</code>. <strong><em>The system takes a snapshot of the widget and periodically</em></strong>, will try to update it. If it updates successfully, the function calls the system-provided completion block with the NCUpdateResult.NewData enumeration. If the update wasn’t successful, then the existing snapshot is used.</p>