title: Update UI thread from portable class library
body: <p>I have an MVVM Cross application running on Windows Phone 8 which I recently ported across to using Portable Class Libraries.</p>
  
  <p>The view models are within the portable class library and one of them exposes a property which enables and disables a PerformanceProgressBar from the Silverlight for WP toolkit through data binding.</p>
  
  <p>When the user presses a button a RelayCommand kicks off a background process which sets the property to true which should enable the progress bar and does the background processing.</p>
  
  <p>Before I ported it to a PCL I was able to invoke the change from the UI thread to ensure the progress bar got enabled, but the Dispatcher object isn't available in a PCL.  How can I work around this?</p>
  
  <p>Thanks</p>
  
  <p>Dan</p>