title: LongListSelector cannot change ItemsSource
body: <p>I can't seem to be able to change the <em>ItemsSource</em> in <em>LongListSelector</em>. After doing some research, it appears that <em>LongListSelector</em> ignores any changes in <em>ItemsSource</em>. Am I correct?g
  If yes, is there a workaround?</p>
  
  <p>This what I'm trying to do. I have an option to sort the City list either by country or cities. Something like this:</p>
  
  <pre><code>var groupByCountry = from City in App.ViewModelCity.Items
                                  group city by city.GroupByCountry into c
                                  orderby c.Key
                                  select new Group&lt;Currency&gt;(c.Key, c);
  
  var groupByCity = from City in App.ViewModelCity.Items
                                  group city by city.GroupByCity into c
                                  orderby c.Key
                                  select new Group&lt;City&gt;(c.Key, c);
  </code></pre>
  
  <p>SO basically both are the same, just the grouping is different. My idea was to simple change the <em>ItemsSource</em> to either <strong>groupByCountry</strong> or <strong>groupByCity</strong>, depending on the user choice. Unfortunately, changing the <em>ItemsSource</em> has no effect. It simply won't update. Only when the app is restarted (with the preferred source stored) the <em>LongListSelector</em> is updated.</p>
  
  <p>So is there a way to force <em>LongListSelector</em> to update the <em>ItemsSource</em>? If not, what would be a good workaround?</p>