title: xamarin.forms : ListView not updating as expexcted
body: <p>I have built a app using Xamarin.Forms that I am displaying in Windows.
  Requirement: 
  - tap on item in the ListView > 
  - the displayed value for the item changes in the UI</p>
  
  <p>Here is the relevant xaml
  NOTE: I have set the ListView.ItemsSource from code behind</p>
  
  <pre><code>  &lt;ListView.ItemTemplate &gt; 
    &lt;StackLayout  VerticalOptions="Center" HorizontalOptions="Center"  &gt;
    &lt;Label Text="Passo dopo passo" VerticalOptions="Center" HorizontalOptions="Center" /&gt;
    &lt;Button Clicked="GetGroceries" Text="Get Groceries" &gt;&lt;/Button&gt;
    &lt;ListView x:Name="lvGroceries" ItemTapped="GroceryItemTapped" &gt;
    &lt;ListView.ItemTemplate &gt; 
  </code></pre>
  
  <p>Here is the c# code behind, which runs fine when the item is tapped, but the UI does not change.</p>
  
  <pre><code>
      public ObservableCollection oc;
      public void GroceryItemTapped (object o, ItemTappedEventArgs e )
      {
       if (e.Item == null ) {
           return;
       }
       var g = ((GroceryItem)e.Item);
       // oc is populated and exactly what I expect here
       foreach (var gr in oc)
       {
           // the next line grabs the item I want
           if (gr.GroceryId == g.GroceryId)
           {
               gr.GroceryName = "snot";
           }  } }
  
      // when the above code runs, the data is already in oc and this code ran
      lvGroceries.ItemsSource = oc;
  
  </code></pre>
  
  <p>Ideas?</p>