title: Navigation Back & MVVM - How to refresh WP8 page databinding
body: <p>I'm doing a WP8 App (C#/XAML).</p>
  
  <p>In my view I specify a button, which contect is set by binding with a callback, for the start of an app, when the VM is not fully loaded.</p>
  
  <p>MVVM looks like:</p>
  
  <pre><code>ViewModel
  ---------
   + Model
     -----
      +Property
  </code></pre>
  
  <p>And is created in <code>App.xaml.cs</code> like this:</p>
  
  <pre><code>public static MainViewModel ViewModel
      {
          get
          {
              if (viewModel == null)
              {
                  viewModel = new MainViewModel();
              }
              return viewModel;
          }
      }
  </code></pre>
  
  <p>And set to the page as datacontext in contructor of the page:</p>
  
  <pre><code>DataContext = App.ViewModel;
  </code></pre>
  
  <p>And button:</p>
  
  <pre><code>&lt;Button x:Name="btn" Content="{Binding Model.Property, FallBackValue='click to load'}" .../&gt;
  </code></pre>
  
  <ul>
  <li><p>At the start, the <code>btn</code> does not have a value to put in it's content, because model is empty.</p></li>
  <li><p>When the <code>btn</code> is clicked, it loads the model. It fills model with data and navigates to another page which shows that data.</p></li>
  <li><p>And when i navigate back (through hardware back button) I'd like <code>btn</code> to use the value from binding instead of a fallback, because the value is already set. But It doesn't use it and still uses the one provided by FallbackValue argument of binding.</p></li>
  </ul>
  
  <p><strong>How to ensure, that the page "refreshes" an uses actual values provided by ViewModel?</strong></p>