title: Property from another ViewModel doesn't change
body: <p>I need to change a value of property <code>TextSource</code> from another ViewModel, so I did this to get ReaderViewModel.</p>
  
  <p><strong>ViewerViewModel.cs</strong>:</p>
  
  <pre class="lang-cs prettyprint-override"><code>        private ReaderViewModel _readerViewModel = new ReaderViewModel();
          public ReaderViewModel ReaderViewModel
          {
              get =&gt; _readerViewModel;
              set
              {
                  _readerViewModel = value;
                  OnPropertyChanged("");
              }
          }
  </code></pre>
  
  <p>As well as changing the property's value to something:</p>
  
  <pre class="lang-cs prettyprint-override"><code>ReaderViewModel.TextSource = "some string";
  </code></pre>
  
  <p><strong>ReaderViewModel.cs</strong>:</p>
  
  <pre class="lang-cs prettyprint-override"><code>        private string _textSource;
          public string TextSource
          {
              get =&gt; _textSource;
              set
              {
                  _textSource = value;
                  OnPropertyChanged();
              }
          }
  </code></pre>
  
  <p><strong>ReaderPage.xaml</strong>:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;Page.DataContext&gt;
     &lt;viewModels:ReaderViewModel/&gt;
  &lt;/Page.DataContext&gt;
  
     ...
  
  &lt;Grid&gt;
     &lt;TextBox Text="{Binding TextSource}"/&gt;
  &lt;Grid/&gt;
  </code></pre>
  
  <p>But it don't work at all, TextBox is still empty. I can't really figure out, where's the problem. The DataContext for ReaderPage is already set to the right ViewModel.</p>