title: How Validation.HasError property is updated for user input validation?
body: <p>In <a href="http://www.codeproject.com/Tips/784331/WPF-MVVM-Validation-ViewModel-using-IDataErrorInfo" rel="nofollow">this tutorial</a> there is a text box that shows a pink background for invalid data. This is the wpf code:</p>
  
  <pre><code>&lt;TextBox Text="{Binding Aid,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay,ValidatesOnDataErrors=True}" 
               Canvas.Left="95" Canvas.Top="60" Width="297"&gt;
  
  &lt;TextBox.Style&gt;
              &lt;Style TargetType="TextBox"&gt;
                  &lt;Style.Triggers&gt;
                      &lt;Trigger  Property="Validation.HasError" Value="True"&gt;
                          &lt;Setter Property="Background" Value="Pink"/&gt;
                          ...
                      &lt;/Trigger&gt;
                  &lt;/Style.Triggers&gt;
              &lt;/Style&gt;
          &lt;/TextBox.Style&gt;
  
          &lt;Validation.ErrorTemplate&gt;
              &lt;ControlTemplate&gt;                    
                  &lt;StackPanel&gt;
                      &lt;AdornedElementPlaceholder x:Name="textBox" ToolTip="{Binding [0].ErrorContent}"/&gt;
                  &lt;/StackPanel&gt;
              &lt;/ControlTemplate&gt;
          &lt;/Validation.ErrorTemplate&gt;
  
      &lt;/TextBox&gt;
  </code></pre>
  
  <p>But I don't get how Validation.HasError is updated as the user inputs the value?</p>