title: Wpf binding update happens too late
body: <p>I use MVVM. There is a slow command. I want to use a log to indicate where is the execution.
  xaml:</p>
  
  <pre><code> &lt;TextBlock Height="200" Text="{Binding Log.Content, UpdateSourceTrigger=PropertyChanged}"&gt;&lt;/TextBlock&gt;
  </code></pre>
  
  <p>code:</p>
  
  <pre><code>Log.addLine("Ledger initialized.");
  // slow1 operation
  Log.addLine("slow1 operation.");
  </code></pre>
  
  <p>The textblock is only updated when the whole command is finished. Both message appears at the same time when the GUI is active.</p>