title: Bind Button.IsEnabled to a property in a TextBox's data source
body: <p>I have a <code>Foo</code> class:</p>
  
  <pre><code>public class Foo
  {
      public string Value { get; set; }
      public string IsDirty { get; private set; }
  }
  </code></pre>
  
  <p>and I have xaml with a <code>TextBox</code> and a <code>Button</code> bound to <code>Foo</code>:</p>
  
  <pre><code>&lt;TextBox Text="{Binding Value, UpdateSourceTrigger=PropertyChanged}" ... /&gt;
  &lt;Button IsEnabled="{Binding IsDirty}" ... /&gt;
  </code></pre>
  
  <p>Once the text in the <code>TextBox</code> is changed (updated on <code>KeyDown</code>), <code>Foo.IsDirty</code> becomes true (until a save button is clicked).</p>
  
  <p>Right now, <code>Button.IsEnabled</code> is not changing when <code>Foo.IsDirty</code> changes.</p>
  
  <p>How might I change the binding on the <code>Button</code> so that it becomes enabled as soon as <code>Foo.IsDirty = true</code> and vice versa?</p>
  
  <p>Thanks!</p>