title: KnockOut run function on value changed
body: <p>Got simple viewModel:</p>
  
  <pre><code>function viewModel() {
      enabled: ko.observable(false);
      ...
  }
  </code></pre>
  
  <p>and some binding like:</p>
  
  <pre><code>&lt;input data-bind="hasFocus: enabled" /&gt;
  </code></pre>
  
  <p>and i want run some function on focus, and other on focus lost( or enabled = true/false) but run when value changes.
  Any help?</p>