title: Angular2 Reactive Forms - Disable form control dynamically from condition
body: <p>I have a select control that I want to disable dynamically based on a condition:</p>
  
  <pre><code>this.activityForm = this.formBuilder.group({
    docType: [{ value: '2', disabled: this.activeCategory != 'document' }, Validators.required]
  });
  </code></pre>
  
  <p>However, docType doesn't become enabled even though at some point this.activeCategory becomes 'document'.</p>
  
  <p>How do I fix this?</p>