title: extjs6 bind label visibility
body: <p>I am looking to bind the visibility of a button to a parameter with some difficulty. In xamarin I would add binding in xaml to a parameter, then in viewmodel I would have a getter/setter and simply put my simple true/false formula in the getter.  </p>
  
  <p>What would be the equivalent in extjs. Right now in my view I have,</p>
  
  <pre><code>{
     xtype: 'button',
     text: 'Upload LiveEnd',
     //itemId: 'buttonUploadLiveEnd',
     handler: 'onUploadLiveEnding',
     margin: '5px 5px 0 0',
     bind: {
       hidden: '{param}'
     }
  }
  </code></pre>
  
  <p>In my viewmodel I have a parameter</p>
  
  <pre><code>data: {
     param: true
  }
  </code></pre>
  
  <p>That works to hide button, But when I try to change the variable in my controller it doesn't change the view even though I think the parameter is being changed. Is there something simpler like getter/setter from xamarin?</p>