title: How to access the iterating-index in a sapUI5 Aggregation Binding?
body: <p>In sapUI5 I have a JSONModel containing</p>
  
  <pre><code>{
    "element1": {
      "columnName": "col1",
      "values":[
        {"name": "value 1 - 1", "selected": true},
        {"name": "value 1 - 2", "selected": true},
        {"name": "value 1 - 3", "selected": true},
        {"name": "value 1 - 4", "selected": true}
      ]
    },
    "element2": {
      "columnName": "col2",
      "values":[
        {"name": "value 2 - 1", "selected": true},
        {"name": "value 2 - 2", "selected": true},
        {"name": "value 2 - 3", "selected": true},
        {"name": "value 2 - 4", "selected": true}
      ]
    }
  }
  </code></pre>
  
  <p>and a XML-View bound to this model iterating over each element and each elements value (to display a checkbox for each value):</p>
  
  <pre><code>&lt;mvc:View
      ...
      xmlns:app="http://schemas.sap.com/sapui5/extension/sap.ui.core.CustomData/1"
      xmlns="sap.m"
  &gt;
      &lt;VBox items="{/}"&gt;
          &lt;VBox items="{values}"&gt;
              &lt;CheckBox 
                  text={name}
                  app:xxColumName="{/columnName}" &lt;!-- this is null - how to add columnName here? --&gt;
              &gt;&lt;/CheckBox&gt;
          &lt;/VBox&gt;
      &lt;/VBox&gt;
  &lt;/mvc:View&gt;
  </code></pre>
  
  <p>How can I access &amp; bind the <code>columnName</code> value from the JSON-element to each checkbox?</p>
  
  <p>Is there a syntax like <code>{/$idex/columnName}</code> so I can 'break out' of the 2nd loops namespace?</p>
  
  <p><strong>Edit:</strong>
  <a href="https://jsbin.com/humadoxuto/edit?html,output" rel="nofollow">JS-Bin is here.</a></p>