title: Where 'index' property is defined, as Repeater is used to instantiate an array of items, each them can access the index
body: <p>I guess <strong>index</strong> is an attached property, but where is it defined and why accessing the <strong>index</strong> is not as <code>ListView.isCurrentItem</code>?</p>
  
  <p>From the official documentation, <a href="http://doc.qt.io/qt-5/qml-qtquick-repeater.html" rel="nofollow noreferrer">description of <strong>delegate</strong> property of Repeater</a>:</p>
  
  <blockquote>
    <p>Delegates are exposed to a read-only index property that indicates the index of the delegate within the repeater.</p>
  </blockquote>
  
  <p>I am more confused. Why are delegates exposed to a property, and by what operation does this happen?</p>