angular.js bind to iframe's loading state

Question

how is it possible to have a boolean scope variable that is true when an iframe is loading/reloading?

JS:

$scope.item = {
  src :  "http:example.com",
  isLoading:false
  }

HTML:

<div ngclass="'loading' : item.isLoading">

  <iframe ng-src="item.src">

<div>