How to deep watch an array in angularjs?

There is an array of objects in my scope, I want to watch all the values of each object.

This is my code:

function TodoCtrl($scope) {
  $scope.columns = [
      { field:'title', displayName: 'TITLE'},
      { field: 'content', displayName: 'CONTENT' }
  ];
   $scope.$watch('columns', function(newVal) {
       alert('columns changed');
   });
}

But when I modify the values, e.g. I change TITLE to TITLE2, the alert('columns changed') never popped.

How to deep watch the objects inside an array?

There is a live demo: http://jsfiddle.net/SYx9b/