$scope array and $scope.$apply in angularjs

I'm learning AngularJs and I'm facing a problem.

I want to show an array of items in a view. First in the controller I define the array using $scope.arrName = [], then I receive data from a third party service and add it to the array with $scope.arrName.push(data[i]). I figured it out that in order to my view render the changes I have to insert $scope.$apply after pushing data to the array.

Is there a way to modify the array and render the changes without $scope.$apply? Why code breaks if I change $scope.arraName with this.arrName? (it throws Uncaught TypeError: Cannot read property 'push' of undefined)

Reference code

Controller

angular.module('testApp')
  .controller('testCtrl', function($scope) {

    $scope.objects = [];

    var service = new thirdPartyService();

    service.getData(function(err, data) {
      if (err) {
        console.log('Error');
      } else {
        for (var i = 0; i < data.Contents.length; i++) {
          $scope.objects.push(data.Contents[i].Key);
          $scope.$apply();
        }
      }
    });
  });

View

Just an ng-repeat to show the data