Angular-js $scope variable is not updated in view

I am new to angular-js and building a simple to-do application. I am creating a task and displaying the created task in a html table using ng-repeat. But the problem is that after posting the data, $scope.tasks variable is updated on controller side, but not in view. The view updates after refreshing the web page only and the task is added to html table. How can I make the view update after creating the task. Thanks in advance. Here is my code:

In my controller:

var app = angular.module('MyApp', ['ngMaterial', 'ngMessages']);
app.controller('DemoCtrl', function($scope,$mdSidenav,$mdDialog,$interval,$http,$mdToast) {
     $scope.tasks = []; 
   _refreshTaskData();  //initial refresh

  $scope.submitForm = function() {

        var description = "";
        var taskId = "";

        $scope.formData = {
                taskId: $scope.taskId,
                description: $scope.description,
         };

        $http({
            method: "POST",
            url: 'savetask',
            data:  angular.toJson($scope.formData),
            headers : {
                'Content-Type': 'application/json',
            }
        }).then(_success, _error);
      };

    function  _refreshTaskData() {
        $http({
                method : 'GET',
                url : 'getTask',

            }).then(function(res) { // success
                $scope.tasks = res.data;
            }, function(res) { // error
                console.log("Error: " + res.status + " : " + res.data);
            });
    }

    function _success(res) {
         $mdDialog.hide();
         console.log('in success function');
        _refreshTaskData(); ;
    }

    function _error(res) {
           //error handling
     }
}); 

In my view:

<table>
   <tr ng-repeat=" t in tasks">
      <td>{{t.id}}</td>
      <td>{{t.description}}</td>
   </tr>
</table>