how delegate-handle work in ionic + angularjs?

could you please tell me how delegate-handle work in ionic actually I make a demo in code pen .I want to scroll the contend to top when button click .But my code is not working when I scroll my contend and then press button but my scroll not goes to top why ? http://codepen.io/anon/pen/wavoXb

var app=angular.module('app',['ionic']);
app.controller('MainCtrl',function($scope){
   $scope.scrollMainToTop = function() {
    $ionicScrollDelegate.$getByHandle('mainScroll').scrollTop();
  };
  $scope.scrollSmallToTop = function() {
    $ionicScrollDelegate.$getByHandle('small').scrollTop();
  };
})

You need to inject $ionicScrollDelegate in the controller. 

   app.controller('MainCtrl',function($scope,$ionicScrollDelegate){
   $scope.scrollMainToTop = function() {
    $ionicScrollDelegate.$getByHandle('mainScroll').scrollTop();
  };
  $scope.scrollSmallToTop = function() {
    $ionicScrollDelegate.$getByHandle('small').scrollTop();
  };