AngularJS a scope variable is not updated in an anonymous callback function in directive

I've been trying to make a cubism.js graph with polled data (every second) from REST API.

It seems like a correct value is passed to the directive, but it is not passed to an anonymous callback function.

If I am understanding correctly, the anonymous callback function in directive should have an access to the parameter variable 'val'.

The correct value is updated in the view, and the rest of the directive. However, right now it seems like the parameter value, 'val', which was updated from the controller when the promise resolves to a callback function 'notifyCall(data)', is not being updated at an anonymous callback function of directive.

Methods I tried

• putting the 'random' function inside of notifyCallback function in the controller. -> didn't work
• wrapping $scope.$apply on $scope.sysHeapMemoryPercent -> $digest error
• using scope.watch in the directive instead of attrs.$observe -> had a same result as attrs.$observe method

How can I solve this problem? Do I need to use a closure?

Controller

'use strict';
angular.module('adminControllers')
.controller('pollerController', ['$scope', '$resource', 'poller', function ($scope, $resource, poller) {

  $scope.sysHeapMemoryPercent=0;

  // ......
  // Some codes that makes polling API calls every second
  // using $resource and Angular Poller
  // ......

  myPoller.promise.then({}, {}, notifyCallback); 

  function notifyCallback(data){
    $scope.sysHeapMemoryPercent = Number(data[0].value.used) / Number(data[0].value.max);
    }
}]);

Directive

'use strict';
angular.module('adminControllers')
 .directive('sysHeapMemoryDirective', function ($timeout) {

 return {
 restrict: 'AE',
 link: function (scope, element, attrs) {
    var context = cubism.context().serverDelay(0).clientDelay(0).step(1000).size(594);


  attrs.$observe('memory', function (newVal,oldVal) {
    console.log(oldVal); // prints undefined every second
    console.log(newVal); // prints the correct value every second

    var constantNum = 0.123;
    compute(constNum); // works
    compute(newVal); // Doesn't work. Some child values are returned as null
  });


  function compute(val) {
    var value = 0, values = [],
    i = 0, last; 

    console.log(val); // prints the correct value every second

    return context.metric(function (start, stop, step, callback) {
      start = +start, stop = +stop;

      console.log(val); // prints 0 every second

      if (isNaN(last)) last = start;
      while (last < stop) {
        last += step;
        values.push(val);
      }
      callback(null, values = values.slice((start - stop) / step));
    }, "Demo");
  }
}
};
});

HTML

<div ng-controller="pollerController">
 {{sysHeapMemoryPercent}}  // successfully shows the updated value every second
   <sys-heap-memory-directive memory="{{sysHeapMemoryPercent}}"> </sys-heap-memory-directive>
</div>