function interval(fn, delay, count, invokeApply) {
      var setInterval = $window.setInterval,
          clearInterval = $window.clearInterval,
          iteration = 0,
          skipApply = (isDefined(invokeApply) && !invokeApply),
          deferred = (skipApply ? $$q : $q).defer(),
          promise = deferred.promise;

      count = isDefined(count) ? count : 0;

      promise.then(null, null, fn);

      promise.$$intervalId = setInterval(function tick() {
        deferred.notify(iteration++);

        if (count > 0 && iteration >= count) {
          deferred.resolve(iteration);
          clearInterval(promise.$$intervalId);
          delete intervals[promise.$$intervalId];
        }

        if (!skipApply) $rootScope.$apply();

      }, delay);

      intervals[promise.$$intervalId] = deferred;

      return promise;
    }
@ngdoc service
@name $interval

@description
Angular's wrapper for `window.setInterval`. The `fn` function is executed every `delay`
milliseconds.

The return value of registering an interval function is a promise. This promise will be
notified upon each tick of the interval, and will be resolved after `count` iterations, or
run indefinitely if `count` is not defined. The value of the notification will be the
number of iterations that have run.
To cancel an interval, call `$interval.cancel(promise)`.

In tests you can use {@link ngMock.$interval#flush `$interval.flush(millis)`} to
move forward by `millis` milliseconds and trigger any functions scheduled to run in that
time.

<div class="alert alert-warning">
**Note**: Intervals created by this service must be explicitly destroyed when you are finished
with them.  In particular they are not automatically destroyed when a controller's scope or a
directive's element are destroyed.
You should take this into consideration and make sure to always cancel the interval at the
appropriate moment.  See the example below for more details on how and when to do this.
</div>

@param {function()} fn A function that should be called repeatedly.
@param {number} delay Number of milliseconds between each function call.
@param {number=} [count=0] Number of times to repeat. If not set, or 0, will repeat
  indefinitely.
@param {boolean=} [invokeApply=true] If set to `false` skips model dirty checking, otherwise
  will invoke `fn` within the {@link ng.$rootScope.Scope#$apply $apply} block.
@returns {promise} A promise which will be notified on each iteration.

@example
<example module="intervalExample">
<file name="index.html">
  <script>
    angular.module('intervalExample', [])
      .controller('ExampleController', ['$scope', '$interval',
        function($scope, $interval) {
          $scope.format = 'M/d/yy h:mm:ss a';
          $scope.blood_1 = 100;
          $scope.blood_2 = 120;

          var stop;
          $scope.fight = function() {
            // Don't start a new fight if we are already fighting
            if ( angular.isDefined(stop) ) return;

            stop = $interval(function() {
              if ($scope.blood_1 > 0 && $scope.blood_2 > 0) {
                $scope.blood_1 = $scope.blood_1 - 3;
                $scope.blood_2 = $scope.blood_2 - 4;
              } else {
                $scope.stopFight();
              }
            }, 100);
          };

          $scope.stopFight = function() {
            if (angular.isDefined(stop)) {
              $interval.cancel(stop);
              stop = undefined;
            }
          };

          $scope.resetFight = function() {
            $scope.blood_1 = 100;
            $scope.blood_2 = 120;
          };

          $scope.$on('$destroy', function() {
            // Make sure that the interval is destroyed too
            $scope.stopFight();
          });
        }])
      // Register the 'myCurrentTime' directive factory method.
      // We inject $interval and dateFilter service since the factory method is DI.
      .directive('myCurrentTime', ['$interval', 'dateFilter',
        function($interval, dateFilter) {
          // return the directive link function. (compile function not needed)
          return function(scope, element, attrs) {
            var format,  // date format
                stopTime; // so that we can cancel the time updates

            // used to update the UI
            function updateTime() {
              element.text(dateFilter(new Date(), format));
            }

            // watch the expression, and update the UI on change.
            scope.$watch(attrs.myCurrentTime, function(value) {
              format = value;
              updateTime();
            });

            stopTime = $interval(updateTime, 1000);

            // listen on DOM destroy (removal) event, and cancel the next UI update
            // to prevent updating time after the DOM element was removed.
            element.on('$destroy', function() {
              $interval.cancel(stopTime);
            });
          }
        }]);
  </script>

  <div>
    <div ng-controller="ExampleController">
      Date format: <input ng-model="format"> <hr/>
      Current time is: <span my-current-time="format"></span>
      <hr/>
      Blood 1 : <font color='red'>{{blood_1}}</font>
      Blood 2 : <font color='red'>{{blood_2}}</font>
      <button type="button" data-ng-click="fight()">Fight</button>
      <button type="button" data-ng-click="stopFight()">StopFight</button>
      <button type="button" data-ng-click="resetFight()">resetFight</button>
    </div>
  </div>

</file>
</example>