angular 倒计时 只第一次暂停有效

效果：http://runjs.cn/detail/iyvy6dl7

打开后，第一次点stop是生效的，但开始后再点stop就无效了。

  <div ng-controller="ListController">
        {{ t }}
        <p></p>
        <button type="button" ng-click="stop()">stop</button>
        <button type="button" ng-click="start()">start</button>
  </div>

    'use strict';
    var myModule = angular.module("myModule",[])
    .controller('ListController', ['$interval','$scope', function($interval, $scope){

        var t, isPause, timePromise;

        $scope.t = 100;  

        var runTiming = function(){
          timePromise = $interval(function(){
              $scope.t -= 1;
          }, 1000, 100);
        }

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

        $scope.start = function(){
          if(angular.isDefined(timePromise)) return;
            timePromise = runTiming();
            $scope.isPause = false;
        }

        runTiming();  
      
    }])

---

var runTiming = function(){
          timePromise = $interval(function(){
              $scope.t -= 1;
          }, 1000, 100);
          return timePromise;
        }