正在使用$scope处理$interval.cancel。$watch不起作用

data.staus 是 complete

  angular.module('timerApp', ['timerApp.controllers']);
  angular.module('timerApp.controllers', []).controller('timerController', ['$scope', '$interval',
      function($scope, $interval) {
        var timer;
        var time = 10;
        $scope.countdown = time;
        $scope.stopTimer = function() 
        {
        $interval.cancel(timer);
         };



 $scope.startTimer = function() {
  timer = $interval(function() {
  $scope.countdown--;
//res is response from my backend
   var data = res;
   if (data.status = "complete"){
      $scope.timerFlag = true;
     }
    }, 15000, time);
         timer.then(function() {
        $scope.countdown = time;
      });
    };

  }

$scope.$watch(){
if($scope.timerFlag == true){
 $scope.stopTimer();

}

}
]);