AngularJS：将函数以及范围传递给工厂

Question

我正在用angularjs工厂实现websockets。

angular.
module('core.socket').
factory('Socket', ['$rootScope', function($rootScope){
    var ws;
    var stack = [];
    var onmessageDefer;
    function socket(url){
      ws = new WebSocket(url);
      ws.onopen = function(event) {
          for (i in stack) {
              ws.send(stack[i]);
          }
          stack = [];
          if (onmessageDefer) {
              ws.onmessage = onmessageDefer;
              onmessageDefer = null;
          }
      };
    }

    socket.prototype = {
      send: function(data) {
          data = JSON.stringify(data);
          if (ws.readyState == 1) {
              ws.send(data);
          } else {
              stack.push(data);
          }
      },
      onmessage: function(callback) {
          if (ws.readyState == 1) {
              ws.onmessage = callback;
          } else {
              onmessageDefer = callback;
          }
      }
    }

    return socket;
}]);

Socket工厂通过以下方式传递给控制器​​：

angular.
module('common').
controller('noteVBoxController', ['$scope', '$location','$element', 'Socket',
 function($scope, $location, $element, Socket){
  var self = this;
  self.socket = new Socket("ws://192.168.225.59:8010/ws/");
  $scope.submit = function(){
    self.socket.send({
      type: "place_note",
      data: {
        noteId: $scope.note.id
      }
    });
  };
  self.socket.onmessage(function(event) {
      var data = JSON.parse(event.data);
      $scope.note.top_note=data["message"];
  });

}]);

如您所见，回调函数已传递给工厂，该函数无法使用当前的$ scope。请帮忙。

主要问题是，如果我将此控制器与多个指令元素一起使用，则最后一个元素的作用域仅在onmessage函数内部可用。