_on_message函数

该函数处理来与thingsbaord消息，主要流程如下：

1. 消息解码：

   content = TBUtility.decode(message)

2. 调用父类的_on_decoded_message函数

3. 调用自己类的_on_decoded_message函数

父类（网关设备）的_on_decoded_message函数，主要流程如下：
四种类型的消息处理：

1. “v1/devices/me/rpc/request/“

   request_id = message.topic[len(RPC_REQUEST_TOPIC):len(message.topic)]
    if self.__device_on_server_side_rpc_response:
        # __device_on_server_side_rpc_response回调是gatewayservice设置的
        self.__device_on_server_side_rpc_response(request_id, content)

2. “v1/devices/me/rpc/response/“

   with self._lock:
    # __device_client_rpc_dict字典里保存了客户端远程调用服务端的request_id以及回调函数
    request_id = int(message.topic[len(RPC_RESPONSE_TOPIC):len(message.topic)])
    callback = self.__device_client_rpc_dict.pop(request_id)
   callback(request_id, content, None)

3. “v1/devices/me/attributes”

todo…

1. “v1/devices/me/attributes/response/“

   with self._lock:
    # _attr_request_dict字典里保存了客户端获取服务端的属性的request_id以及回调函数
    req_id = int(message.topic[len(ATTRIBUTES_TOPIC + "/response/"):])
    callback = self._attr_request_dict.pop(req_id)
   callback(content, None)

   自己类（网关客户端）的_on_decoded_message函数，主要流程如下：
   同上，三种类型的消息处理：

2. “v1/gateway/attributes/response”

   with self._lock:
    req_id = content["id"]
    # pop callback and use it
    if self._attr_request_dict[req_id]:
        self._attr_request_dict.pop(req_id)(content, None)

3. “v1/gateway/attributes”

todo…

1. “v1/gateway/rpc”

   if self.devices_server_side_rpc_request_handler:
    self.devices_server_side_rpc_request_handler(self, content)