code: def _depth_image_callback(self, image_msg):
          
          encoding = image_msg.encoding
          try:
              depth_arr = self._bridge.imgmsg_to_cv2(image_msg, encoding)
              import pdb; pdb.set_trace()
  
          except CvBridgeError as e:
              rospy.logerr(e)
          depth = np.array(depth_arr*MM_TO_METERS, np.float32)
          self._cur_depth_im = DepthImage(depth, self._frame)
docs: subscribe to depth image topic and keep it up to date