code: def update_time(self, ev):
      t = ptime.time()
      self._time += t - self._last_time
      self._last_time = t
      self.shared_program["time"] = self._time
      self.update()
  
file_path: vispy/examples/demo/scene/flow_lines.py
imports: from __future__ import division
  from vispy import app, scene, visuals, gloo
  from vispy.util import ptime
  import numpy
label_desc: Use image node_id 5 for calling the VectorFieldVisual obj's underlying member method code with example usage: obj.update_time(ev) without return types