text: import time
  import threading
  import cv2
  try:
      from greenlet import getcurrent as get_ident
  except ImportError:
      try:
          from thread import get_ident
      except ImportError:
          from _thread import get_ident
  
  
  class CameraEvent(object):
      """An Event-like class that signals all active clients when a new frame is
      available.
      """
      def __init__(self):
          self.events = {}
  
      def wait(self):
          """Invoked from each client's thread to wait for the next frame."""
          ident = get_ident()
          if ident not in self.events:
              # this is a new client
              # add an entry for it in the self.events dict
              # each entry has two elements, a threading.Event() and a timestamp
              self.events[ident] = [threading.Event(), time.time()]
          return self.events[ident][0].wait()
  
      def set(self):
          """Invoked by the camera thread when a new frame is available."""
          now = time.time()
          remove = None
          for ident, event in self.events.items():
              if not event[0].isSet():
                  # if this client's event is not set, then set it
                  # also update the last set timestamp to now
                  event[0].set()
                  event[1] = now
              else:
                  # if the client's event is already set, it means the client
                  # did not process a previous frame
                  # if the event stays set for more than 5 seconds, then assume
                  # the client is gone and remove it
                  if now - event[1] > 5:
                      remove = ident
          if remove:
              del self.events[remove]
  
      def clear(self):
          """Invoked from each client's thread after a frame was processed."""
          self.events[get_ident()][0].clear()
  
  
  class BaseCamera(object):
      thread = None  # background thread that reads frames from camera
      frame = None  # current frame is stored here by background thread
      last_access = 0  # time of last client access to the camera
      event = CameraEvent()
  
      def __init__(self):
          """Start the background camera thread if it isn't running yet."""
          if BaseCamera.thread is None:
              BaseCamera.last_access = time.time()
  
              # start background frame thread
              BaseCamera.thread = threading.Thread(target=self._thread)
              BaseCamera.thread.start()
  
              # wait until frames are available
              while self.get_frame() is None:
                  time.sleep(0)
  
      def get_frame(self):
          """Return the current camera frame."""
          BaseCamera.last_access = time.time()
  
          # wait for a signal from the camera thread
          BaseCamera.event.wait()
          BaseCamera.event.clear()
  
          return BaseCamera.frame
  
      @staticmethod
      def frames():
          """"Generator that returns frames from the camera."""
          raise RuntimeError('Must be implemented by subclasses.')
  
      @classmethod
      def _thread(cls):
          """Camera background thread."""
          print('Starting camera thread.')
          frames_iterator = cls.frames()
          for frame in frames_iterator:
              BaseCamera.frame = frame
              BaseCamera.event.set()  # send signal to clients
              time.sleep(0)
  
              # if there hasn't been any clients asking for frames in
              # the last 10 seconds then stop the thread
              # if time.time() - BaseCamera.last_access > 10:
              #     frames_iterator.close()
              #     print('Stopping camera thread due to inactivity.')
              #     break
          BaseCamera.thread = None
  