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- import time
- import threading
- try:
- from greenlet import getcurrent as get_ident
- except ImportError:
- try:
- from threading 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
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