from multiprocessing import Process, Queue


class BaseCamera(object):
    process = {} # background process that reads frames from camera
    frame = {} # frame queue
    last_frame = {}

    def __init__(self, device):
        """Start the background camera process if it isn't running yet."""
        self.device = device
        BaseCamera.last_frame[self.device] = None

        if self.device not in BaseCamera.process:
            BaseCamera.process[self.device] = None

        if BaseCamera.process[self.device] is None:

            BaseCamera.frame[device] = Queue(100)

            # start background frame process
            BaseCamera.process[self.device] = Process(target=self._process, args=(self.device))
            BaseCamera.process[self.device].start()

            # wait until frames are available
            BaseCamera.last_frame[self.device] = self.get_frame()

    def get_frame(self):
        """Return the current camera frame."""

        if  BaseCamera.last_frame[self.device] is None:
            BaseCamera.last_frame[self.device] = BaseCamera.frame[self.device].get(block=True)
            return BaseCamera.last_frame[self.device]
        elif not BaseCamera.frame[self.device].empty():
            BaseCamera.last_frame[self.device] = BaseCamera.frame[self.device].get()
            return BaseCamera.last_frame[self.device]
        else:
            return BaseCamera.last_frame[self.device]

    def frames(self):
        """"Generator that returns frames from the camera."""
        raise NotImplementedError('Must be implemented by subclasses.')

    def _process(self, device):
        """Camera background process."""
        frames_iterator = self.frames()
        for frame in frames_iterator:
            BaseCamera.frame[device].put(frame, block=True)

        BaseCamera.process[device] = None