message_loader_thread.py

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import threading


class MessageLoaderThread(threading.Thread):
    """
    Waits for a new playhead position on the given topic, then loads the message at that position and notifies the view threads.

    One thread per topic.  Maintains a cache of recently loaded messages.
    """
    def __init__(self, timeline, topic):
        threading.Thread.__init__(self)

        self.timeline = timeline
        self.topic = topic

        self.topic_playhead_position = None

        self._message_cache_capacity = 50
        self._message_cache = {}
        self._message_cache_keys = []

        self._stop_flag = False

        self.setDaemon(True)
        self.start()

    def reset(self):
        self.bag_playhead_position = None

    def run(self):
        while not self._stop_flag:
            # Wait for a new entry
            cv = self.timeline._playhead_positions_cvs[self.topic]
            with cv:
                while (self.topic not in self.timeline._playhead_positions) or (self.topic_playhead_position == self.timeline._playhead_positions[self.topic]):
                    cv.wait()
                    if self._stop_flag:
                        return
                playhead_position = self.timeline._playhead_positions[self.topic]

            self.topic_playhead_position = playhead_position

            # Don't bother loading the message if there are no listeners
            if not self.timeline.has_listeners(self.topic):
                continue

            # Load the message
            if playhead_position is None:
                msg_data = None
            else:
                msg_data = self._get_message(playhead_position)

            # Inform the views
            messages_cv = self.timeline._messages_cvs[self.topic]
            with messages_cv:
                self.timeline._messages[self.topic] = msg_data
                messages_cv.notify_all()      # notify all views that a message is loaded

    def _get_message(self, position):
        key = str(position)
        if key in self._message_cache:
            return self._message_cache[key]

        msg_data = self.timeline.read_message(self.topic, position)

        self._message_cache[key] = msg_data
        self._message_cache_keys.append(key)

        if len(self._message_cache) > self._message_cache_capacity:
            oldest_key = self._message_cache_keys[0]
            del self._message_cache[oldest_key]
            self._message_cache_keys.remove(oldest_key)

        return msg_data

    def stop(self):
        self._stop_flag = True
        cv = self.timeline._playhead_positions_cvs[self.topic]
        with cv:
            cv.notify_all()