subscribers.py

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from threading import Lock
from rospy import Subscriber, logerr
from rostopic import get_topic_type
from rosbridge_library.internal import ros_loader, message_conversion
from rosbridge_library.internal.topics import TopicNotEstablishedException
from rosbridge_library.internal.topics import TypeConflictException

""" Manages and interfaces with ROS Subscriber objects.  A single subscriber
is shared between multiple clients
"""


class MultiSubscriber():
    """ Handles multiple clients for a single subscriber.

    Converts msgs to JSON before handing them to callbacks.  Due to subscriber
    callbacks being called in separate threads, must lock whenever modifying
    or accessing the subscribed clients. """

    def __init__(self, topic, msg_type=None):
        """ Register a subscriber on the specified topic.

        Keyword arguments:
        topic    -- the name of the topic to register the subscriber on
        msg_type -- (optional) the type to register the subscriber as.  If not
        provided, an attempt will be made to infer the topic type

        Throws:
        TopicNotEstablishedException -- if no msg_type was specified by the
        caller and the topic is not yet established, so a topic type cannot
        be inferred
        TypeConflictException        -- if the msg_type was specified by the
        caller and the topic is established, and the established type is
        different to the user-specified msg_type

        """
        # First check to see if the topic is already established
        topic_type = get_topic_type(topic)[0]

        # If it's not established and no type was specified, exception
        if msg_type is None and topic_type is None:
            raise TopicNotEstablishedException(topic)

        # Use the established topic type if none was specified
        if msg_type is None:
            msg_type = topic_type

        # Load the message class, propagating any exceptions from bad msg types
        msg_class = ros_loader.get_message_class(msg_type)

        # Make sure the specified msg type and established msg type are same
        if topic_type is not None and topic_type != msg_class._type:
            raise TypeConflictException(topic, topic_type, msg_class._type)

        # Create the subscriber and associated member variables
        self.subscriptions = {}
        self.lock = Lock()
        self.topic = topic
        self.msg_class = msg_class
        self.subscriber = Subscriber(topic, msg_class, self.callback)

    def unregister(self):
        self.subscriber.unregister()
        with self.lock:
            self.subscriptions.clear()

    def verify_type(self, msg_type):
        """ Verify that the subscriber subscribes to messages of this type.

        Keyword arguments:
        msg_type -- the type to check this subscriber against

        Throws:
        Exception -- if ros_loader cannot load the specified msg type
        TypeConflictException -- if the msg_type is different than the type of
        this publisher

        """
        if not ros_loader.get_message_class(msg_type) is self.msg_class:
            raise TypeConflictException(self.topic,
                                        self.msg_class._type, msg_type)
        return

    def subscribe(self, client_id, callback):
        """ Subscribe the specified client to this subscriber.

        Keyword arguments:
        client_id -- the ID of the client subscribing
        callback  -- this client's callback, that will be called for incoming
        messages

        """
        with self.lock:
            self.subscriptions[client_id] = callback
            # If the topic is latched, add_callback will immediately invoke
            # the given callback.
            self.subscriber.impl.add_callback(self.callback, [callback])
            self.subscriber.impl.remove_callback(self.callback, [callback])

    def unsubscribe(self, client_id):
        """ Unsubscribe the specified client from this subscriber

        Keyword arguments:
        client_id -- the ID of the client to unsubscribe

        """
        with self.lock:
            del self.subscriptions[client_id]

    def has_subscribers(self):
        """ Return true if there are subscribers """
        with self.lock:
            ret = len(self.subscriptions) != 0
            return ret

    def callback(self, msg, callbacks=None):
        """ Callback for incoming messages on the rospy.Subscriber

        Converts the incoming msg to JSON, then passes the JSON to the
        registered subscriber callbacks.

        Keyword Arguments:
        msg - the ROS message coming from the subscriber
        callbacks - subscriber callbacks to invoke

        """
        # Try to convert the msg to JSON
        json = None
        try:
            json = message_conversion.extract_values(msg)
        except Exception as exc:
            logerr("Exception while converting messages in subscriber callback : %s", exc)
            return
        
        # Get the callbacks to call
        if not callbacks:
            with self.lock:
                callbacks = self.subscriptions.values()

        # Pass the JSON to each of the callbacks
        for callback in callbacks:
            try:
                callback(json)
            except Exception as exc:
                # Do nothing if one particular callback fails except log it
                logerr("Exception calling subscribe callback: %s", exc)
                pass


class SubscriberManager():
    """
    Keeps track of client subscriptions
    """

    def __init__(self):
        self._subscribers = {}

    def subscribe(self, client_id, topic, callback, msg_type=None):
        """ Subscribe to a topic

        Keyword arguments:
        client_id -- the ID of the client making this subscribe request
        topic     -- the name of the topic to subscribe to
        callback  -- the callback to call for incoming messages on the topic
        msg_type  -- (optional) the type of the topic

        """
        if not topic in self._subscribers:
            self._subscribers[topic] = MultiSubscriber(topic, msg_type)

        if msg_type is not None:
            self._subscribers[topic].verify_type(msg_type)

        self._subscribers[topic].subscribe(client_id, callback)

    def unsubscribe(self, client_id, topic):
        """ Unsubscribe from a topic

        Keyword arguments:
        client_id -- the ID of the client to unsubscribe
        topic     -- the topic to unsubscribe from

        """
        if not topic in self._subscribers:
            return

        self._subscribers[topic].unsubscribe(client_id)

        if not self._subscribers[topic].has_subscribers():
            self._subscribers[topic].unregister()
            del self._subscribers[topic]


manager = SubscriberManager()