django之signal机制再探

djangobb中的signal post_save信号调用send函数时，为什么它会对与topic、post相关的其他models进行修改？同一个信号，例如post_save（保存过后的处理），是所有model共用的，当调用send时，它怎么找到相关model的reciever？

信号在连接receiver时，会按dispatch_uuid与receiver生成一个元组后作为[]的一项

import weakref
import threading

from django.dispatch import saferef

WEAKREF_TYPES = (weakref.ReferenceType, saferef.BoundMethodWeakref)

def _make_id(target):#获得发送者的id
    if hasattr(target, 'im_func'):
        return (id(target.im_self), id(target.im_func))
    return id(target)

class Signal(object):
    """
    Base class for all signals

    Internal attributes:

        receivers
            { receriverkey (id) : weakref(receiver) }
    """

    def __init__(self, providing_args=None):#信号对象的成员，每个信号都有一个lock
        """
        Create a new signal.

        providing_args
            A list of the arguments this signal can pass along in a send() call.
        """
        self.receivers = []
        if providing_args is None:
            providing_args = []
        self.providing_args = set(providing_args)
        self.lock = threading.Lock()

    def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
        """
        Connect receiver to sender for signal.

        Arguments:

            receiver
                A function or an instance method which is to receive signals.
                Receivers must be hashable objects.

                If weak is True, then receiver must be weak-referencable (more
                precisely saferef.safeRef() must be able to create a reference
                to the receiver).

                Receivers must be able to accept keyword arguments.

                If receivers have a dispatch_uid attribute, the receiver will
                not be added if another receiver already exists with that
                dispatch_uid.

            sender
                The sender to which the receiver should respond. Must either be
                of type Signal, or None to receive events from any sender.

            weak
                Whether to use weak references to the receiver. By default, the
                module will attempt to use weak references to the receiver
                objects. If this parameter is false, then strong references will
                be used.

            dispatch_uid
                An identifier used to uniquely identify a particular instance of
                a receiver. This will usually be a string, though it may be
                anything hashable.
        """
        from django.conf import settings

        # If DEBUG is on, check that we got a good receiver
        if settings.DEBUG:
            import inspect
            assert callable(receiver), "Signal receivers must be callable."

            # Check for **kwargs
            # Not all callables are inspectable with getargspec, so we'll
            # try a couple different ways but in the end fall back on assuming
            # it is -- we don't want to prevent registration of valid but weird
            # callables.
            try:
                argspec = inspect.getargspec(receiver)
            except TypeError:
                try:
                    argspec = inspect.getargspec(receiver.__call__)
                except (TypeError, AttributeError):
                    argspec = None
            if argspec:
                assert argspec[2] is not None, \
                    "Signal receivers must accept keyword arguments (**kwargs)."

        if dispatch_uid:
            lookup_key = (dispatch_uid, _make_id(sender))
        else:
            lookup_key = (_make_id(receiver), _make_id(sender))

        if weak:
            receiver = saferef.safeRef(receiver, onDelete=self._remove_receiver)

        self.lock.acquire()
        try:
            for r_key, _ in self.receivers:
                if r_key == lookup_key:
                    break
            else:
                self.receivers.append((lookup_key, receiver))
        finally:
            self.lock.release()

    def disconnect(self, receiver=None, sender=None, weak=True, dispatch_uid=None):
        """
        Disconnect receiver from sender for signal.

        If weak references are used, disconnect need not be called. The receiver
        will be remove from dispatch automatically.

        Arguments:

            receiver
                The registered receiver to disconnect. May be none if
                dispatch_uid is specified.

            sender
                The registered sender to disconnect

            weak
                The weakref state to disconnect

            dispatch_uid
                the unique identifier of the receiver to disconnect
        """
        if dispatch_uid:
            lookup_key = (dispatch_uid, _make_id(sender))
        else:
            lookup_key = (_make_id(receiver), _make_id(sender))

        self.lock.acquire()
        try:
            for index in xrange(len(self.receivers)):
                (r_key, _) = self.receivers[index]
                if r_key == lookup_key:
                    del self.receivers[index]
                    break
        finally:
            self.lock.release()

    def send(self, sender, **named):
        """
        Send signal from sender to all connected receivers.

        If any receiver raises an error, the error propagates back through send,
        terminating the dispatch loop, so it is quite possible to not have all
        receivers called if a raises an error.

        Arguments:

            sender
                The sender of the signal Either a specific object or None.

            named
                Named arguments which will be passed to receivers.

        Returns a list of tuple pairs [(receiver, response), ... ].
        """
        responses = []
        if not self.receivers:
            return responses

        for receiver in self._live_receivers(_make_id(sender)):
            response = receiver(signal=self, sender=sender, **named)#调用接收器函数，接收器函数必须按参数
                                                                     顺序
            responses.append((receiver, response))
        return responses

    def send_robust(self, sender, **named):
        """
        Send signal from sender to all connected receivers catching errors.

        Arguments:

            sender
                The sender of the signal. Can be any python object (normally one
                registered with a connect if you actually want something to
                occur).

            named
                Named arguments which will be passed to receivers. These
                arguments must be a subset of the argument names defined in
                providing_args.

        Return a list of tuple pairs [(receiver, response), ... ]. May raise
        DispatcherKeyError.

        If any receiver raises an error (specifically any subclass of
        Exception), the error instance is returned as the result for that
        receiver.
        """
        responses = []
        if not self.receivers:
            return responses

        # Call each receiver with whatever arguments it can accept.
        # Return a list of tuple pairs [(receiver, response), ... ].
        for receiver in self._live_receivers(_make_id(sender)):
            try:
                response = receiver(signal=self, sender=sender, **named)
            except Exception, err:
                responses.append((receiver, err))
            else:
                responses.append((receiver, response))
        return responses

    def _live_receivers(self, senderkey):
        """
        Filter sequence of receivers to get resolved, live receivers.

        This checks for weak references and resolves them, then returning only
        live receivers.
        """
        none_senderkey = _make_id(None)
        receivers = []

        for (receiverkey, r_senderkey), receiver in self.receivers:
            if r_senderkey == none_senderkey or r_senderkey == senderkey:
                if isinstance(receiver, WEAKREF_TYPES):
                    # Dereference the weak reference.
                    receiver = receiver()
                    if receiver is not None:
                        receivers.append(receiver)
                else:
                    receivers.append(receiver)
        return receivers

    def _remove_receiver(self, receiver):
        """
        Remove dead receivers from connections.
        """

        self.lock.acquire()
        try:
            to_remove = []
            for key, connected_receiver in self.receivers:
                if connected_receiver == receiver:
                    to_remove.append(key)
            for key in to_remove:
                last_idx = len(self.receivers) - 1
                # enumerate in reverse order so that indexes are valid even
                # after we delete some items
                for idx, (r_key, _) in enumerate(reversed(self.receivers)):
                    if r_key == key:
                        del self.receivers[last_idx-idx]
        finally:
            self.lock.release()

def receiver(signal, **kwargs):
    """
    A decorator for connecting receivers to signals. Used by passing in the
    signal and keyword arguments to connect::

        @receiver(post_save, sender=MyModel)
        def signal_receiver(sender, **kwargs):
            ...

    """
    def _decorator(func):
        signal.connect(func, **kwargs)
        return func
    return _decorator

@receiver(post_save, sender=Topic, dispatch_uid='djangobb_topic_save')
def topic_saved(instance, **kwargs):
    topic = instance
    forum = topic.forum
    forum.topic_count = forum.topics.count()
    forum.updated = topic.updated
    forum.post_count = forum.posts.count()
    forum.last_post_id = topic.last_post_id
    forum.save(force_update=True)