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Fixed #11441 -- Improved signal topic guide, particularly regarding the weak and dispatch_uid parameters to the Signal.connect method. Thanks to Mike_A and sayane for the report, and gremmie for the draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14337 bcc190cf-cafb-0310-a4f2-bffc1f526a37
This commit is contained in:
Gabriel Hurley 2010-10-24 08:52:00 +00:00
parent 6737570aeb
commit e4c0c377dd

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@ -46,15 +46,38 @@ You can also `define and send your own custom signals`_; see below.
Listening to signals
====================
To receive a signal, you need to register a *receiver* function that gets called
when the signal is sent. Let's see how this works by registering a signal that
To receive a signal, you need to register a *receiver* function that gets
called when the signal is sent by using the
:meth:`.Signal.connect` method:
.. method:: Signal.connect(receiver, [sender=None, weak=True, dispatch_uid=None])
:param receiver: The callback function which will be connected to this
signal. See :ref:`receiver-functions` for more information.
:param sender: Specifies a particular sender to receive signals from. See
:ref:`connecting-to-specific-signals` for more information.
:param weak: Django stores signal handlers as weak references by
default. Thus, if your receiver is a local function, it may be
garbage collected. To prevent this, pass ``weak=False`` when you call
the signal's ``connect()`` method.
:param dispatch_uid: A unique identifier for a signal receiver in cases
where duplicate signals may be sent. See
:ref:`preventing-duplicate-signals` for more information.
Let's see how this works by registering a signal that
gets called after each HTTP request is finished. We'll be connecting to the
:data:`~django.core.signals.request_finished` signal.
.. _receiver-functions:
Receiver functions
------------------
First, we need to define a receiver function. A receiver can be any Python function or method:
First, we need to define a receiver function. A receiver can be any Python
function or method:
.. code-block:: python
@ -77,6 +100,8 @@ This would be wrong -- in fact, Django will throw an error if you do so. That's
because at any point arguments could get added to the signal and your receiver
must be able to handle those new arguments.
.. _connecting-receiver-functions:
Connecting receiver functions
-----------------------------
@ -115,6 +140,8 @@ The ``receiver`` decorator was added in Django 1.3.
to be sent. This makes your app's ``models.py`` a good place to put
registration of signal handlers.
.. _connecting-to-specific-signals:
Connecting to signals sent by specific senders
----------------------------------------------
@ -146,10 +173,34 @@ Different signals use different objects as their senders; you'll need to consult
the :doc:`built-in signal documentation </ref/signals>` for details of each
particular signal.
.. _preventing-duplicate-signals:
Preventing duplicate signals
----------------------------
In some circumstances, the module in which you are connecting signals may be
imported multiple times. This can cause your receiver function to be
registered more than once, and thus called multiples times for a single signal
event.
If this behavior is problematic (such as when using signals to
send an e-mail whenever a model is saved), pass a unique identifier as
the ``dispatch_uid`` argument to identify your receiver function. This
identifier will usually be a string, although any hashable object will
suffice. The end result is that your receiver function will only be
bound to the signal once for each unique ``dispatch_uid`` value.
.. code-block:: python
from django.core.signals import request_finished
request_finished.connect(my_callback, dispatch_uid="my_unique_identifier")
Defining and sending signals
============================
Your applications can take advantage of the signal infrastructure and provide its own signals.
Your applications can take advantage of the signal infrastructure and provide
its own signals.
Defining signals
----------------
@ -176,9 +227,14 @@ Remember that you're allowed to change this list of arguments at any time, so ge
Sending signals
---------------
.. method:: Signal.send(sender, **kwargs)
There are two ways to send send signals in Django.
To send a signal, call :meth:`Signal.send`. You must provide the ``sender`` argument, and may provide as many other keyword arguments as you like.
.. method:: Signal.send(sender, **kwargs)
.. method:: Signal.send_robust(sender, **kwargs)
To send a signal, call either :meth:`Signal.send` or :meth:`Signal.send_robust`.
You must provide the ``sender`` argument, and may provide as many other keyword
arguments as you like.
For example, here's how sending our ``pizza_done`` signal might look:
@ -191,4 +247,26 @@ For example, here's how sending our ``pizza_done`` signal might look:
pizza_done.send(sender=self, toppings=toppings, size=size)
...
Both ``send()`` and ``send_robust()`` return a list of tuple pairs
``[(receiver, response), ... ]``, representing the list of called receiver
functions and their response values.
``send()`` differs from ``send_robust()`` in how exceptions raised by receiver
functions are handled. ``send()`` does *not* catch any exceptions raised by
receivers; it simply allows errors to propagate. Thus not all receivers may
be notified of a signal in the face of an error.
``send_robust()`` catches all errors derived from Python's ``Exception`` class,
and ensures all receivers are notified of the signal. If an error occurs, the
error instance is returned in the tuple pair for the receiver that raised the error.
Disconnecting signals
=====================
.. method:: Signal.disconnect([receiver=None, sender=None, weak=True, dispatch_uid=None])
To disconnect a receiver from a signal, call :meth:`Signal.disconnect`. The
arguments are as described in :meth:`.Signal.connect`.
The *receiver* argument indicates the registered receiver to disconnect. It may
be ``None`` if ``dispatch_uid`` is used to identify the receiver.