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--- fw:djangoadv [2014/06/02 11:35]
alfred [Customizing authorization]
+++ fw:djangoadv [2020/05/09 09:25] (actual)
@@ Línea 59: / Línea 59: @@
 ===== Signals =====
+==== Signals in Django ====
+Django provides a way of intercept special notifications...
+  * ''django.db.models.signals.pre_save'' & ''django.db.models.signals.post_save'', sent before or after a model’s save() method is called.
+  * ''django.db.models.signals.pre_delete'' & ''django.db.models.signals.post_delete'', sent before or after a model’s delete() method or queryset’s delete() method is called.
+  * ''django.db.models.signals.m2m_changed'', sent when a ManyToManyField on a model is changed.
+  * ''django.core.signals.request_started'' & ''django.core.signals.request_finished'', sent when Django starts or finishes an HTTP request.
+There are [[https://docs.djangoproject.com/en/dev/ref/signals/|ohters]] like ''pre_init'', ''class_prepared'', ''pre_migrate'', ''pre_syncdb'', ''got_request_exception'', ''connection_created''...
+==== Connecting signals ====
+The way to intercept a signal is using the ''connect()'' function or the ''receiver()'' decorator:
+<code python>
+def my_callback(sender, **kwargs):
+    print("Request finished!")
+# .......
+from django.core.signals import request_finished
+request_finished.connect(my_callback)
+# .......
+from django.core.signals import request_finished
+from django.dispatch import receiver
+@receiver(request_finished)
+def my_callback(sender, **kwargs):
+    print("Request finished!")
+</code>
+The function ''Signal.connect'' has the next argument:
+<code python>
+Signal.connect(receiver[, sender=None, weak=True, dispatch_uid=None])
+</code>
+Its arguments mean:
+  * Receiver argument is a function defined as: ''def my_callback(sender, <nowiki>**kwargs</nowiki>)''.
+  * Sender argument concretes a sender class.
+  * Weak argument avoid Django storing signal handlers as weak references, which is done 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.
+  * dispatch_uid, its an unique identifier for a signal receiver in cases where duplicate signals may be sent.
+<code python>
+from myapp.models import MyModel
+@receiver(pre_save, sender=MyModel)
+def my_handler(sender, **kwargs):
+...
+request_finished.connect(my_callback, dispatch_uid="my_unique_identifier")
+</code>
+==== Defining and sending signals ====
+Signals are ''django.dispatch.Signal([providing_args=list])'' instances. ''providing_args'' is a list of the names of arguments the signal will provide to listeners. To declare a ''pizza_done'' signal which provides receivers with toppics and size arguments:
+<code python>
+import django.dispatch
+pizza_done = django.dispatch.Signal(providing_args=["toppings", "size"])
+</code>
+To send a signal, call either ''Signal.send()'' or ''Signal.send_robust()''. Both return a list of tuple pairs ''[(receiver, response), ... ]''. They differ in how exceptions raised by receiver functions. ''send()'' does not catch any exceptions raised by receivers; it simply allows errors to propagate. Whereas ''send_robust()'' catches all errors derived from Python’s Exception class, and ensures all receivers are notified of the signal.
+<code python>
+class PizzaStore(object):
+    ...
+    def send_pizza(self, toppings, size):
+        pizza_done.send(sender=self.__class__, toppings=toppings, size=size)
+</code>
+To disconnect a receiver from a signal, call ''Signal.disconnect()''.

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