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Name	Size	Mode	Actions
api/	-	0755	rm
api.rst.txt	237	0644	edit dl rm
configure.rst.txt	1052	0644	edit dl rm
event.rst.txt	3663	0644	edit dl rm
factory.rst.txt	3104	0644	edit dl rm
hacking.rst.txt	10562	0644	edit dl rm
hooks.rst.txt	2791	0644	edit dl rm
index.rst.txt	306	0644	edit dl rm
narr.rst.txt	12647	0644	edit dl rm
persistentregistry.rst.txt	207	0644	edit dl rm
socketexample.rst.txt	21767	0644	edit dl rm
testlayer.rst.txt	3445	0644	edit dl rm
zcml.rst.txt	33599	0644	edit dl rm

Edit: /usr/share/doc/python3-zope-component/html/_sources/event.rst.txt (3663B)

Events
======

The Component Architecture provides a way to dispatch events to event
handlers.  Event handlers are registered as *subscribers*
a.k.a. *handlers*.

Before we can start we need to import ``zope.component.event`` to make
the dispatching effective:

.. doctest::

>>> import zope.component.event

Consider two event classes:

.. doctest::

>>> class Event1(object):
   ...     pass

>>> class Event2(Event1):
   ...     pass

Now consider two handlers for these event classes:

.. doctest::

>>> called = []

>>> import zope.component
   >>> @zope.component.adapter(Event1)
   ... def handler1(event):
   ...     called.append(1)

>>> @zope.component.adapter(Event2)
   ... def handler2(event):
   ...     called.append(2)

We can register them with the Component Architecture:

.. doctest::

>>> zope.component.provideHandler(handler1)
   >>> zope.component.provideHandler(handler2)

Now let's go through the events.  We'll see that the handlers have been
called accordingly:

.. doctest::

>>> from zope.event import notify
   >>> notify(Event1())
   >>> called
   [1]

>>> del called[:]
   >>> notify(Event2())
   >>> called.sort()
   >>> called
   [1, 2]

Object events
-------------

The ``objectEventNotify`` function is a subscriber to dispatch
ObjectEvents to interested adapters.

First create an object class:

.. doctest::

>>> class IUseless(zope.interface.Interface):
   ...     """Useless object"""

>>> class UselessObject(object):
   ...     """Useless object"""
   ...     zope.interface.implements(IUseless)

Then create an event class:

.. doctest::

>>> class IObjectThrownEvent(zope.component.interfaces.IObjectEvent):
   ...     """An object has been thrown away"""

>>> class ObjectThrownEvent(zope.component.interfaces.ObjectEvent):
   ...     """An object has been thrown away"""
   ...     zope.interface.implements(IObjectThrownEvent)

Create an object and an event:

.. doctest::

>>> hammer = UselessObject()
   >>> event = ObjectThrownEvent(hammer)

Then notify the event to the subscribers.
Since the subscribers list is empty, nothing happens.

.. doctest::

>>> zope.component.event.objectEventNotify(event)

Now create an handler for the event:

.. doctest::

>>> events = []
   >>> def record(*args): #*
   ...     events.append(args)

>>> zope.component.provideHandler(record, [IUseless, IObjectThrownEvent])

The event is notified to the subscriber:

.. doctest::

>>> zope.component.event.objectEventNotify(event)
   >>> events == [(hammer, event)]
   True

Following test demonstrates how a subscriber can raise an exception
to prevent an action.

.. doctest::

>>> zope.component.provideHandler(zope.component.event.objectEventNotify)

Let's create a container:

.. doctest::

>>> class ToolBox(dict):
   ...     def __delitem__(self, key):
   ...         notify(ObjectThrownEvent(self[key]))
   ...         return super(ToolBox,self).__delitem__(key)

>>> container = ToolBox()

And put the object into the container:

.. doctest::

>>> container['Red Hammer'] = hammer

Create an handler function that will raise an error when called:

.. doctest::

>>> class Veto(Exception):
   ...     pass

>>> def callback(item, event):
   ...     assert(item == event.object)
   ...     raise Veto

.. doctest::

>>> zope.component.provideHandler(callback, [IUseless, IObjectThrownEvent])

Then if we try to remove the object, an ObjectThrownEvent is fired:

.. doctest::

>>> del container['Red Hammer']
   ... # doctest: +NORMALIZE_WHITESPACE
   Traceback (most recent call last):
   ...
       raise Veto
   Veto

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