zope.component.events: Event dispatching

The Component Architecture provides a way to dispatch events to event handlers using zope.event.notify(). 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:

>>> import zope.component.event

Consider two event classes:

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

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

Now consider two handlers for these event classes:

>>> 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:

>>> 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:

>>> 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.

zope.component.event.objectEventNotify(event)

Dispatch ObjectEvents to interested adapters.

Note

This function is automatically registered as a subscriber for zope.interface.interfaces.IObjectEvent when the ZCML configuration for this package is loaded.

First create an object class:

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

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

Then create an event class:

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

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

Create an object and an event:

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

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

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

Now create an handler for the event:

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

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

The event is notified to the subscriber:

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

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

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

Let’s create a container:

>>> 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:

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

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

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

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

Register the handler:

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

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

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