How to use events to hook the Archetypes creation process

Introduction¶

Times have changed since the days of at_post_create_script(). Here is the way to hook into Zope3 (or Five's) event system in order to execute code during the Archetypes content creation and or editing process.

In the old days the only way to execute code during the object creation process for Archetypes was to add a method to your content type called at_post_create_script. In this script you would add any code that should execute after Archetypes was done creating the object.

The new method for hooking the Archetypes object creation and editing process is to use Zope3 style events, like Products.Archetypes.interfaces.IObjectInitializedEvent.

Prerequisites¶

Have a content type handy so we can add a post creation hook to it. To learn how to create a content type, check previous sections of this manual.

We're going to use a content type called ExampleContent with the interface IExampleContent for this how to. The code structure will look like this:

                        tutorial/configure.zcml
tutorial/interfaces.py
tutorial/content/examplecontent.py

                      

Step by step¶

First let's create the interface for our ExampleContent type. In interfaces.py, add:

                        from zope.interfaces import Interface

class IExampleContent(Interface):
    ''' Interface for the ExampleContent type
    '''

You can store the implementation for your event handlers anywhere but for the purpose of this example we're going to put it in the same module as the ExampleContent type:

                        from zope.interface import implements
from Products.ATContentTypes import atct

def addSubFolder(obj, event):
    obj.invokeFactory(type_name='Folder', id='subfolder')

class ExampleContent(atct.ATFolder):
    implements(IExampleContent)
    portal_type = archetype_name = 'ExampleContent'  # <-- this is no longer needed with GenericSetup.

                      

All we need to do now is register the addSubFolder method as a handler for Products.Archetypes.interfaces.IObjectInitializedEvent and for anything implementing the IExampleContent interface. We do this in a configure.zcml file:

                        <subscriber for=".interfaces.IExampleContent
                 Products.Archetypes.interfaces.IObjectInitializedEvent"
            handler=".content.examplecontent.addSubFolder" />

                      

Notice that there are two interfaces in the "for" attribute. This is because we are registering a multi-adapter. Now when you add an ExampleContent type the addSubFolder method will be executed after Archetypes has created the object. The object itself will be passed to the handler and we can use the object reference to make additional modifications, in this case adding a sub folder.

You can register as many handlers as you need.

Warnings from your future¶

Having implemented all of your content type's event hooks you might then run off and try using invokeFactory somewhere in your code only to realize that your IObjectInitializedEvent handlers are not being executed. This is because invokeFactory does not notify Zope's event system that new objects are being created. You have to provide these notifications yourself. So here is an example:

                        import zope.event
from Products.Archetypes.event import ObjectInitializedEvent
some_folder.invokeFactory(type_name='ExampleContent', id='foobar')
obj = getattr(some_folder, 'foobar')
zope.event.notify(ObjectInitializedEvent(obj))

                      

This will both create your object and invoke any IObjectInitializedEvent handlers you have registered. Notice that we are importing ObjectInitializedEvent, not the interface IObjectInitializedEvent. We want to actually instantiate an event passing it our newly created object as the single parameter and then pass the event to zope.event.notify. From there, Zope takes care of figuring out which handlers need to execute.

So for example, in our addSubFolder method above, any events registered for the folder we created would not fire. To complete our hook in this case we should provide a notification for our newly created folder. Archetypes or other products may be expecting notifications so when using invokeFactory, always send an IObjectInitializedEvent for the object. The complete method looks like this:

def addSubFolder(obj, event):

obj.invokeFactory(type_name='Folder', id='subfolder') folder = getattr(obj, 'subfolder') zope.event.notify(ObjectInitializedEvent(folder))

Further information¶

The IObjectInitializedEvent is fired once during the objects creation process. To hook the editing process for an object use IObjectEditedEvent.

The Sending and handling events tutorial is a little out of date but provides a broader explanation of the underlying mechanics. Walking through Five to Zope 3 - Events is another great introduction to events handling.