Content Providers

“Content Provider” is a term from the Java world that refers to components that can provide HTML content. It means nothing more! How the content is found and returned is totally up to the implementation. The Zope 3 touch to the concept is that content providers are multi-adapters that are looked up by the context, request (and thus the layer/skin), and view they are displayed in.

The second important concept of content providers are their two-phase rendering design. In the first phase the state of the content provider is prepared and, if applicable, any data the provider is responsible for is updated.

>>> from zope.contentprovider import interfaces

So let’s create a simple content provider:

>>> import zope.interface
>>> import zope.component
>>> from zope.publisher.interfaces import browser

>>> @zope.interface.implementer(interfaces.IContentProvider)
... @zope.component.adapter(zope.interface.Interface,
...                         browser.IDefaultBrowserLayer,
...                         zope.interface.Interface)
... class MessageBox(object):
...     message = u'My Message'
...
...     def __init__(self, context, request, view):
...         self.__parent__ = view
...
...     def update(self):
...         pass
...
...     def render(self):
...         return u'<div class="box">%s</div>' % self.message
...
...     def __repr__(self):
...         return '<MessageBox object at %x>' % id(self)

The update() method is executed during phase one. Since no state needs to be calculated and no data is modified by this simple content provider, it is an empty implementation. The render() method implements phase 2 of the process. We can now instantiate the content provider (manually) and render it:

>>> box = MessageBox(None, None, None)
>>> print(box.render())
<div class="box">My Message</div>

Since our content provider did not require the context, request or view to create its HTML content, we were able to pass trivial dummy values into the constructor. Also note that the provider must have a parent (using the __parent__ attribute) specified at all times. The parent must be the view the provider appears in.

I agree, this functionally does not seem very useful now. The constructor and the update() method seem useless and the returned content is totally static. However, we implemented a contract for content providers that other code can rely on. Content providers are (commonly) instantiated using the context, request and view they appear in and are required to always generate its HTML using those three components.

Two-Phased Content Providers

Let’s now have a look at a content provider that actively uses the two-phase rendering process. The simpler scenario is the case where the content provider updates a content component without affecting anything else. So let’s create a content component to be updated,

>>> class Article(object):
...     title = u'initial'
>>> article = Article()

and the content provider that is updating the title:

>>> @zope.interface.implementer(interfaces.IContentProvider)
... @zope.component.adapter(zope.interface.Interface,
...                         browser.IDefaultBrowserLayer,
...                         zope.interface.Interface)
... class ChangeTitle(object):
...     fieldName = 'ChangeTitle.title'
...
...     def __init__(self, context, request, view):
...         self.__parent__ = view
...         self.context, self.request = context, request
...
...     def update(self):
...         if self.fieldName in self.request:
...             self.context.title = self.request[self.fieldName]
...
...     def render(self):
...         return u'<input name="%s" value="%s" />' % (self.fieldName,
...                                                     self.context.title)

Using a request, let’s now instantiate the content provider and go through the two-phase rendering process:

>>> from zope.publisher.browser import TestRequest
>>> request = TestRequest()
>>> changer = ChangeTitle(article, request, None)
>>> changer.update()
>>> print(changer.render())
<input name="ChangeTitle.title" value="initial" />

Let’s now enter a new title and render the provider:

>>> request = TestRequest(form={'ChangeTitle.title': u'new title'})
>>> changer = ChangeTitle(article, request, None)
>>> changer.update()
>>> print(changer.render())
<input name="ChangeTitle.title" value="new title" />
>>> print(article.title)
new title

So this was easy. Let’s now look at a case where one content provider’s update influences the content of another. Let’s say we have a content provider that displays the article’s title:

>>> @zope.interface.implementer(interfaces.IContentProvider)
... @zope.component.adapter(zope.interface.Interface,
...                         browser.IDefaultBrowserLayer,
...                         zope.interface.Interface)
... class ViewTitle(object):
...
...     def __init__(self, context, request, view):
...         self.context, self.__parent__ = context, view
...
...     def update(self):
...         pass
...
...     def render(self):
...         return u'<h1>Title: %s</h1>' % self.context.title

Let’s now say that the ShowTitle content provider is shown on a page before the ChangeTitle content provider. If we do the full rendering process for each provider in sequence, we get the wrong result:

>>> request = TestRequest(form={'ChangeTitle.title': u'newer title'})

>>> viewer = ViewTitle(article, request, None)
>>> viewer.update()
>>> print(viewer.render())
<h1>Title: new title</h1>

>>> changer = ChangeTitle(article, request, None)
>>> changer.update()
>>> print(changer.render())
<input name="ChangeTitle.title" value="newer title" />

So the correct way of doing this is to first complete phase 1 (update) for all providers, before executing phase 2 (render):

>>> request = TestRequest(form={'ChangeTitle.title': u'newest title'})

>>> viewer = ViewTitle(article, request, None)
>>> changer = ChangeTitle(article, request, None)

>>> viewer.update()
>>> changer.update()

>>> print(viewer.render())
<h1>Title: newest title</h1>

>>> print(changer.render())
<input name="ChangeTitle.title" value="newest title" />

UpdateNotCalled Errors

Since calling update before any other method that mutates the provider or any other data is so important to the correct functioning of the API, the developer has the choice to raise the UpdateNotCalled error, if any method is called before update() (with exception of the constructor):

>>> @zope.interface.implementer(interfaces.IContentProvider)
... @zope.component.adapter(zope.interface.Interface,
...                         browser.IDefaultBrowserLayer,
...                         zope.interface.Interface)
... class InfoBox(object):
...
...     def __init__(self, context, request, view):
...         self.__parent__ = view
...         self.__updated = False
...
...     def update(self):
...         self.__updated = True
...
...     def render(self):
...         if not self.__updated:
...             raise interfaces.UpdateNotCalled
...         return u'<div>Some information</div>'

>>> info = InfoBox(None, None, None)

>>> info.render()
Traceback (most recent call last):
...
UpdateNotCalled: ``update()`` was not called yet.

>>> info.update()

>>> print(info.render())
<div>Some information</div>

zope.contentprovider.interfaces

Content provider interfaces