zope.contentprovider 3.4.0

Motivation and Design Goals

Before diving into the features of this package let me take up a few bytes of text to explain the use cases that drove us to develop this package (also others) and how the API documented below fulfills/solves those use cases. When we started developing Zope 3, it was from a desire to decentralize functionality and thus the complexity of the Python code. And we were successful! The component architecture is a marvelous piece of software that hopefully will allow us to build scalable solutions for a very long time. However, when it comes to user interface design, in this case specifically HTML pages, we have failed to provide the features and patterns of assembeling a page from configured components.

Looking up views for a particular content component and a request just simply does not work by itself. The content inside the page is still monolithic. One attempt to solve this problem are METAL macros, which allow you to insert other TAL snippets into your main template. But macros have two shortcomings. For one there is a “hard-coded” one-to-one mapping between a slot and the macro that fills that slot, which makes it impossible to register several macros for a given location. The second problem is that macros are not views in their own right; thus they cannot provide functionality that is independent of the main template’s view.

A second approach to modular UI design are rendering pipes. Rendering pipes have the great advantage that they can reach all regions of the page during every step of the rendering process. For example, if we have a widget in the middle of the page that requires some additional Javascript, then it is easy for a rendering unit to insert the Javascript file link in the HTML header of the page. This type of use case is very hard to solve using page templates. However, pipes are not the answer to componentized user interface, since they cannot simply deal with registering random content for a given page region. In fact, I propose that pipelines are orthogonal to content providers, the concept introducted below. A pipeline framework could easily use functionality provided by this and other packages to provide component-driven UI design.

So our goal is clear: Bring the pluggability of the component architecture into page templates and user interface design. Zope is commonly known to reinvent the wheel, develop its own terminology and misuse other’s terms. For example, the Plone community has a very different understanding of what a “portlet” is compared to the commonly accepted meaning in the corporate world, which derives its definition from JSR 168. Therefore an additional use case of the design of this package was to stick with common terms and use them in their original meaning – well, given a little extra twist.

The most basic user interface component in the Web application Java world is the “content provider” [1]. A content provider is simply responsible for providing HTML content for a page. This is equivalent to a view that does not provide a full page, but just a snippet, much like widgets or macros. Once there is a way to configure those content providers, we need a way to inserting them into our page templates. In our implementation this is accomplished using a new TALES namespace that allows to insert content providers by name. But how, you might wonder, does this provide a componentized user interface? On the Zope 3 level, each content provider is registered as a presentation component discriminted by the context, request and view it will appear in. Thus different content providers will be picked for different configurations.

Okay, that’s pretty much everything there is to say about content providers. What, we are done? Hold on, what about defining regions of pages and filling them configured UI snippets. The short answer is: See the zope.viewlet pacakge. But let me also give you the long answer. This and the other pacakges were developed using real world use cases. While doing this, we noticed that not every project would need, for example, all the features of a portlet, but would still profit from lower-level features. Thus we decided to declare clear boundaries of functionality and providing each level in a different package. This particualr package is only meant to provide the interface between the content provider world and page templates.

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.

So let’s create a simple content provider:

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

>>> class MessageBox(object):
...     zope.interface.implements(interfaces.IContentProvider)
...     zope.component.adapts(zope.interface.Interface,
...                           browser.IDefaultBrowserLayer,
...                           zope.interface.Interface)
...     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

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)
>>> box.render()
u'<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.

The TALES provider Expression

The provider expression will look up the name of the content provider, call it and return the HTML content. The first step, however, will be to register our content provider with the component architecture:

>>> zope.component.provideAdapter(MessageBox, name='mypage.MessageBox')

The content provider must be registered by name, since the TALES expression uses the name to look up the provider at run time.

Let’s now create a view using a page template:

>>> import os, tempfile
>>> temp_dir = tempfile.mkdtemp()
>>> templateFileName = os.path.join(temp_dir, 'template.pt')
>>> open(templateFileName, 'w').write('''
... <html>
...   <body>
...     <h1>My Web Page</h1>
...     <div class="left-column">
...       <tal:block replace="structure provider:mypage.MessageBox" />
...     </div>
...     <div class="main">
...       Content here
...     </div>
...   </body>
... </html>
... ''')

As you can see, we exprect the provider expression to simply look up the content provider and insert the HTML content at this place.

Next we register the template as a view (browser page) for all objects:

>>> from zope.app.pagetemplate.simpleviewclass import SimpleViewClass
>>> FrontPage = SimpleViewClass(templateFileName, name='main.html')

>>> zope.component.provideAdapter(
...     FrontPage,
...     (zope.interface.Interface, browser.IDefaultBrowserLayer),
...     zope.interface.Interface,
...     name='main.html')

Let’s create a content object that can be viewed:

>>> class Content(object):
...     zope.interface.implements(zope.interface.Interface)

>>> content = Content()

Finally we look up the view and render it. Note that a BeforeUpdateEvent is fired - this event should always be fired before any contentprovider is updated.

>>> from zope.publisher.browser import TestRequest
>>> events = []
>>> zope.component.provideHandler(events.append, (None, ))
>>> request = TestRequest()

>>> view = zope.component.getMultiAdapter((content, request),
...                                       name='main.html')
>>> print view().strip()
<html>
  <body>
    <h1>My Web Page</h1>
    <div class="left-column">
      <div class="box">My Message</div>
    </div>
    <div class="main">
      Content here
    </div>
  </body>
</html>

>>> events
[<zope.contentprovider.interfaces.BeforeUpdateEvent object at ...>]

The event holds the provider and the request.

>>> events[0].request
<zope.publisher.browser.TestRequest instance URL=http://127.0.0.1>
>>> events[0].object
<MessageBox object at ...>

Cleanup

>>> import shutil
>>> shutil.rmtree(temp_dir)