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Writing Viewlets

Get the code!

Get the code for this chapter (:doc:`More info <code>`):

git checkout viewlets_1

In this part you will:

  • Display data from a behavior in a viewlet

Topics covered:

  • Viewlets

A viewlet for the social behavior

.. only:: not presentation

    A viewlet is not a view but a snippet of HTML and logic that can be put in various places in the site. These places are called ``viewletmanager``.

Social viewlet

.. only:: not presentation

    Let's add a link to the site that uses the information that we collected using the social behavior.

We register the viewlet in :file:`browser/configure.zcml`.

<browser:viewlet
  name="social"
  for="ploneconf.site.behaviors.social.ISocial"
  manager="plone.app.layout.viewlets.interfaces.IBelowContentTitle"
  class=".viewlets.SocialViewlet"
  layer="zope.interface.Interface"
  template="templates/social_viewlet.pt"
  permission="zope2.View"
  />

for, manager, layer and permission are constraints that limit the contexts in which the viewlet is loaded and rendered, by filtering out all the contexts that do not match those constraints.

.. only:: not presentation

    This registers a viewlet called ``social``.
    It is visible on all content that implements the interface :py:class:`ISocial` from our behavior.
    It is also good practice to bind it to a specific ``layer``, so it only shows up if our add-on is actually installed.  We will return to this in a later chapter.

The viewlet class :py:class:`SocialViewlet` is expected in a file :file:`browser/viewlets.py`.

from plone.app.layout.viewlets import ViewletBase

class SocialViewlet(ViewletBase):
    pass
.. only:: not presentation

    This class does nothing except rendering the associated template (That we have yet to write)

Let's add the missing template :file:`templates/social_viewlet.pt`.

<div id="social-links">
    <a href="#"
       class="lanyrd-link"
       tal:define="link view/lanyrd_link"
       tal:condition="link"
       tal:attributes="href link">
         See this talk on Lanyrd!
    </a>
</div>
.. only:: not presentation

    As you can see this is not a valid HTML document. That is not needed, because we don't want a complete view here, just a html snippet.

    There is a :samp:`tal:define` statement, querying for :samp:`view/lanyrd_link`. Same as for views, viewlets have access to their class in page templates, as well.

We have to extend the Social Viewlet now to add the missing attribute:

.. only:: not presentation

    .. sidebar:: Why not to access context directly

        In this example, :samp:`ISocial(self.context)` does return the context directly. It is still good to use this idiom for two reasons:

          #. It makes it clear that we only want to use the ISocial aspect of the object
          #. If we decide to use a factory, for example to store our attributes in an annotation, we would `not` get back our context, but the adapter.

        Therefore in this example you could simply write :samp:`return self.context.lanyrd`.


from plone.app.layout.viewlets import ViewletBase
from ploneconf.site.behaviors.social import ISocial

class SocialViewlet(ViewletBase):

    def lanyrd_link(self):
        adapted = ISocial(self.context)
        return adapted.lanyrd

So far, we

  • register the viewlet to content that has the ISocial Interface.
  • adapt the object to its behavior to be able to access the fields of the behavior
  • return the link

Exercise 1

Register a viewlet 'number_of_talks' in the footer that is only visible to admins (the permission you are looking for is :py:class:`cmf.ManagePortal`). Use only a template (no class) to display the number of talks already submitted. Hint: Use Acquisition to get the catalog (You know, you should not do this but there is plenty of code out there that does it...)

Solution

Register the viewlet in :file:`browser/configure.zcml`

<browser:viewlet
  name="number_of_talks"
  for="*"
  manager="plone.app.layout.viewlets.interfaces.IPortalFooter"
  layer="zope.interface.Interface"
  template="templates/number_of_talks.pt"
  permission="cmf.ManagePortal"
  />

For the for and layer-parameters * is shorthand for :py:class:`zope.interface.Interface` and the same effect as omitting them: The viewlet will be shown for all types of pages and for all Plone sites within your Zope instance.

Add the template :file:`browser/templates/number_of_talks.pt`:

<div class="number_of_talks"
     tal:define="catalog python:context.portal_catalog;
                 talks python:len(catalog(portal_type='talk'));">
    There are <span tal:replace="talks" /> talks.
</div>

:samp:`python:context.portal_catalog` will return the catalog through Acquisition. Be careful if you want to use path expressions: :samp:`content/portal_catalog` calls the catalog (and returns all brains). You need to prevent this by using :samp:`nocall:content/portal_catalog`.

Relying on Acquisition is a bad idea. It would be much better to use the helper view plone_tools from :file:`plone/app/layout/globals/tools.py` to get the catalog.

<div class="number_of_talks"
     tal:define="catalog context/@@plone_tools/catalog;
                 talks python:len(catalog(portal_type='talk'));">
    There are <span tal:replace="talks" /> talks.
</div>

:samp:`context/@@plone_tools/catalog` traverses to the view plone_tools and calls its method :py:meth:`catalog`. In python it would look like this:

<div class="number_of_talks"
     tal:define="catalog python:context.restrictedTraverse('plone_tools').catalog();
                 talks python:len(catalog(portal_type='talk'));">
    There are <span tal:replace="talks" /> talks.
</div>

It is not a good practice to query the catalog within a template since even simple logic like this should live in Python. But it is very powerful if you are debugging or need a quick and dirty solution.

In Plone 5 you could even write it like this:

<?python

from plone import api
catalog = api.portal.get_tool('portal_catalog')
talks_amount = len(catalog(portal_type='talk'))

?>

<div class="number_of_talks">
    There are ${talks_amount} talks.
</div>

Exercise 2

Register a viewlet 'days_to_conference' in the header. Use a class and a template to display the number of days until the conference. You get bonus points if you display it in a nice format (think "In 2 days" and "Last Month") by using either javascript or a python library.

Solution

In :file:`configure.zcml`:

<browser:viewlet
  name="days_to_conference"
  for="*"
  manager="plone.app.layout.viewlets.interfaces.IPortalHeader"
  layer="*"
  class=".viewlets.DaysToConferenceViewlet"
  template="templates/days_to_conference.pt"
  permission="zope2.View"
  />

In :file:`viewlets.py`:

from plone.app.layout.viewlets import ViewletBase
from datetime import datetime
import arrow

CONFERENCE_START_DATE = datetime(2015, 10, 12)


class DaysToConferenceViewlet(ViewletBase):

    def date(self):
        return CONFERENCE_START_DATE

    def human(self):
        return arrow.get(CONFERENCE_START_DATE).humanize()

Setting the date in python is not very user-friendly. In the chapter :ref:`registry-label` you learn how store global configuration and easily create controlpanels.

And in :file:`templates/days_to_conference.pt`:

<div class="days_to_conf">
    ${python: view.human()}
</div>

Or using the moment pattern in Plone 5:

<div class="pat-moment"
     data-pat-moment="format: relative">
    ${python: view.date()}
</div>