24. Writing Viewlets¶
In this part you will:
- Display data from a behavior in a viewlet
Topics covered:
- Viewlets
A viewlet for the social behavior¶
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
.
- Inspect existing viewlets and their managers by going to http://localhost:8080/Plone/@@manage-viewlets.
- We already customized a viewlet (
colophon.pt
). Now we add a new one. - Viewlets don’t save data (portlets do)
- Viewlets have no user interface (portlets do)
Exercise 1¶
Register a viewlet ‘number_of_talks’ in the footer that is only visible to admins (the permission you are looking for is 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 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 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 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>
python:context.portal_catalog
will return the catalog through Acquisition. Be careful if you want to use path expressions: content/portal_catalog
calls the catalog (and returns all brains). You need to prevent this by using nocall:content/portal_catalog
.
Relying on Acquisition is a bad idea. It would be much better to use the helper view plone_tools
from 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>
context/@@plone_tools/catalog
traverses to the view plone_tools
and calls its method 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 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 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 Manage Settings with Registry, Controlpanels and Vocabularies you learn how store global configuration and easily create controlpanels.
And in 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>
Social viewlet¶
Let’s add a link to the site that uses the information that we collected using the social behavior.
We register the viewlet in
browser/configure.zcml
.for
,manager
,layer
andpermission
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.This registers a viewlet called
social
. It is visible on all content that implements the interfaceISocial
from our behavior. It is also good practice to bind it to a specificlayer
, so it only shows up if our add-on is actually installed. We will return to this in a later chapter.The viewlet class
SocialViewlet
is expected in a filebrowser/viewlets.py
.This class does nothing except rendering the associated template (That we have yet to write)
Let’s add the missing template
templates/social_viewlet.pt
.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
tal:define
statement, querying forview/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:
Why not to access context directly
In this example,
ISocial(self.context)
does return the context directly. It is still good to use this idiom for two reasons:Therefore in this example you could simply write
return self.context.lanyrd
.So far, we