zope.viewlet 4.0.0

``zope.viewlet`` README
=======================

Viewlets provide a generic framework for building pluggable user interfaces.


Detailed Documentation
**********************



=============================
Viewlets and Viewlet Managers
=============================

Let's start with some motivation. Using content providers allows us to insert
one piece of HTML content. In most Web development, however, you are often
interested in defining some sort of region and then allow developers to
register content for those regions.

>>> from zope.viewlet import interfaces


Design Notes
------------

As mentioned above, besides inserting snippets of HTML at places, we more
frequently want to define a region in our page and allow specialized content
providers to be inserted based on configuration. Those specialized content
providers are known as viewlets and are only available inside viewlet
managers, which are just a more complex example of content providers.

Unfortunately, the Java world does not implement this layer separately. The
viewlet manager is most similar to a Java "channel", but we decided against
using this name, since it is very generic and not very meaningful. The viewlet
has no Java counterpart, since Java does not implement content providers using
a component architecture and thus does not register content providers
specifically for viewlet managers, which I believe makes the Java
implementation less useful as a generic concept. In fact, the main design
goal in the Java world is the implementation of reusable and shareable
portlets. The scope for Zope 3 is larger, since we want to provide a generic
framework for building pluggable user interfaces.


The Viewlet Manager
-------------------

In this implementation of viewlets, those regions are just content providers
called viewlet managers that manage a special type of content providers known
as viewlets. Every viewlet manager handles the viewlets registered for it:

>>> class ILeftColumn(interfaces.IViewletManager):
... """Viewlet manager located in the left column."""

You can then create a viewlet manager using this interface now:

>>> from zope.viewlet import manager
>>> LeftColumn = manager.ViewletManager('left', ILeftColumn)

Now we have to instantiate it:

>>> import zope.interface
>>> @zope.interface.implementer(zope.interface.Interface)
... class Content(object):
... pass
>>> content = Content()

>>> from zope.publisher.browser import TestRequest
>>> request = TestRequest()

>>> from zope.publisher.interfaces.browser import IBrowserView
>>> @zope.interface.implementer(IBrowserView)
... class View(object):
... def __init__(self, context, request):
... pass
>>> view = View(content, request)

>>> leftColumn = LeftColumn(content, request, view)

So initially nothing gets rendered:

>>> leftColumn.update()
>>> leftColumn.render()
u''

But now we register some viewlets for the manager

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

>>> @zope.interface.implementer(interfaces.IViewlet)
... class WeatherBox(object):
...
... def __init__(self, context, request, view, manager):
... self.__parent__ = view
...
... def update(self):
... pass
...
... def render(self):
... return u'<div class="box">It is sunny today!</div>'
...
... def __repr__(self):
... return '<WeatherBox object at %x>' % id(self)

>>> # Create a security checker for viewlets.
>>> from zope.security.checker import NamesChecker, defineChecker
>>> viewletChecker = NamesChecker(('update', 'render'))
>>> defineChecker(WeatherBox, viewletChecker)

>>> zope.component.provideAdapter(
... WeatherBox,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, ILeftColumn),
... interfaces.IViewlet, name='weather')

>>> from zope.location.interfaces import ILocation
>>> @zope.interface.implementer(interfaces.IViewlet,
... ILocation)
... class SportBox(object):
...
... def __init__(self, context, request, view, manager):
... self.__parent__ = view
...
... def update(self):
... pass
...
... def render(self):
... return u'<div class="box">Patriots (23) : Steelers (7)</div>'

>>> defineChecker(SportBox, viewletChecker)

>>> zope.component.provideAdapter(
... SportBox,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, ILeftColumn),
... interfaces.IViewlet, name='sport')

and thus the left column is filled. Note that also events get fired
before viewlets are updated. We register a simple handler to
demonstrate this behaviour.

>>> from zope.contentprovider.interfaces import IBeforeUpdateEvent
>>> events = []
>>> def handler(ev):
... events.append(ev)
>>> zope.component.provideHandler(handler, (IBeforeUpdateEvent,))
>>> leftColumn.update()
>>> sorted([(ev, ev.object.__class__.__name__) for ev in events],
... key=lambda x: x[1])
[(<zope.contentprovider.interfaces.BeforeUpdateEvent...>, 'SportBox'),
(<zope.contentprovider.interfaces.BeforeUpdateEvent...>, 'WeatherBox')]

>>> print(leftColumn.render())
<div class="box">Patriots (23) : Steelers (7)</div>
<div class="box">It is sunny today!</div>

But this is of course pretty lame, since there is no way of specifying how the
viewlets are put together. But we have a solution. The second argument of the
``ViewletManager()`` function is a template in which we can specify how the
viewlets are put together:

>>> import os, tempfile
>>> temp_dir = tempfile.mkdtemp()
>>> leftColTemplate = os.path.join(temp_dir, 'leftCol.pt')
>>> with open(leftColTemplate, 'w') as file:
... _ = file.write('''
... <div class="left-column">
... <tal:block repeat="viewlet options/viewlets"
... replace="structure viewlet/render" />
... </div>
... ''')

>>> LeftColumn = manager.ViewletManager('left', ILeftColumn,
... template=leftColTemplate)
>>> leftColumn = LeftColumn(content, request, view)

TODO: Fix this silly thing; viewlets should be directly available.

As you can see, the viewlet manager provides a global ``options/viewlets``
variable that is an iterable of all the available viewlets in the correct
order:

>>> leftColumn.update()
>>> print(leftColumn.render().strip())
<div class="left-column">
<div class="box">Patriots (23) : Steelers (7)</div>
<div class="box">It is sunny today!</div>
</div>

If a viewlet provides ILocation the ``__name__`` attribute of the
viewlet is set to the name under which the viewlet is registered.

>>> [getattr(viewlet, '__name__', None) for viewlet in leftColumn.viewlets]
[u'sport', None]


You can also lookup the viewlets directly for management purposes:

>>> leftColumn['weather']
<WeatherBox ...>
>>> leftColumn.get('weather')
<WeatherBox ...>

The viewlet manager also provides the __contains__ method defined in
IReadMapping:

>>> 'weather' in leftColumn
True

>>> 'unknown' in leftColumn
False

If the viewlet is not found, then the expected behavior is provided:

>>> leftColumn['stock']
Traceback (most recent call last):
...
ComponentLookupError: No provider with name `stock` found.

>>> leftColumn.get('stock') is None
True

Customizing the default Viewlet Manager
---------------------------------------

One important feature of any viewlet manager is to be able to filter and sort
the viewlets it is displaying. The default viewlet manager that we have been
using in the tests above, supports filtering by access availability and
sorting via the viewlet's ``__cmp__()`` method (default). You can easily
override this default policy by providing a base viewlet manager class.

In our case we will manage the viewlets using a global list:

>>> shown = ['weather', 'sport']

The viewlet manager base class now uses this list:

>>> class ListViewletManager(object):
...
... def filter(self, viewlets):
... viewlets = super(ListViewletManager, self).filter(viewlets)
... return [(name, viewlet)
... for name, viewlet in viewlets
... if name in shown]
...
... def sort(self, viewlets):
... viewlets = dict(viewlets)
... return [(name, viewlets[name]) for name in shown]

Let's now create a new viewlet manager:

>>> LeftColumn = manager.ViewletManager(
... 'left', ILeftColumn, bases=(ListViewletManager,),
... template=leftColTemplate)
>>> leftColumn = LeftColumn(content, request, view)

So we get the weather box first and the sport box second:

>>> leftColumn.update()
>>> print(leftColumn.render().strip())
<div class="left-column">
<div class="box">It is sunny today!</div>
<div class="box">Patriots (23) : Steelers (7)</div>
</div>

Now let's change the order...

>>> shown.reverse()

and the order should switch as well:

>>> leftColumn.update()
>>> print(leftColumn.render().strip())
<div class="left-column">
<div class="box">Patriots (23) : Steelers (7)</div>
<div class="box">It is sunny today!</div>
</div>

Of course, we also can remove a shown viewlet:

>>> weather = shown.pop()
>>> leftColumn.update()
>>> print(leftColumn.render().strip())
<div class="left-column">
<div class="box">Patriots (23) : Steelers (7)</div>
</div>


WeightOrderedViewletManager
---------------------------

The weight ordered viewlet manager offers ordering viewlets by a additional
weight argument. Viewlets which doesn't provide a weight attribute will get
a weight of 0 (zero).

Let's define a new column:

>>> class IWeightedColumn(interfaces.IViewletManager):
... """Column with weighted viewlet manager."""

First register a template for the weight ordered viewlet manager:

>>> weightedColTemplate = os.path.join(temp_dir, 'weightedColTemplate.pt')
>>> with open(weightedColTemplate, 'w') as file:
... _ = file.write('''
... <div class="weighted-column">
... <tal:block repeat="viewlet options/viewlets"
... replace="structure viewlet/render" />
... </div>
... ''')

And create a new weight ordered viewlet manager:

>>> from zope.viewlet.manager import WeightOrderedViewletManager
>>> WeightedColumn = manager.ViewletManager(
... 'left', IWeightedColumn, bases=(WeightOrderedViewletManager,),
... template=weightedColTemplate)
>>> weightedColumn = WeightedColumn(content, request, view)

Let's create some viewlets:

>>> from zope.viewlet import viewlet
>>> class FirstViewlet(viewlet.ViewletBase):
...
... weight = 1
...
... def render(self):
... return u'<div>first</div>'

>>> class SecondViewlet(viewlet.ViewletBase):
...
... weight = 2
...
... def render(self):
... return u'<div>second</div>'

>>> class ThirdViewlet(viewlet.ViewletBase):
...
... weight = 3
...
... def render(self):
... return u'<div>third</div>'

>>> class UnWeightedViewlet(viewlet.ViewletBase):
...
... def render(self):
... return u'<div>unweighted</div>'

>>> defineChecker(FirstViewlet, viewletChecker)
>>> defineChecker(SecondViewlet, viewletChecker)
>>> defineChecker(ThirdViewlet, viewletChecker)
>>> defineChecker(UnWeightedViewlet, viewletChecker)

>>> zope.component.provideAdapter(
... ThirdViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IWeightedColumn),
... interfaces.IViewlet, name='third')

>>> zope.component.provideAdapter(
... FirstViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IWeightedColumn),
... interfaces.IViewlet, name='first')

>>> zope.component.provideAdapter(
... SecondViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IWeightedColumn),
... interfaces.IViewlet, name='second')

>>> zope.component.provideAdapter(
... UnWeightedViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IWeightedColumn),
... interfaces.IViewlet, name='unweighted')

And check the order:

>>> weightedColumn.update()
>>> print(weightedColumn.render().strip())
<div class="weighted-column">
<div>unweighted</div>
<div>first</div>
<div>second</div>
<div>third</div>
</div>


ConditionalViewletManager
-------------------------

The conditional ordered viewlet manager offers ordering viewlets by a
additional weight argument and filters by the available attribute if a
supported by the viewlet. Viewlets which doesn't provide a available attribute
will not get skipped. The default weight value for viewlets which doesn't
provide a weight attribute is 0 (zero).

Let's define a new column:

>>> class IConditionalColumn(interfaces.IViewletManager):
... """Column with weighted viewlet manager."""

First register a template for the weight ordered viewlet manager:

>>> conditionalColTemplate = os.path.join(temp_dir,
... 'conditionalColTemplate.pt')
>>> with open(conditionalColTemplate, 'w') as file:
... _ = file.write('''
... <div class="conditional-column">
... <tal:block repeat="viewlet options/viewlets"
... replace="structure viewlet/render" />
... </div>
... ''')

And create a new conditional viewlet manager:

>>> from zope.viewlet.manager import ConditionalViewletManager
>>> ConditionalColumn = manager.ViewletManager(
... 'left', IConditionalColumn, bases=(ConditionalViewletManager,),
... template=conditionalColTemplate)
>>> conditionalColumn = ConditionalColumn(content, request, view)

Let's create some viewlets. We also use the previous viewlets supporting no
weight and or no available attribute:

>>> from zope.viewlet import viewlet
>>> class AvailableViewlet(viewlet.ViewletBase):
...
... weight = 4
...
... available = True
...
... def render(self):
... return u'<div>available</div>'

>>> class UnAvailableViewlet(viewlet.ViewletBase):
...
... weight = 5
...
... available = False
...
... def render(self):
... return u'<div>not available</div>'

>>> defineChecker(AvailableViewlet, viewletChecker)
>>> defineChecker(UnAvailableViewlet, viewletChecker)

>>> zope.component.provideAdapter(
... ThirdViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IConditionalColumn),
... interfaces.IViewlet, name='third')

>>> zope.component.provideAdapter(
... FirstViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IConditionalColumn),
... interfaces.IViewlet, name='first')

>>> zope.component.provideAdapter(
... SecondViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IConditionalColumn),
... interfaces.IViewlet, name='second')

>>> zope.component.provideAdapter(
... UnWeightedViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IConditionalColumn),
... interfaces.IViewlet, name='unweighted')

>>> zope.component.provideAdapter(
... AvailableViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IConditionalColumn),
... interfaces.IViewlet, name='available')

>>> zope.component.provideAdapter(
... UnAvailableViewlet,
... (zope.interface.Interface, IDefaultBrowserLayer,
... IBrowserView, IConditionalColumn),
... interfaces.IViewlet, name='unavailable')

And check the order:

>>> conditionalColumn.update()
>>> print(conditionalColumn.render().strip())
<div class="conditional-column">
<div>unweighted</div>
<div>first</div>
<div>second</div>
<div>third</div>
<div>available</div>
</div>


Viewlet Base Classes
--------------------

To make the creation of viewlets simpler, a set of useful base classes and
helper functions are provided.

The first class is a base class that simply defines the constructor:

>>> base = viewlet.ViewletBase('context', 'request', 'view', 'manager')
>>> base.context
'context'
>>> base.request
'request'
>>> base.__parent__
'view'
>>> base.manager
'manager'

But a default ``render()`` method implementation is not provided:

>>> base.render()
Traceback (most recent call last):
...
NotImplementedError: `render` method must be implemented by subclass.

If you have already an existing class that produces the HTML content in some
method, then the ``SimpleAttributeViewlet`` might be for you, since it can be
used to convert any class quickly into a viewlet:

>>> class FooViewlet(viewlet.SimpleAttributeViewlet):
... __page_attribute__ = 'foo'
...
... def foo(self):
... return 'output'

The `__page_attribute__` attribute provides the name of the function to call for
rendering.

>>> foo = FooViewlet('context', 'request', 'view', 'manager')
>>> foo.foo()
'output'
>>> foo.render()
'output'

If you specify `render` as the attribute an error is raised to prevent
infinite recursion:

>>> foo.__page_attribute__ = 'render'
>>> foo.render()
Traceback (most recent call last):
...
AttributeError: render

The same is true if the specified attribute does not exist:

>>> foo.__page_attribute__ = 'bar'
>>> foo.render()
Traceback (most recent call last):
...
AttributeError: 'FooViewlet' object has no attribute 'bar'

To create simple template-based viewlets you can use the
``SimpleViewletClass()`` function. This function is very similar to its view
equivalent and is used by the ZCML directives to create viewlets. The result
of this function call will be a fully functional viewlet class. Let's start by
simply specifying a template only:

>>> template = os.path.join(temp_dir, 'demoTemplate.pt')
>>> with open(template, 'w') as file:
... _ = file.write('''<div>contents</div>''')

>>> Demo = viewlet.SimpleViewletClass(template)
>>> print(Demo(content, request, view, manager).render())
<div>contents</div>

Now let's additionally specify a class that can provide additional features:

>>> class MyViewlet(object):
... myAttribute = 8

>>> Demo = viewlet.SimpleViewletClass(template, bases=(MyViewlet,))
>>> MyViewlet in Demo.__bases__
True
>>> Demo(content, request, view, manager).myAttribute
8

The final important feature is the ability to pass in further attributes to
the class:

>>> Demo = viewlet.SimpleViewletClass(
... template, attributes={'here': 'now', 'lucky': 3})
>>> demo = Demo(content, request, view, manager)
>>> demo.here
'now'
>>> demo.lucky
3

As for all views, they must provide a name that can also be passed to the
function:

>>> Demo = viewlet.SimpleViewletClass(template, name='demoViewlet')
>>> demo = Demo(content, request, view, manager)
>>> demo.__name__
'demoViewlet'

In addition to the the generic viewlet code above, the package comes with two
viewlet base classes and helper functions for inserting CSS and Javascript
links into HTML headers, since those two are so very common. I am only going
to demonstrate the helper functions here, since those demonstrations will
fully demonstrate the functionality of the base classes as well.

The viewlet will look up the resource it was given and tries to produce the
absolute URL for it:

>>> class JSResource(object):
... def __init__(self, request):
... self.request = request
...
... def __call__(self):
... return '/@@/resource.js'

>>> zope.component.provideAdapter(
... JSResource,
... (IDefaultBrowserLayer,),
... zope.interface.Interface, name='resource.js')

>>> JSViewlet = viewlet.JavaScriptViewlet('resource.js')
>>> print(JSViewlet(content, request, view, manager).render().strip())
<script type="text/javascript" src="/@@/resource.js"></script>


There is also a javascript viewlet base class which knows how to render more
then one javascript resource file:

>>> class JSSecondResource(object):
... def __init__(self, request):
... self.request = request
...
... def __call__(self):
... return '/@@/second-resource.js'

>>> zope.component.provideAdapter(
... JSSecondResource,
... (IDefaultBrowserLayer,),
... zope.interface.Interface, name='second-resource.js')

>>> JSBundleViewlet = viewlet.JavaScriptBundleViewlet(('resource.js',
... 'second-resource.js'))
>>> print(JSBundleViewlet(content, request, view, manager).render().strip())
<script type="text/javascript"
src="/@@/resource.js"> </script>
<script type="text/javascript"
src="/@@/second-resource.js"> </script>


The same works for the CSS resource viewlet:

>>> class CSSResource(object):
... def __init__(self, request):
... self.request = request
...
... def __call__(self):
... return '/@@/resource.css'

>>> zope.component.provideAdapter(
... CSSResource,
... (IDefaultBrowserLayer,),
... zope.interface.Interface, name='resource.css')

>>> CSSViewlet = viewlet.CSSViewlet('resource.css')
>>> print(CSSViewlet(content, request, view, manager).render().strip())
<link type="text/css" rel="stylesheet"
href="/@@/resource.css" media="all" />

You can also change the media type and the rel attribute:

>>> CSSViewlet = viewlet.CSSViewlet('resource.css', media='print', rel='css')
>>> print(CSSViewlet(content, request, view, manager).render().strip())
<link type="text/css" rel="css" href="/@@/resource.css"
media="print" />

There is also a bundle viewlet for CSS links:

>>> class CSSPrintResource(object):
... def __init__(self, request):
... self.request = request
...
... def __call__(self):
... return '/@@/print-resource.css'

>>> zope.component.provideAdapter(
... CSSPrintResource,
... (IDefaultBrowserLayer,),
... zope.interface.Interface, name='print-resource.css')

>>> items = []
>>> items.append({'path':'resource.css', 'rel':'stylesheet', 'media':'all'})
>>> items.append({'path':'print-resource.css', 'media':'print'})
>>> CSSBundleViewlet = viewlet.CSSBundleViewlet(items)
>>> print(CSSBundleViewlet(content, request, view, manager).render().strip())
<link type="text/css" rel="stylesheet"
href="/@@/resource.css" media="all" />
<link type="text/css" rel="stylesheet"
href="/@@/print-resource.css" media="print" />


A Complex Example
-----------------

The Data
~~~~~~~~

So far we have only demonstrated simple (maybe overly trivial) use cases of
the viewlet system. In the following example, we are going to develop a
generic contents view for files. The step is to create a file component:

>>> class IFile(zope.interface.Interface):
... data = zope.interface.Attribute('Data of file.')

>>> @zope.interface.implementer(IFile)
... class File(object):
... def __init__(self, data=''):
... self.__name__ = ''
... self.data = data

Since we want to also provide the size of a file, here a simple implementation
of the ``ISized`` interface:

>>> from zope import size
>>> @zope.interface.implementer(size.interfaces.ISized)
... @zope.component.adapter(IFile)
... class FileSized(object):
...
... def __init__(self, file):
... self.file = file
...
... def sizeForSorting(self):
... return 'byte', len(self.file.data)
...
... def sizeForDisplay(self):
... return '%i bytes' %len(self.file.data)

>>> zope.component.provideAdapter(FileSized)

We also need a container to which we can add files:

>>> class Container(dict):
... def __setitem__(self, name, value):
... value.__name__ = name
... super(Container, self).__setitem__(name, value)

Here is some sample data:

>>> container = Container()
>>> container['test.txt'] = File('Hello World!')
>>> container['mypage.html'] = File('<html><body>Hello World!</body></html>')
>>> container['data.xml'] = File('<message>Hello World!</message>')


The View
~~~~~~~~

The contents view of the container should iterate through the container and
represent the files in a table:

>>> contentsTemplate = os.path.join(temp_dir, 'contents.pt')
>>> with open(contentsTemplate, 'w') as file:
... _ = file.write('''
... <html>
... <body>
... <h1>Contents</h1>
... <div tal:content="structure provider:contents" />
... </body>
... </html>
... ''')

>>> from zope.browserpage.simpleviewclass import SimpleViewClass
>>> Contents = SimpleViewClass(contentsTemplate, name='contents.html')


The Viewlet Manager
~~~~~~~~~~~~~~~~~~~

Now we have to write our own viewlet manager. In this case we cannot use the
default implementation, since the viewlets will be looked up for each
different item:

>>> shownColumns = []

>>> @zope.interface.implementer(interfaces.IViewletManager)
... class ContentsViewletManager(object):
... index = None
...
... def __init__(self, context, request, view):
... self.context = context
... self.request = request
... self.__parent__ = view
...
... def update(self):
... rows = []
... for name, value in sorted(self.context.items()):
... rows.append(
... [zope.component.getMultiAdapter(
... (value, self.request, self.__parent__, self),
... interfaces.IViewlet, name=colname)
... for colname in shownColumns])
... [entry.update() for entry in rows[-1]]
... self.rows = rows
...
... def render(self, *args, **kw):
... return self.index(*args, **kw)

Now we need a template to produce the contents table:

>>> tableTemplate = os.path.join(temp_dir, 'table.pt')
>>> with open(tableTemplate, 'w') as file:
... _ = file.write('''
... <table>
... <tr tal:repeat="row view/rows">
... <td tal:repeat="column row">
... <tal:block replace="structure column/render" />
... </td>
... </tr>
... </table>
... ''')

From the two pieces above, we can generate the final viewlet manager class and
register it (it's a bit tedious, I know):

>>> from zope.browserpage import ViewPageTemplateFile
>>> ContentsViewletManager = type(
... 'ContentsViewletManager', (ContentsViewletManager,),
... {'index': ViewPageTemplateFile(tableTemplate)})

>>> zope.component.provideAdapter(
... ContentsViewletManager,
... (Container, IDefaultBrowserLayer, zope.interface.Interface),
... interfaces.IViewletManager, name='contents')

Since we have not defined any viewlets yet, the table is totally empty:

>>> contents = Contents(container, request)
>>> print(contents().strip())
<html>
<body>
<h1>Contents</h1>
<div>
<table>
<tr>
</tr>
<tr>
</tr>
<tr>
</tr>
</table>
</div>
</body>
</html>


The Viewlets and the Final Result
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now let's create a first viewlet for the manager...

>>> class NameViewlet(object):
...
... def __init__(self, context, request, view, manager):
... self.__parent__ = view
... self.context = context
...
... def update(self):
... pass
...
... def render(self):
... return self.context.__name__

and register it:

>>> zope.component.provideAdapter(
... NameViewlet,
... (IFile, IDefaultBrowserLayer,
... zope.interface.Interface, interfaces.IViewletManager),
... interfaces.IViewlet, name='name')

Note how you register the viewlet on ``IFile`` and not on the container. Now
we should be able to see the name for each file in the container:

>>> print(contents().strip())
<html>
<body>
<h1>Contents</h1>
<div>
<table>
<tr>
</tr>
<tr>
</tr>
<tr>
</tr>
</table>
</div>
</body>
</html>

Waaa, nothing there! What happened? Well, we have to tell our user preferences
that we want to see the name as a column in the table:

>>> shownColumns = ['name']

>>> print(contents().strip())
<html>
<body>
<h1>Contents</h1>
<div>
<table>
<tr>
<td>
data.xml
</td>
</tr>
<tr>
<td>
mypage.html
</td>
</tr>
<tr>
<td>
test.txt
</td>
</tr>
</table>
</div>
</body>
</html>

Let's now write a second viewlet that will display the size of the object for
us:

>>> class SizeViewlet(object):
...
... def __init__(self, context, request, view, manager):
... self.__parent__ = view
... self.context = context
...
... def update(self):
... pass
...
... def render(self):
... return size.interfaces.ISized(self.context).sizeForDisplay()

>>> zope.component.provideAdapter(
... SizeViewlet,
... (IFile, IDefaultBrowserLayer,
... zope.interface.Interface, interfaces.IViewletManager),
... interfaces.IViewlet, name='size')

After we added it to the list of shown columns,

>>> shownColumns = ['name', 'size']

we can see an entry for it:

>>> print(contents().strip())
<html>
<body>
<h1>Contents</h1>
<div>
<table>
<tr>
<td>
data.xml
</td>
<td>
31 bytes
</td>
</tr>
<tr>
<td>
mypage.html
</td>
<td>
38 bytes
</td>
</tr>
<tr>
<td>
test.txt
</td>
<td>
12 bytes
</td>
</tr>
</table>
</div>
</body>
</html>

If we switch the two columns around,

>>> shownColumns = ['size', 'name']

the result will be

>>> print(contents().strip())
<html>
<body>
<h1>Contents</h1>
<div>
<table>
<tr>
<td>
31 bytes
</td>
<td>
data.xml
</td>
</tr>
<tr>
<td>
38 bytes
</td>
<td>
mypage.html
</td>
</tr>
<tr>
<td>
12 bytes
</td>
<td>
test.txt
</td>
</tr>
</table>
</div>
</body>
</html>


Supporting Sorting
~~~~~~~~~~~~~~~~~~

Oftentimes you also want to batch and sort the entries in a table. Since those
two features are not part of the view logic, they should be treated with
independent components. In this example, we are going to only implement
sorting using a simple utility:

>>> class ISorter(zope.interface.Interface):
...
... def sort(values):
... """Sort the values."""

>>> @zope.interface.implementer(ISorter)
... class SortByName(object):
...
... def sort(self, values):
... return sorted(values, key=lambda x: x.__name__)

>>> zope.component.provideUtility(SortByName(), name='name')

>>> @zope.interface.implementer(ISorter)
... class SortBySize(object):
...
... def sort(self, values):
... return sorted(
... values,
... key=lambda x: size.interfaces.ISized(x).sizeForSorting())

>>> zope.component.provideUtility(SortBySize(), name='size')

Note that we decided to give the sorter utilities the same name as the
corresponding viewlet. This convention will make our implementation of the
viewlet manager much simpler:

>>> sortByColumn = ''

>>> @zope.interface.implementer(interfaces.IViewletManager)
... class SortedContentsViewletManager(object):
... index = None
...
... def __init__(self, context, request, view):
... self.context = context
... self.request = request
... self.__parent__ = view
...
... def update(self):
... values = self.context.values()
...
... if sortByColumn:
... sorter = zope.component.queryUtility(ISorter, sortByColumn)
... if sorter:
... values = sorter.sort(values)
...
... rows = []
... for value in values:
... rows.append(
... [zope.component.getMultiAdapter(
... (value, self.request, self.__parent__, self),
... interfaces.IViewlet, name=colname)
... for colname in shownColumns])
... [entry.update() for entry in rows[-1]]
... self.rows = rows
...
... def render(self, *args, **kw):
... return self.index(*args, **kw)

As you can see, the concern of sorting is cleanly separated from generating
the view code. In MVC terms that means that the controller (sort) is logically
separated from the view (viewlets). Let's now do the registration dance for
the new viewlet manager. We simply override the existing registration:

>>> SortedContentsViewletManager = type(
... 'SortedContentsViewletManager', (SortedContentsViewletManager,),
... {'index': ViewPageTemplateFile(tableTemplate)})

>>> zope.component.provideAdapter(
... SortedContentsViewletManager,
... (Container, IDefaultBrowserLayer, zope.interface.Interface),
... interfaces.IViewletManager, name='contents')

Finally we sort the contents by name:

>>> shownColumns = ['name', 'size']
>>> sortByColumn = 'name'

>>> print(contents().strip())
<html>
<body>
<h1>Contents</h1>
<div>
<table>
<tr>
<td>
data.xml
</td>
<td>
31 bytes
</td>
</tr>
<tr>
<td>
mypage.html
</td>
<td>
38 bytes
</td>
</tr>
<tr>
<td>
test.txt
</td>
<td>
12 bytes
</td>
</tr>
</table>
</div>
</body>
</html>

Now let's sort by size:

>>> sortByColumn = 'size'

>>> print(contents().strip())
<html>
<body>
<h1>Contents</h1>
<div>
<table>
<tr>
<td>
test.txt
</td>
<td>
12 bytes
</td>
</tr>
<tr>
<td>
data.xml
</td>
<td>
31 bytes
</td>
</tr>
<tr>
<td>
mypage.html
</td>
<td>
38 bytes
</td>
</tr>
</table>
</div>
</body>
</html>

That's it! As you can see, in a few steps we have built a pretty flexible
contents view with selectable columns and sorting. However, there is a lot of
room for extending this example:

- Table Header: The table header cell for each column should be a different
type of viewlet, but registered under the same name. The column header
viewlet also adapts the container not the item. The header column should
also be able to control the sorting.

- Batching: A simple implementation of batching should work very similar to
the sorting feature. Of course, efficient implementations should somehow
combine batching and sorting more effectively.

- Sorting in ascending and descending order: Currently, you can only sort from
the smallest to the highest value; however, this limitation is almost
superficial and can easily be removed by making the sorters a bit more
flexible.

- Further Columns: For a real application, you would want to implement other
columns, of course. You would also probably want some sort of fallback for
the case that a viewlet is not found for a particular container item and
column.


Cleanup
-------

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


================================
The ``viewletManager`` Directive
================================

The ``viewletManager`` directive allows you to quickly register a new viewlet
manager without worrying about the details of the ``adapter``
directive. Before we can use the directives, we have to register their
handlers by executing the package's meta configuration:

>>> from zope.configuration import xmlconfig
>>> context = xmlconfig.string('''
... <configure i18n_domain="zope">
... <include package="zope.viewlet" file="meta.zcml" />
... </configure>
... ''')

Now we can register a viewlet manager:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewletManager
... name="defaultmanager"
... permission="zope.Public"
... />
... </configure>
... ''', context=context)

Let's make sure the directive has really issued a sensible adapter
registration; to do that, we create some dummy content, request and view
objects:

>>> import zope.interface
>>> @zope.interface.implementer(zope.interface.Interface)
... class Content(object):
... pass
>>> content = Content()

>>> from zope.publisher.browser import TestRequest
>>> request = TestRequest()

>>> from zope.publisher.browser import BrowserView
>>> view = BrowserView(content, request)

Now let's lookup the manager. This particular registration is pretty boring:

>>> import zope.component
>>> from zope.viewlet import interfaces
>>> manager = zope.component.getMultiAdapter(
... (content, request, view),
... interfaces.IViewletManager, name='defaultmanager')

>>> manager
<zope.viewlet.manager.<ViewletManager providing IViewletManager> object ...>
>>> interfaces.IViewletManager.providedBy(manager)
True
>>> manager.template is None
True
>>> manager.update()
>>> manager.render()
u''

However, this registration is not very useful, since we did specify a specific
viewlet manager interface, a specific content interface, specific view or
specific layer. This means that all viewlets registered will be found.

The first step to effectively using the viewlet manager directive is to define
a special viewlet manager interface:

>>> class ILeftColumn(interfaces.IViewletManager):
... """Left column of my page."""

Now we can register register a manager providing this interface:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewletManager
... name="leftcolumn"
... permission="zope.Public"
... provides="zope.viewlet.directives.ILeftColumn"
... />
... </configure>
... ''', context=context)

>>> manager = zope.component.getMultiAdapter(
... (content, request, view), ILeftColumn, name='leftcolumn')

>>> manager
<zope.viewlet.manager.<ViewletManager providing ILeftColumn> object ...>
>>> ILeftColumn.providedBy(manager)
True
>>> manager.template is None
True
>>> manager.update()
>>> manager.render()
u''

Next let's see what happens, if we specify a template for the viewlet manager:

>>> import os, tempfile
>>> temp_dir = tempfile.mkdtemp()

>>> leftColumnTemplate = os.path.join(temp_dir, 'leftcolumn.pt')
>>> with open(leftColumnTemplate, 'w') as file:
... _ = file.write('''
... <div class="column">
... <div class="entry"
... tal:repeat="viewlet options/viewlets"
... tal:content="structure viewlet" />
... </div>
... ''')

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewletManager
... name="leftcolumn"
... permission="zope.Public"
... provides="zope.viewlet.directives.ILeftColumn"
... template="%s"
... />
... </configure>
... ''' %leftColumnTemplate, context=context)

>>> manager = zope.component.getMultiAdapter(
... (content, request, view), ILeftColumn, name='leftcolumn')

>>> manager
<zope.viewlet.manager.<ViewletManager providing ILeftColumn> object ...>
>>> ILeftColumn.providedBy(manager)
True
>>> manager.template
<BoundPageTemplateFile of ...<ViewletManager providing ILeftColumn> ...>>
>>> manager.update()
>>> print(manager.render().strip())
<div class="column">
</div>

Additionally you can specify a class that will serve as a base to the default
viewlet manager or be a viewlet manager in its own right. In our case we will
provide a custom implementation of the ``sort()`` method, which will sort by a
weight attribute in the viewlet:

>>> class WeightBasedSorting(object):
... def sort(self, viewlets):
... return sorted(viewlets, key=lambda x: getattr(x[1], 'weight', 0))

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewletManager
... name="leftcolumn"
... permission="zope.Public"
... provides="zope.viewlet.directives.ILeftColumn"
... template="%s"
... class="zope.viewlet.directives.WeightBasedSorting"
... />
... </configure>
... ''' %leftColumnTemplate, context=context)

>>> manager = zope.component.getMultiAdapter(
... (content, request, view), ILeftColumn, name='leftcolumn')

>>> manager
<zope.viewlet.manager.<ViewletManager providing ILeftColumn> object ...>
>>> manager.__class__.__bases__
(<class 'zope.viewlet.directives.WeightBasedSorting'>,
<class 'zope.viewlet.manager.ViewletManagerBase'>)
>>> ILeftColumn.providedBy(manager)
True
>>> manager.template
<BoundPageTemplateFile of ...<ViewletManager providing ILeftColumn> ...>>
>>> manager.update()
>>> print(manager.render().strip())
<div class="column">
</div>

Finally, if a non-existent template is specified, an error is raised:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewletManager
... name="leftcolumn"
... permission="zope.Public"
... template="foo.pt"
... />
... </configure>
... ''', context=context)
Traceback (most recent call last):
...
ZopeXMLConfigurationError: File "<string>", line 3.2-7.8
ConfigurationError: ('No such file', '...foo.pt')


=========================
The ``viewlet`` Directive
=========================

Now that we have a viewlet manager, we have to register some viewlets for
it. The ``viewlet`` directive is similar to the ``viewletManager`` directive,
except that the viewlet is also registered for a particular manager interface,
as seen below:

>>> weatherTemplate = os.path.join(temp_dir, 'weather.pt')
>>> with open(weatherTemplate, 'w') as file:
... _ = file.write('''
... <div>sunny</div>
... ''')

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewlet
... name="weather"
... manager="zope.viewlet.directives.ILeftColumn"
... template="%s"
... permission="zope.Public"
... extra_string_attributes="can be specified"
... />
... </configure>
... ''' % weatherTemplate, context=context)

If we look into the adapter registry, we will find the viewlet:

>>> viewlet = zope.component.getMultiAdapter(
... (content, request, view, manager), interfaces.IViewlet,
... name='weather')
>>> viewlet.render().strip()
u'<div>sunny</div>'
>>> viewlet.extra_string_attributes
u'can be specified'

The manager now also gives us the output of the one and only viewlet:

>>> manager.update()
>>> print(manager.render().strip())
<div class="column">
<div class="entry">
<div>sunny</div>
</div>
</div>

Let's now ensure that we can also specify a viewlet class:

>>> class Weather(object):
... weight = 0

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewlet
... name="weather2"
... for="*"
... manager="zope.viewlet.directives.ILeftColumn"
... template="%s"
... class="zope.viewlet.directives.Weather"
... permission="zope.Public"
... />
... </configure>
... ''' % weatherTemplate, context=context)

>>> viewlet = zope.component.getMultiAdapter(
... (content, request, view, manager), interfaces.IViewlet,
... name='weather2')
>>> viewlet().strip()
u'<div>sunny</div>'

Okay, so the template-driven cases work. But just specifying a class should
also work:

>>> class Sport(object):
... weight = 0
... def __call__(self):
... return u'Red Sox vs. White Sox'

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewlet
... name="sport"
... for="*"
... manager="zope.viewlet.directives.ILeftColumn"
... class="zope.viewlet.directives.Sport"
... permission="zope.Public"
... />
... </configure>
... ''', context=context)

>>> viewlet = zope.component.getMultiAdapter(
... (content, request, view, manager), interfaces.IViewlet, name='sport')
>>> viewlet()
u'Red Sox vs. White Sox'

It should also be possible to specify an alternative attribute of the class to
be rendered upon calling the viewlet:

>>> class Stock(object):
... weight = 0
... def getStockTicker(self):
... return u'SRC $5.19'

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewlet
... name="stock"
... for="*"
... manager="zope.viewlet.directives.ILeftColumn"
... class="zope.viewlet.directives.Stock"
... attribute="getStockTicker"
... permission="zope.Public"
... />
... </configure>
... ''', context=context)

>>> viewlet = zope.component.getMultiAdapter(
... (content, request, view, manager), interfaces.IViewlet,
... name='stock')
>>> viewlet.render()
u'SRC $5.19'

A final feature the ``viewlet`` directive is that it supports the
specification of any number of keyword arguments:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewlet
... name="stock2"
... permission="zope.Public"
... class="zope.viewlet.directives.Stock"
... weight="8"
... />
... </configure>
... ''', context=context)

>>> viewlet = zope.component.getMultiAdapter(
... (content, request, view, manager), interfaces.IViewlet,
... name='stock2')
>>> viewlet.weight
u'8'


Error Scenarios
---------------

Neither the class or template have been specified:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewlet
... name="testviewlet"
... manager="zope.viewlet.directives.ILeftColumn"
... permission="zope.Public"
... />
... </configure>
... ''', context=context)
Traceback (most recent call last):
...
ZopeXMLConfigurationError: File "<string>", line 3.2-7.8
ConfigurationError: Must specify a class or template

The specified attribute is not ``__call__``, but also a template has been
specified:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewlet
... name="testviewlet"
... manager="zope.viewlet.directives.ILeftColumn"
... template="test_viewlet.pt"
... attribute="faux"
... permission="zope.Public"
... />
... </configure>
... ''', context=context)
Traceback (most recent call last):
...
ZopeXMLConfigurationError: File "<string>", line 3.2-9.8
ConfigurationError: Attribute and template cannot be used together.

Now, we are not specifying a template, but a class that does not have the
specified attribute:

>>> context = xmlconfig.string('''
... <configure xmlns="http://namespaces.zope.org/browser" i18n_domain="zope">
... <viewlet
... name="testviewlet"
... manager="zope.viewlet.directives.ILeftColumn"
... class="zope.viewlet.directives.Sport"
... attribute="faux"
... permission="zope.Public"
... />
... </configure>
... ''', context=context)
Traceback (most recent call last):
...
ZopeXMLConfigurationError: File "<string>", line 3.2-9.8
ConfigurationError: The provided class doesn't have the specified attribute


Cleanup
-------

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


Changes
=======

4.0.0 (2014-12-24)
------------------

- Add support for PyPy and PyPy3.

- Add support for Python 3.4.

- Add support for testing on Travis.


4.0.0a1 (2013-02-24)
--------------------

- Add support for Python 3.3.

- Replace deprecated ``zope.component.adapts`` usage with equivalent
``zope.component.adapter`` decorator.

- Replace deprecated ``zope.interface.implements`` usage with equivalent
``zope.interface.implementer`` decorator.

- Drop support for Python 2.4 and 2.5.


3.7.2 (2010-05-25)
------------------

- Fix unit tests broken under Python 2.4 by the switch to the standard
library ``doctest`` module.


3.7.1 (2010-04-30)
------------------

- Remove use of 'zope.testing.doctest' in favor of stdlib's 'doctest.

- Fix dubious quoting in metadirectives.py. Closes
https://bugs.launchpad.net/zope2/+bug/143774.


3.7.0 (2009-12-22)
------------------

- Depend on ``zope.browserpage`` in favor of ``zope.app.pagetemplate``.


3.6.1 (2009-08-29)
------------------

- Fix unit tests in README.txt.


3.6.0 (2009-08-02)
------------------

- Optimize the the script tag for the JS viewlet. This makes YSlow happy.

- Remove ZCML slugs and old zpkg-related files.

- Drop all testing dependncies except ``zope.testing``.


3.5.0 (2009-01-26)
------------------

- Remove the dependency on ``zope.app.publisher`` by moving four simple helper
functions into this package and making the interface for describing the
ZCML content provider directive explicit.

- Typo fix in CSSViewlet docstring.


3.4.2 (2008-01-24)
------------------

- Re-release of 3.4.1 because of brown bag release.


3.4.1 (2008-01-21)
------------------

- Implement missing ``__contains__`` method in IViewletManager

- Implement additional viewlet managers offering weight ordered sorting

- Implement additional viewlet managers offering conditional filtering


3.4.1a (2007-4-22)
------------------

- Add a missing ',' behind ``zope.i18nmessageid``.

- Recreate the ``README.txt`` removing everything except for the overview.


3.4.0 (2007-10-10)
------------------

- Initial release independent of the main Zope tree.