.. _Views:

Principles

----------

We'll start with a description of the interface providing a basic

understanding of the available classes and methods, then detail the

view selection principle.

A `View` is an object responsible for the rendering of data from the

model into an end-user consummable form. They typically churn out an

XHTML stream, but there are views concerned with email other non-html

outputs.

.. _views_base_class:

Discovering possible views

~~~~~~~~~~~~~~~~~~~~~~~~~~

It is possible to configure the web user interface to have a left box

showing all the views than can be applied to the current result set.

To enable this, click on your login at the top right corner. Chose

"user preferences", then "boxes", then "possible views box" and check

"visible = yes" before validating your changes.

The views listed there we either not selected because of a lower

score, or they were deliberately excluded by the main template logic.

Basic class for views

~~~~~~~~~~~~~~~~~~~~~

Class `View` (`cubicweb.view`)

```````````````````````````````

This class is an abstraction of a view class, used as a base class for

every renderable object such as views, templates and other user

interface components.

A `View` is instantiated to render a result set or part of a result

set. `View` subclasses may be parametrized using the following class

attributes:

* `templatable` indicates if the view may be embedded in a main

  template or if it has to be rendered standalone (i.e. pure XML views

  must not be embedded in the main template of HTML pages)

* if the view is not templatable, it should set the `content_type`

  class attribute to the correct MIME type (text/xhtml being the

  default)

* the `category` attribute may be used in the interface to regroup

  related view kinds together

A view writes to its output stream thanks to its attribute `w` (the

append method of an `UStreamIO`, except for binary views).

At instantiation time, the standard `_cw` and `cw_rset` attributes are

added and the `w` attribute will be set at rendering time.

The basic interface for views is as follows (remember that the result

set has a tabular structure with rows and columns, hence cells):

* `render(**context)`, render the view by calling `call` or

  `cell_call` depending on the context

* `call(**kwargs)`, call the view for a complete result set or null

  (the default implementation calls `cell_call()` on each cell of the

  result set)

* `cell_call(row, col, **kwargs)`, call the view for a given cell of a

  result set (`row` and `col` being integers used to access the cell)

* `url()`, returns the URL enabling us to get the view with the current

  result set

* `wview(__vid, rset, __fallback_vid=None, **kwargs)`, call the view of

  identifier `__vid` on the given result set. It is possible to give a

  fallback view identifier that will be used if the requested view is

  not applicable to the result set.

* `html_headers()`, returns a list of HTML headers to be set by the

  main template

* `page_title()`, returns the title to use in the HTML header `title`

Other basic view classes

````````````````````````

Here are some of the subclasses of `View` defined in `cubicweb.common.view`

that are more concrete as they relate to data rendering within the application:

* `EntityView`, view applying to lines or cell containing an entity (e.g. an eid)

* `StartupView`, start view that does not require a result set to apply to

* `AnyRsetView`, view applicable to any result set

Examples of views class

```````````````````````

- Using `templatable`, `content_type` and HTTP cache configuration

.. sourcecode:: python

    class RSSView(XMLView):

        __regid__ = 'rss'

        title = _('rss')

        templatable = False

        content_type = 'text/xml'

        http_cache_manager = MaxAgeHTTPCacheManager

        cache_max_age = 60*60*2 # stay in http cache for 2 hours by default

- Using a custom selector

.. sourcecode:: python

    class SearchForAssociationView(EntityView):

        """view called by the edition view when the user asks

        to search for something to link to the edited eid

        """

        __regid__ = 'search-associate'

        title = _('search for association')

        __select__ = one_line_rset() & match_search_state('linksearch') & implements('Any')

XML views, binaries views...

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For views generating other formats than HTML (an image generated dynamically

for example), and which can not simply be included in the HTML page generated

by the main template (see above), you have to:

* set the attribute `templatable` of the class to `False`

* set, through the attribute `content_type` of the class, the MIME

  type generated by the view to `application/octet-stream` or any

  relevant and more specialised mime type

For views dedicated to binary content creation (like dynamically generated

images), we have to set the attribute `binary` of the class to `True` (which

implies that `templatable == False`, so that the attribute `w` of the view could be

replaced by a binary flow instead of unicode).