Breadcrumbs
-----------

Breadcrumbs are a navigation component to help the user locate himself
along a path of entities.

Display
~~~~~~~

Breadcrumbs are displayed by default in the header section (see
:ref:`the_main_template_sections`).  With the default main
template, the header section is composed by the logo, the application
name, breadcrumbs and, at the most right, the login box. Breadcrumbs
are displayed just next to the application name, thus breadcrumbs
begin with a separator.

Here is the header section of the CubicWeb's forge:

.. image:: ../../images/breadcrumbs_header.png

There are three breadcrumbs components defined in
:mod:`cubicweb.web.views.ibreadcrumbs`:

- `BreadCrumbEntityVComponent`: displayed for a result set with one line
  if the entity implements the ``IBreadCrumbs`` interface.
- `BreadCrumbETypeVComponent`: displayed for a result set with more than
  one line, but with all entities of the same type which implement the
  ``IBreadCrumbs`` interface.
- `BreadCrumbAnyRSetVComponent`: displayed for any other result set.

Building breadcrumbs
~~~~~~~~~~~~~~~~~~~~

The ``IBreadCrumbs`` interface is defined in the
:mod:`cubicweb.interfaces` module. It specifies that an entity which
implements this interface must have a ``breadcrumbs`` method.

.. note::

   Redefining the breadcrumbs is the hammer way to do it. Another way
   is to define the `parent` method on an entity (as defined in the
   `ITree` interface). If available, it will be used to compute
   breadcrumbs.

Here is the API of the ``breadcrumbs`` method:

.. automethod:: cubicweb.interfaces.IBreadCrumbs.breadcrumbs

If the breadcrumbs method return a list of entities, the
``cubicweb.web.views.ibreadcrumbs.BreadCrumbView`` is used to display
the elements.

By default, for any entity, if recurs=True, breadcrumbs method returns
a list of entities, else a list of a simple string.

In order to see a hierarchical breadcrumbs, entities must have a
``parent`` method which returns the parent entity. By default this
method doesn't exist on entity, given that it can not be guessed.