.. _link-plugin:

===========
Link Plugin
===========

**djangocms-cascade** ships with its own link plugin. This is because other plugins from the
Cascade eco-system, such as the **BootstrapButtonPlugin**, the **BootstrapImagePlugin** or the
**BootstrapPicturePlugin** also require a functionality in order to set links to internal- and
external URLs. Since we do not want to duplicate the linking functionality for each of those
plugins, it has been moved into its own mixin-classes. Therefore we will use the terminology
**TextLinkPlugin** when referring to text-based links.

The de-facto plugin for links, djangocms-link_ can't be used as a base class for these plugins,
hence an alternative implementation has been created within the Cascade framework. The link related
data is stored in a various fields in our main JSON field (named ``glossary``).


Prerequisites
=============

Before using this plugin, assure that ``'cmsplugin_cascade.link'`` is member of the list or
tuple ``CMSPLUGIN_CASCADE_PLUGINS`` in the project's ``settings.py``.

|simple-link-element|

.. |simple-link-element| image:: _static/simple-link-element.png

The behavior of this Plugin is what you expect from a Link editor. The field **Link Content** is the
text displayed between the opening and closing ``<a>`` tag. If used in combination with
djangocms-text-ckeditor_ the field automatically is filled out.

By changing the **Link type**, the user can choose between different types of Links:

 * Internal Links pointing to another page inside the CMS.
 * External Links pointing to a valid Internet URL.
 * Files from **django-filer** to download.
 * Links pointing to a valid e-mail address.
 * Optionally any other linkable object, if another Django application extends the Link-Plugin (see
   below for details).

The optional field **Title** can be used to add a ``title="some value"`` attribute to the
``<a ...>`` element.

With **Link Target**, the user can specify, whether the linked content shall open in the current
window or if the browser shall open a new window.


Link Plugin with Sharable Fields
================================

If your web-site contains many links pointing onto a few external URLs, you might want to refer to
them by a symbolic name, rather than having to reenter the URL repeatedly. With
**djangocms-cascade** this can be achieved easily by declaring some of the plugin's fields as
*sharable*.

Assure that ``INSTALLED_APPS`` contains ``'cmsplugin_cascade.sharable'``, then redefine the
**TextLinkPlugin** to have sharable fields in ``settings.py``:

.. code-block:: python

	CMSPLUGIN_CASCADE = {
	    ...
	    'plugins_with_sharables':
	        …
	        'TextLinkPlugin': ['link_type', 'ext_url'],
	        …
	    },
	    ...
	}

This will change the Link Plugin's editor slightly. Note the extra field added to the bottom of the
form.

|sharable-link-element|

.. |sharable-link-element| image:: _static/sharable-link-element.png

The URL for this link entity now is stored in a central entity. This feature is useful, if for
instance the URL of an external web page may change in the future. Then the administrator can change
that link in the administration area once, rather than having to go through all the pages and check
if that link was used.

To retain the Link settings, click onto the checkbox *Remember these settings as: ...* and give it
a name of your choice. The next time your create a Shared Link element, you may select a previously
named settings from the select field *Shared Settings*. Since these settings can be shared among
other plugins, these input fields are disabled and can't be changed anymore.


Changing shared settings
------------------------

The settings of a shared plugin can be changed globally, for all plugins using them. To edit such a
shared setting, in the Django Admin, go into the list view for
**Home › django CMS Cascade › Shared between Plugins** and choose the named shared settings.

Please note, that each plugin type can specify which fields shall be sharable between plugins of
the same type. In this example, only the Link itself is shared, but one could configure
**djangocms-cascade** to also share the link's ``title``, the ``target``, and other tags.

Then only these fields are editable in the detail view **Shared between Plugins**. The interface
for other shared plugin may vary substantially, depending of their type definition.


Extending the Link Plugin
=========================

While programming third party modules for Django, one might have to access a model instance through
a URL and thus add the method get_absolute_url_ to that Django model. Since such a URL is neither a
CMS page, nor a URL to an external web page, it would be convenient to access that model using a
special Link type.

For example, in django-shop_ we can allow to link directly to a product, sold by the shop.
This is achieved by reconfiguring the Link Plugin inside Cascade with:

.. code-block:: python

	CMSPLUGIN_CASCADE = {
	    …
	    'link_plugin_classes': (
	        'shop.cascade.plugin_base.CatalogLinkPluginBase',
	        'shop.cascade.plugin_base.CatalogLinkForm',
	    ),
	    …
	}

The tuple specified through ``link_plugin_classes`` replaces the base class for the **LinkPlugin**
class and the form class used by its editor.

Here we replace the two built-in classes :class:`cmsplugin_cascade.link.plugin_base.DefaultLinkPluginBase`
and :class:`cmsplugin_cascade.link.forms.LinkForm` by alternative implementations.

.. code-block:: python
	:caption: shop/cascade/plugin_base.py

	from entangled.forms import get_related_object
	from cmsplugin_cascade.link.plugin_base import LinkPluginBase

	class CatalogLinkPluginBase(LinkPluginBase):
	    @classmethod
	    def get_link(cls, obj):
	        link_type = obj.glossary.get('link_type')
	        if link_type == 'product':
	            relobj = get_related_object(obj.glossary, 'product')
	            if relobj:
	                return relobj.get_absolute_url()
	        else:
	            return super().get_link(obj) or link_type

This class handles links of type "Product" and creates a URL pointing onto a Django model implementing
the method ``get_absolute_url``.

Additionally, we have to override the form class used by the Link plugin editor:

.. code-block:: python
	:caption: shop/cascade/plugin_base.py

	from cms.plugin_pool import plugin_pool
	from django.forms import models
	from shop.models.product import ProductModel

	class CatalogLinkForm(LinkForm):
	    LINK_TYPE_CHOICES = [
	        ('cmspage', _("CMS Page")),
	        ('product', _("Product")),
	        ('download', _("Download File")),
	        ('exturl', _("External URL")),
	        ('email', _("Mail To")),
	    ]

	    product = models.ModelChoiceField(
	        label=_("Product"),
	        queryset=ProductModel.objects.all(),
	        required=False,
	        help_text=_("An internal link onto a product from the catalog"),
	    )

	    class Meta:
	        entangled_fields = {'glossary': ['product']}

Now the select box for **Link type** will offer one additional option named "Product". When this is
selected, the page administrator can select one product in the shop and the link will point onto
its proper detail page.


Using Links in your own Plugins
===============================

Many HTML components allow to link onto other resources, for instance images, the button element,
icons, etc. Since we don't want the reimplement the linking functionality for each of them,
**djangocms-cascade** offers a few base classes, which can be used by those plugin. As an example,
let's implement a simple button plugin.

.. code-block:: python
	:caption: myproject/cascade/button.py

	from django.forms import models
	from cms.plugin_pool import plugin_pool
	from cmsplugin_cascade.link.config import LinkPluginBase, LinkFormMixin
	from cmsplugin_cascade.link.plugin_base import LinkElementMixin

	class ButtonForm(LinkFormMixin):
	    require_link = False

	    button_content = models.CharField(
	        label=_("Button Content"),
	    )

	    class Meta:
	        entangled_fields = {'glossary': ['link_content']}

	class ButtonPlugin(LinkPluginBase):
	    name = _("Button")
	    model_mixins = (LinkElementMixin,)
	    form = ButtonForm
	    render_template = 'myproject/button.html'
	    allow_children = False

	plugin_pool.register_plugin(ButtonPlugin)

What we see here is, that our ``ButtonForm``, which is used by our ``ButtonPlugin`` inherits from
a base form offering all the fields required to link somewhere. Sine the button may just display
some content, but without linking anywhere, we make that optional by setting ``require_link`` to
``False``. The box for selecting the "Link Type" then adds "No Link" to its set of options.

We don't even have to bother, whether our custom button can point onto links types specified by yet
another third party app, and not handled by **djangocms-cascade** – All these additional link types
are handled automatically by the configuration setting ``CMSPLUGIN_CASCADE['link_plugin_classes']``
as explained in the previous section.


.. _djangocms-link: https://github.com/divio/djangocms-link
.. _djangocms-text-ckeditor: https://github.com/divio/djangocms-text-ckeditor
.. _get_absolute_url: https://docs.djangoproject.com/en/stable/ref/models/instances/#get-absolute-url
.. _django-shop: https://github.com/awesto/django-shop