Comprehensive Theming
=====================

Any application, including Otto, can be loosely divided into two parts:

- the user interface ("how it looks"),
- and the application logic ("how it works").

For example, when considering a product in Otto,
the user interface consists of how the products are laid out on the page, how the selectors look,
how the checkout button is labelled, what sort of fonts and colors are used to display the text, and so on.
The application logic consists of how Otto adjusts product price based on the discount coupons,
and how it records that information to be displayed in the future.

Theming consists of changing the user interface without changing the application logic.
When setting up an E-Commerce website, the website operator often wants to use their own logo, modify the color scheme,
change links in the header and footer for SEO (search engine optimization) purposes, and so on.
However, although the user interface may look different, the application logic must remain the same so that Otto
continues to work properly. A well-designed theme preserves the general layout and structure of the user interface,
so that users of the website still find it familiar and easy to use.
Be careful about making sweeping changes to the user interface without warning: your users will be very confused!

.. note::
    Comprehensive Theming can be disabled by setting ``ENABLE_COMPREHENSIVE_THEMING`` to ``False``.

    .. code-block:: python

        ENABLE_COMPREHENSIVE_THEMING = False

---------------
Theme Structure
---------------
From a technical perspective, theming consists of overriding core

* Templates,
* Static Assets and
* Sass

with themed versions of those resources. Every theme must conform to a directory structure.
This structure mirrors the Otto directory structure and looks like this

.. code-block:: text

    my-theme
        ├── README.rst
        ├── static
        |      └── images
        |      |     └── logo.png
        |      |
        |      └── sass
        |            └── partials
        |                   └── utilities
        |                           └── _variables.scss
        |
        └── templates
                └── oscar
                |     └── dashboard
                |             └── index.html
                └── 404.html

-----------
- Templates
-----------
Any template included in ``ecommerce/templates`` directory is themable. However, make sure not to override
class names or id values of html elements inside a template, as these are used by javascript and/or css and overriding
these could cause unwanted behavior.

---------------
- Static Assets
---------------
Any static asset included in ``ecommerce/static`` can be overridden except css files present in ``ecommerce/static/css``.
Css styles can be overridden via sass overrides explained below.

.. caution::
    Theme names should be unique and no static asset and/or directory name should be same as theme's name.
    Otherwise static assets would not work correctly.

------
- Sass
------
Sass overrides are a little different from static asset or template overrides.
There are two types of styles included in ``ecommerce/static/sass``:

- `base`
- `partials`

.. WARNING::
    Styles present in ``ecommerce/static/sass/base`` should not be overridden as overriding these
    could result in an unexpected behavior.

Any styles included in ``ecommerce/static/sass/partials`` can be overridden.
Styles included in this directory contain variable definitions that are used by main sass files. This is the best place
if you want to update things like header/footer background, fonts etc.

----------------
Enabling a Theme
----------------
To enable a theme, you must first install your theme onto the same server that is running Otto.
If you are using devstack or fullstack to run Otto, you must be sure that the theme is present on the Vagrant virtual machine.
It is up to you where to install the theme on the server, but a good default location is ``/edx/app/ecommerce/ecommerce/themes``.

.. note::
    All themes must reside in the same physical directory.

In order for Otto to use the installed themes, you must specify the location of the theme directory in
Django settings by setting COMPREHENSIVE_THEME_DIRS in your settings file:

.. code-block:: python

    COMPREHENSIVE_THEME_DIRS = ["/edx/app/ecommerce/ecommerce/themes", ]

Where ``/edx/app/ecommerce/ecommerce/themes`` is the path to where you have installed the
themes on your server. You can list any/all theme directories through this setting.

After installing a theme, it is associated with sites by adding appropriate entries to the following tables

- ``Site``
- ``Site Themes``

for local devstack, if Otto server is running at ``localhost:8002`` you can enable a ``my-theme`` by

- Adding a new site with domain ``localhost:8002`` and name "Otto My Theme"
- and a site theme with Theme dir name ``my-theme`` and selecting ``localhost:8002`` from site dropdown.

Otto server can now be started, and ``my-theme`` should be applied now. If you have overridden sass styles and you are not
seeing those overrides then you need to compile sass files as discussed in `Compiling Theme Sass`_.

-----------------
Disabling a Theme
-----------------
Theme can be disabled by removing its corresponding ``Site Theme`` entry using django admin.

--------------------
Compiling Theme Sass
--------------------
Management command ``update_assets`` can be used for compiling and collecting themed sass.

.. code-block:: yaml

    python manage.py update_assets

``update_assets`` accepts the following optional arguments

    :--settings: settings file to use, ``default: ecommerce.settings.devstack``

        .. code-block:: Bash

            python manage.py update_assets --settings=ecommerce.settings.production

    :--themes: Space separated list of themes to compile sass for. 'all' for all themes,
        'no' to skip sass compilation for themes,  ``default: 'all'``

        .. code-block:: Bash

            # compile sass for all themes
            python manage.py update_assets --theme=all

            # compile sass for only given themes, useful for situations if you have installed a new theme
            # and want to compile sass for just this theme
            python manage.py update_assets --themes my-theme second-theme third-theme

            # skip sass compilation for themes, useful for testing changes to system
            # sass, keeping theme styles unchanged
            python manage.py update_assets --theme=no

    :--output-style: Coding style for compiled css files. Possible options are ``nested``, ``expanded``,
        ``compact`` and ``compressed``. ``default: 'nested'``

        .. code-block:: Bash

            python manage.py update_assets --output-style='compressed'

    :--skip-system: This flag disables system sass compilation.

        .. code-block:: Bash

            # useful in cases where you have updated theme sass and system sass is unchanged.
            python manage.py update_assets --skip-system

    :--enable-source-comments: This flag enables source comments in generated css files

        .. code-block:: Bash

            python manage.py update_assets --enable-source-comments

    :--skip-collect: This flag can be used to skip collectstatic call after sass compilation

        .. code-block:: Bash

            # useful if you just want to compile sass, and collectstatic would later be called, may be by a script
            python manage.py update_assets --skip-collect

---------------
Troubleshooting
---------------
If you have gone through the above procedure and you are not seeing theme overrides, you need to make sure

- ``COMPREHENSIVE_THEME_DIRS`` must contain path for the directory containing themes e.g. if your theme is
  ``/edx/app/ecommerce/ecommerce/themes/my-theme`` then correct value for ``COMPREHENSIVE_THEME_DIRS`` is
  ``['/edx/app/ecommerce/ecommerce/themes']``.
- ``domain`` name for site is the name users will put in the browser to access the site, it also includes port number
  e.g. if Otto is running on ``localhost:8002`` then domain should be ``localhost:8002``
- Theme dir name is the name of the directory of you theme, for our ongoing example ``my-theme``
  is the correct theme dir name.