===============================
Write lean easy-to-maintain CSS
===============================

There are many ways to lean and simplify SCSS. The first step is to establish if custom code is
needed at all.

Odoo's webclient has been designed to be modular, meaning that (potentially all) classes can be
shared across views. Check the code before creating a new class. Chances are that there is already a
class or an HTML tag doing exactly what you're looking for.

On top of that, Odoo relies on `Bootstrap
<https://getbootstrap.com/docs/5.1/getting-started/introduction/>`_ (BS), one of the most complete
CSS frameworks available. The framework has been customized in order to match Odoo's design (both
community and enterprise versions), meaning that you can use any BS class directly in Odoo and
achieve a visual result that is consistent with our UI.

.. warning::
   - The fact that a class achieves the desired visual result doesn't necessarily mean that it's the
     right one for the job. Be aware of classes triggering JS behaviors, for example.
   - Be careful about class semantics. Applying a **button class** to a **title** is not only
     semantically wrong, it may also lead to migration issues and visual inconsistencies.

The following sections describe tips to strip-down SCSS lines **when custom-code is the only way to
go**.

.. _tutorials/scss_tips/browser_defaults:

Browser defaults
================

By default, each browser renders content using a *user agent stylesheet*. To overcome
inconsistencies between browsers, some of these rules are overridden by `Bootstrap Reboot
<https://getbootstrap.com/docs/5.1/content/reboot/>`_.

At this stage all "browser-specific-decoration" rules have been stripped away, but a big chunk of
rules defining basic layout information is maintained (or reinforced by *Reboot* for consistency
reasons).

You can rely on these rules.

.. example::

   Applying `display: block;` to a `<div/>` is normally not necessary.

   .. code-block:: css

      div.element {
         display: block;
         /* not needed 99% of the time */
      }

.. example::

   In this instance, you may opt to switching the HTML tag rather than adding a new CSS rule.

   .. code-block:: css

      span.element {
         display: block;
         /* replace <span> with <div> instead
            to get 'display: block' by default */
      }

Here's a non-comprehensive list of default rules:

.. list-table::
   :header-rows: 1

   * - Tag / Attribute
     - Defaults
   * - `<div/>`, `<section/>`, `<header/>`, `<footer/>`...
     - `display: block`
   * - `<span/>`, `<a/>`, `<em/>`, `<b/>`...
     - `display: inline`
   * - `<button/>`, `<label/>`, `<output/>`...
     - `display: inline-block`
   * - `<img/>`, `<svg/>`
     - `vertical-align: middle`
   * - `<summary/>`, `[role="button"]`
     - `cursor: pointer;`
   * - `<q/>`
     - | `:before {content: open-quote}`
       | `:after  {content: close-quote}`
   * - ...
     - ...

.. seealso::
   `Bootstrap Reboot on GitHub
   <https://github.com/twbs/bootstrap/blob/1a6fdfae6b/scss/_reboot.scss>`_

.. _tutorials/scss_tips/html_tags:

HTML tags
=========

It may seem obvious, but the simplest and most **consistent** way of making text look like a title
is to use a header tag (`<h1>`, `<h2>`, ...). Besides reboot rules, mostly all tags carry decorative
styles defined by Odoo.

.. rst-class:: bg-light
.. example::

   .. container:: alert alert-danger

      Don't

      .. tabs::

         .. code-tab:: html XML

            <span class="o_module_custom_title">
               Hello There!
            </span>

            <span class="o_module_custom_subtitle">
               I'm a subtitle.
            </span>

         .. code-tab:: css SCSS

            .o_module_custom_title {
               display: block;
               font-size: 120%;
               font-weight: bold;
               animation: 1s linear 1s mycustomAnimation;
            }

            .o_module_custom_subtitle {
               display: block;
               font-size: 12px;
               font-weight: bold;
               animation: 2s linear 1s mycustomAnimation;
            }

   .. container:: alert alert-success

      Do

      .. tabs::

         .. code-tab:: html XML

            <h5 class="o_module_custom_title">
               Hello There!
            </h5>

            <div class="o_module_custom_subtitle">
               <b><small>I'm a subtitle.</small></b>
            </div>

         .. code-tab:: css SCSS

            .o_module_custom_title {
               animation: 1s linear 1s mycustomAnimation;
            }

            .o_module_custom_subtitle {
               animation: 2s linear 1s mycustomAnimation;
            }

.. note::
   Besides reducing the amount of code, a modular-design approach (use classes, tags, mixins...)
   keeps the visual result consistent and easily **maintainable**.

   Following the last example, if Odoo titles' design changes, these changes will be applied in the
   `o_module_custom_title` element too since it's using an `<h5>` tag.

.. _tutorials/scss_tips/utility_classes:

Utility classes
===============

Our framework defines a multitude of utility classes designed to cover almost all
layout/design/interaction needs. The simple fact that a class already exists justifies its use over
custom CSS whenever possible.

Take the example of `position-relative`.

.. code-block:: css

   position-relative {
      position: relative !important;
   }

Since a utility-class is defined, any CSS line with the declaration `position: relative` is
**potentially** redundant.

Odoo relies on the default `Bootstrap utility-classes
<https://getbootstrap.com/docs/5.1/utilities/background/>`_ stack and defines its own using
`Bootstrap API <https://getbootstrap.com/docs/5.1/utilities/api/>`_.

.. seealso::
   - `Bootstrap utility classes <https://getbootstrap.com/docs/5.1/utilities/api/>`_
   - `Odoo custom utilities on github
     <{GITHUB_PATH}/addons/web/static/src/scss/utilities_custom.scss>`_

.. _tutorials/scss_tips/utility_classes/downside:

Handling utility-classes verbosity
----------------------------------

The downside of utility-classes is the potential lack of readability.

.. example::

   .. code-block:: html

      <myComponent t-attf-class="d-flex border px-lg-2 card
      {{props.readonly ? 'o_myComponent_disabled' : ''}}
      card d-lg-block position-absolute {{props.active ?
      'o_myComponent_active' : ''}}  myComponent px-3"/>

To overcome the issue you may combine different approaches:

- in Qweb attributes, only use classes to be toggled *on-the-fly*;
- use new lines for each attribute;
- order classes using the convention `[odoo component] [bootstrap component] [css declaration order]`.

.. example::

   .. code-block:: html

      <myComponent
         t-att-class="{
            o_myComponent_disabled: props.readonly,
            o_myComponent_active: props.active
         }"
         class="myComponent card position-absolute d-flex d-lg-block border px-3 px-lg-2"
      />

.. seealso::
   :ref:`Odoo CSS properties order <contributing/coding_guidelines/scss/properties_order>`