feat: allow overriding unit icons (#21433)

This:
1. Introduces a new override using the `pluggable_override` decorator.
It is now possible to specify a custom way of getting XBlock's icon
by defining `GET_UNIT_ICON_IMPL` in settings.
2. Introduces a way to add custom `XBLOCK_MIXINS` by defining
`XBLOCK_EXTRA_MIXINS` in settings. This allows, e.g. to add
new fields to XBlocks.

common/lib/xmodule/xmodule/seq_module.py

@@ -692,6 +692,9 @@ class SequenceBlock(
         display_items.  Returns a list of dict objects with information about
         the given display_items.
         """
+        # Avoid circular imports.
+        from openedx.core.lib.xblock_utils import get_icon
+
         render_items = not context.get('exclude_units', False)
         is_user_authenticated = self.is_user_authenticated(context)
         completion_service = self.runtime.service(self, 'completion')
@@ -708,8 +711,7 @@ class SequenceBlock(
         ]
         contents = []
         for item in display_items:
-            # NOTE (CCB): This seems like a hack, but I don't see a better method of determining the type/category.
-            item_type = item.get_icon_class()
+            item_type = get_icon(item)
             usage_id = item.scope_ids.usage_id
 
             show_bookmark_button = False

docs/guides/extension_points.rst

@@ -134,6 +134,9 @@ Here are the different integration points that python plugins can use:
    * - XBlock unit tests (``xblock.test.v0``)
      - Assess, Limited
      - XBlocks can also install test code that will then be run alongside the platform's usual python unit tests. It's unclear how well-supported this is at the moment.
+   * - Pluggable override (``edx_django_utils.plugins.pluggable_override.pluggable_override``)
+     - Trial, Stable
+     - This decorator allows overriding any function or method by pointing to an alternative implementation in settings. Read the |pluggable_override docstring|_ to learn more.
 
 .. _Application: https://docs.djangoproject.com/en/3.0/ref/applications/
 .. _Django app plugin documentation: https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/plugins/README.rst
@@ -146,6 +149,8 @@ Here are the different integration points that python plugins can use:
 .. _learning_context.py: https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/xblock/learning_context/learning_context.py
 .. |UserPartition docstring| replace:: ``UserPartition`` docstring
 .. _UserPartition docstring: https://github.com/edx/edx-platform/blob/f8cc58618a39c9f7b8e9e1001eb2d7a10395797e/common/lib/xmodule/xmodule/partitions/partitions.py#L105-L120
+.. |pluggable_override docstring| replace:: ``pluggable_override`` docstring
+.. _pluggable_override docstring: https://github.com/edx/edx-django-utils/blob/master/edx_django_utils/plugins/pluggable_override.py
 
 Platform Look & Feel
 ====================

lms/devstack.yml

@@ -563,6 +563,7 @@ VIDEO_UPLOAD_PIPELINE:
     ROOT_PATH: ''
 WIKI_ENABLED: true
 WRITABLE_GRADEBOOK_URL: null
+XBLOCK_EXTRA_MIXINS: []
 XBLOCK_FS_STORAGE_BUCKET: null
 XBLOCK_FS_STORAGE_PREFIX: null
 XBLOCK_SETTINGS: {}

lms/envs/common.py

@@ -1424,6 +1424,7 @@ from xmodule.x_module import XModuleMixin
 # This should be moved into an XBlock Runtime/Application object
 # once the responsibility of XBlock creation is moved out of modulestore - cpennington
 XBLOCK_MIXINS = (LmsBlockMixin, InheritanceMixin, XModuleMixin, EditInfoMixin)
+XBLOCK_EXTRA_MIXINS = ()
 
 # .. setting_name: XBLOCK_SELECT_FUNCTION
 # .. setting_default: prefer_xmodules

lms/envs/production.py

@@ -1047,3 +1047,6 @@ EXPLICIT_QUEUES = {
 }
 
 LOGO_IMAGE_EXTRA_TEXT = ENV_TOKENS.get('LOGO_IMAGE_EXTRA_TEXT', '')
+
+############## XBlock extra mixins ############################
+XBLOCK_MIXINS += tuple(XBLOCK_EXTRA_MIXINS)

openedx/core/lib/xblock_utils/__init__.py

@@ -19,6 +19,7 @@ from django.contrib.auth.models import User  # lint-amnesty, pylint: disable=imp
 from django.contrib.staticfiles.storage import staticfiles_storage
 from django.urls import reverse
 from django.utils.html import escape
+from edx_django_utils.plugins import pluggable_override
 from lxml import etree, html
 from opaque_keys.edx.asides import AsideUsageKeyV1, AsideUsageKeyV2
 from pytz import UTC
@@ -554,3 +555,13 @@ def hash_resource(resource):
         else:
             md5.update(repr(data).encode('utf-8'))
     return md5.hexdigest()
+
+
+@pluggable_override('OVERRIDE_GET_UNIT_ICON')
+def get_icon(block):
+    """
+    A function that returns the CSS class representing an icon to use for this particular
+    XBlock (in the courseware navigation bar). Mostly used for Vertical/Unit XBlocks.
+    It can be overridden by setting `GET_UNIT_ICON_IMPL` to an alternative implementation.
+    """
+    return block.get_icon_class()

requirements/edx/base.in

@@ -78,7 +78,7 @@ edx-celeryutils
 edx-completion
 edx-django-release-util             # Release utils for the edx release pipeline
 edx-django-sites-extensions
-edx-django-utils>=3.12.0            # Utilities for cache, monitoring, and plugins; 3.12.0+ for set_code_owner_attribute method
+edx-django-utils>=3.16.0            # Utilities for cache, monitoring, and plugins; 3.16.0+ for the `pluggable_override`
 edx-drf-extensions
 edx-enterprise
 edx-event-routing-backends