Settings and theme support

Accessing XBlock specific settings

XBlock utils provide a mixin to simplify accessing instance-wide XBlock-specific configuration settings: XBlockWithSettingsMixin. This mixin aims to provide a common interface for pulling XBlock settings from the LMS SettingsService.

SettingsService allows individual XBlocks to access environment and django settings in an isolated manner:

• XBlock settings are represented as dictionary stored in django settings and populated from environment *.json files (cms.env.json and lms.env.json)
• Each XBlock is associated with a particular key in that dictionary: by default an XBlock's class name is used, but XBlocks can override it using the block_settings_key attribute/property.

Please note that at the time of writing the implementation of SettingsService assumed "good citizenship" behavior on the part of XBlocks, i.e. it does not check for key collisions and allows modifying mutable settings. Both SettingsService and XBlockWithSettingsMixin are not concerned with contents of settings bucket and return them as is. Refer to the SettingsService docstring and implementation for more details.

Using XBlockWithSettingsMixin

In order to use SettingsService and XBlockWithSettingsMixin, a client XBlock must require it via standard XBlock.wants('settings') or XBlock.needs('settings') decorators. The mixins themselves are not decorated as this would not result in all descendant XBlocks to also be decorated.

With XBlockWithSettingsMixin and wants decorator applied, obtaining XBlock settings is as simple as

self.get_xblock_settings()  # returns settings bucket or None
self.get_xblock_settings(default=something)  # returns settings bucket or "something"

In case of missing or inaccessible XBlock settings (i.e. no settings service in runtime, no XBLOCK_SETTINGS in settings, or XBlock settings key is not found) default value is used.

Theming support

XBlock theming support is built on top of XBlock-specific settings. XBlock utils provide ThemableXBlockMixin to streamline using XBlock themes.

XBlock theme support is designed with two major design goals:

• Allow for a different look and feel of an XBlock in different environments.
• Use a pluggable approach to hosting themes, so that adding a new theme will not require forking an XBlock.

The first goal made using SettingsService and XBlockWithSettingsMixin an obvious choice to store and obtain theme configuration. The second goal dictated the configuration format - it is a dictionary (or dictionary-like object) with the following keys:

• package - "top-level" selector specifying package which hosts theme files
• locations - a list of locations within that package

Examples:

# will search for files red.css and small.css in my_xblock package
{
    'package': 'my_xblock',
    'locations': ['red.css', 'small.css']
}

# will search for files public/themes/red.css in my_other_xblock.assets package
default_theme_config = {
    'package': 'my_other_xblock.assets',
    'locations': ['public/themes/red.css']
}

Theme files must be included into package (see python docs for details). At the time of writing it is not possible to fetch theme files from multiple packages.

Note: XBlock themes are not LMS themes - they are just additional CSS files included into an XBlock fragment when the corresponding XBlock is rendered. However, it is possible to misuse this feature to change look and feel of the entire LMS, as contents of CSS files are not checked and might contain selectors that apply to elements outside of the XBlock in question. Hence, it is advised to scope all CSS rules belonging to a theme with a global CSS selector .themed-xblock.<root xblock element class>, e.g. .themed-xblock.poll-block. Note that the themed-xblock class is not automatically added by ThemableXBlockMixin, so one needs to add it manually.

Using ThemableXBlockMixin

In order to use ThemableXBlockMixin, a descendant XBlock must also be a descendant of XBlockWithSettingsMixin (XBlock.wants decorator requirement applies) or provide a similar interface for obtaining the XBlock settings bucket.

There are three configuration parameters that govern ThemableXBlockMixin behavior:

• default_theme_config - default theme configuration in case no theme configuration can be obtained
• theme_key - a key in XBlock settings bucket that stores theme configuration
• block_settings_key - inherited from XBlockWithSettingsMixin if used in conjunction with it

It is safe to omit default_theme_config or set it to None in case no default theme is available. In this case, ThemableXBlockMixin will skip including theme files if no theme is specified via settings.

ThemableXBlockMixin exposes two methods:

• get_theme() - this is used to get theme configuration. Default implementation uses get_xblock_settings and theme_key, descendants are free to override it. Normally, it should not be called directly.
• include_theme_files(fragment) - this method is an entry point to ThemableXBlockMixin functionality. It calls get_theme to obtain theme configuration, fetches theme files and includes them into fragment. fragment must be an XBlock.Fragment instance.

So, having met usage requirements and set up theme configuration parameters, including theme into XBlock fragment is a one liner:

self.include_theme_files(fragment)