`content:export` command added to help with recipe development

Recipes have been able to include default content (Recipe authoring > Provide content) since they were first introduced to core. However, creating that default content has not been supported by core; it was necessary to use the contributed Default Content module to export content from Drupal into the format used by recipes.

As of #3532694: Add a command-line utility to export content in YAML format, Drupal core now includes a command-line tool to export content in that format. It can export a single entity at a time, but it is also possible to export the dependencies of the entity automatically (for example, images attached to it or taxonomy terms it references).

How to use

To use it, run the following from the Drupal root:

php core/scripts/drupal content:export <ENTITY_TYPE_ID> <ENTITY_ID>

For example: php core/scripts/drupal content:export taxonomy_term 40

This will output a dump of the entity's data in YAML format. You can save it to a file like this: php core/scripts/drupal content:export taxonomy_term 40 > recipes/my_recipe/content/taxonomy_term/some-tag.yml

As of #3532951: Support exporting content and its dependencies to a folder structure on disk, you can also export the entity to a directory:

php core/scripts/drupal content:export node 42 --dir=my-content

This will create my-content/node/UUID_OF_NODE_42.yml.

You can export the entity and all of its referenced entities, recursively, to a directory:

php core/scripts/drupal content:export node 42 --with-dependencies --dir=content

This will export the content to a directory called content, divided by entity type into additional subdirectories. It's similar to the drush dcer command, from the contrib Default Content module.

It is possible to export all entities of a particular type, optionally filtered by bundle. For example, to export all blog content:

php core/scripts/drupal content:export node --bundle=blog --with-dependencies --dir=content

Or multiple bundles, by passing the --bundle option more than once:

php core/scripts/drupal content:export node --bundle=blog --bundle=faculty --with-dependencies --dir=content

You can leave out the --bundle option entirely to just export all entities of a particular type:

php core/scripts/drupal content:export media --dir=content

How to integrate with the API

The export command supports core field types, but modules that provide their own field types may need to write a little code to integrate with this command.

You can attach a callback to a particular field type (or a particular field) by writing an event subscriber that listens to \Drupal\Core\DefaultContent\PreExportEvent, which is dispatched before an entity is exported. The subscriber should call the event's setCallback() method, which takes either a field name or a field type (such as field_item:image -- if using a field type, the field_item: prefix must be present), and a callback that takes two arguments: the field item being exported (\Drupal\Core\FieldItemInterface) and an object that collects information about the entity being exported (\Drupal\Core\DefaultContent\ExportMetadata). It should return the exported values for that field item, or NULL if the item shouldn't be exported.

For examples, see the following classes:

• \Drupal\Core\DefaultContent\Exporter
• \Drupal\link\EventSubscriber\DefaultContentSubscriber

The event can also be used to opt a specific field out of being exported:

$event->setExportable('field_name', FALSE);

Computed fields are not exported by default, but you can use that same method to opt them in:

$event->setExportable('some_computed_field', TRUE);

How to use the API

You can export an entity programmatically using the new Exporter service:

use Drupal\Core\DefaultContent\Exporter;

$node = Node::load(32);
$exporter = \Drupal::service(Exporter::class);
$exported_content_as_yaml = (string) $exporter->export($node);

// Export to a file: creates /path/to/content/directory/node/UUID.yml.
$exporter->exportToFile($node, '/path/to/content/directory');

// Export to a directory with all dependencies, divided into subdirectories.
$exporter->exportWithDependencies($node, '/path/to/content');