Use a function to provide default field values

A default value can be set for fields that are attached to a content type (i.e., any field-able entity). The entered value is used to pre-fill the form (i.e., entity) when creating new content. Although having a static starting value is often perfectly fine, true power comes when these values can be set dynamically. Of course, you accomplish this by using hook_form_alter or hook_field_widget_multivalue_form_alter, but these only fulfill when content is created using forms. Therefore, the recommended way is to implement a function to provide the value and set the function's name in field.field.<entity_type>.<bundle_name>.<field_name>.yml for the default_value_callback setting.

…
translatable: false
default_value: {  }
default_value_callback: example_default_value
settings: {  }
field_type: string
…

The code below illustrates an example of the default value callback function. Here we simply return a (translated) string, but the value can, of course, be provided using more complex logic, e.g., based on the entity type/bundle or even using a service to get values from external API. The sky is the limit!

use Drupal\Core\Entity\ContentEntityInterface;
use Drupal\Core\Field\FieldDefinitionInterface;

/**
 * Delivers the default value for the example field.
 *
 * @param \Drupal\Core\Entity\ContentEntityInterface $entity
 *   The entity being created.
 * @param \Drupal\Core\Field\FieldDefinitionInterface $definition
 *   The field definition.
 *
 * @return array
 *   Array providing, for each delta item, initial values for the field's
 *   properties.
 *
 * @see \Drupal\Core\Field\FieldConfigBase::getDefaultValue()
 */
function example_default_value(ContentEntityInterface $entity, FieldDefinitionInterface $definition) {
  // Maybe some logic here, to create the values dynamically or even retrieve
  // the values from an external source.

  return [
    ['value' => t('Some value')],
  ];
}

Don't forget to import the configuration after setting the default_value_callback and rebuild Drupal's caches in order for the function to be found.

Use a function to provide default field values

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