The name of a callback function that returns default values.
The function will be called with the following arguments:
It should return an array of default values, in the same format as the $default_value property.
This property takes precedence on the list of fixed values specified in the $default_value property.
Type: string
protected $default_value_callback = '';
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Field!FieldConfigBase.php/property/FieldConfigBase::default_value_callback/8.1.x