public FieldConfigInterface::setRequired($required)
Sets whether the field can be empty.
If a field is required, an entity needs to have at least a valid, non-empty item in that field's FieldItemList in order to pass validation.
An item is considered empty if its isEmpty() method returns TRUE. Typically, that is if at least one of its required properties is empty.
bool $required: TRUE if the field is required. FALSE otherwise.
$this The current object, for a fluent interface.
public function setRequired($required);
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Field!FieldConfigInterface.php/function/FieldConfigInterface::setRequired/8.1.x