How to store a safevalue of a field in addition to the original?

Question

I have a custom field type that I create with a custom module. It is a plain text field where the user type in a string. I want to retain two copies of this field:

1. The exact string value as typed by the user.
2. A safe value of the same string.

This is how I create the field:

class MyItem extends FieldItemBase {

  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'type' => 'text',
          'size' => 'tiny',
          'not null' => FALSE,
        ],
        'safevalue' => [
          'type' => 'text',
          'size' => 'tiny',
          'not null' => FALSE,
        ],
      ],
    ];
  }
…

}

I assume I should generate a safevalue of the string in hook_node_presave() and somehow get into the entity there (but corrections to these assumptions are welcome).

I have no problem getting the string value as typed by the user and to compute a safe value:

$value = $entity->field_myfield->getString();
$safevalue = makeSafe($value);

But I do not know how to get the $safevalue stored in the database.

Edit

I see that this has two nearly identical answers. Both work fine if the cardinality of the field is one, so I've upvoted both. Since I cannot accept both, I've accepted the one from apaderno for the extra detail about propertyDefinitions().

I am still not able to get this to work with a multivalue field, but I think that probably need to be a separate question.

Answer 1

If safeValue has been defined as field property in propertyDefinitions(), it can be accessed as $entity->field_myfield->safeValue, either to read it or write it.

In Drupal core, an example of field that stores a processed copy of the field value is the TextItem class, whose propertyDefinitions() method contains the following code.

$properties['value'] = DataDefinition::create('string')
  ->setLabel(t('Text'))
  ->setRequired(TRUE);
$properties['format'] = DataDefinition::create('filter_format')->setLabel(t('Text format'));
$properties['processed'] = DataDefinition::create('string')
  ->setLabel(t('Processed text'))
  ->setDescription(t('The text with the text format applied.'))
  ->setComputed(TRUE)
  ->setClass('\\Drupal\\text\\TextProcessed')
  ->setSetting('text source', 'value')
  ->setInternal(FALSE);

That class defines the property as computed. That's why its schema() method doesn't include that property.

Answer 2

For saving:

$value = $entity->field_myfield->getString();
$entity->field_myfield->safevalue = makeSafe($value);

For loading:

$safe_value = $entity->field_myfield->safevalue;