getInitialValueFromField example

    $initial_value = $initial_value_from_field = [];
    $storage_definition_is_new = empty($this->loadFieldSchemaData($storage_definition));
    if ($storage_definition_is_new && $storage_definition instanceof BaseFieldDefinition && $table_mapping->allowsSharedTableStorage($storage_definition)) {
      if (($initial_storage_value = $storage_definition->getInitialValue()) && !empty($initial_storage_value)) {
        // We only support initial values for fields that are stored in shared         // tables (i.e. single-value fields).         // @todo Implement initial value support for multi-value fields in         // https://www.drupal.org/node/2883851.         $initial_value = reset($initial_storage_value);
      }

      if ($initial_value_field_name = $storage_definition->getInitialValueFromField()) {
        // Check that the field used for populating initial values is valid.         if (!isset($this->fieldStorageDefinitions[$initial_value_field_name])) {
          throw new FieldException("Illegal initial value definition on {$storage_definition->getName()}: The field $initial_value_field_name does not exist.");
        }

        if ($storage_definition->getType() !== $this->fieldStorageDefinitions[$initial_value_field_name]->getType()) {
          throw new FieldException("Illegal initial value definition on {$storage_definition->getName()}: The field types do not match.");
        }

        if (!$table_mapping->allowsSharedTableStorage($this->fieldStorageDefinitions[$initial_value_field_name])) {
          throw new FieldException("Illegal initial value definition on {$storage_definition->getName()}: Both fields have to be stored in the shared entity tables.");
        }