setInitialValueFromField example

// The second entity does not have any value for the 'test_single_property'     // field, allowing us to test the 'default_value' parameter of     // \Drupal\Core\Field\BaseFieldDefinition::setInitialValueFromField().     $storage->create([
      'name' => 'Second entity',
    ])->save();

    // Add a base field with an initial value inherited from another field.     $definitions['new_base_field'] = BaseFieldDefinition::create('string')
      ->setName('new_base_field')
      ->setLabel('A new base field')
      ->setInitialValueFromField('name');
    $definitions['another_base_field'] = BaseFieldDefinition::create('string')
      ->setName('another_base_field')
      ->setLabel('Another base field')
      ->setInitialValueFromField('test_single_property', $default_initial_value);

    $this->state->set('entity_test_update.additional_base_field_definitions', $definitions);

    $this->assertFalse($db_schema->fieldExists('entity_test_update', 'new_base_field'), "New field 'new_base_field' does not exist before applying the update.");
    $this->assertFalse($db_schema->fieldExists('entity_test_update', 'another_base_field'), "New field 'another_base_field' does not exist before applying the update.");
    $this->entityDefinitionUpdateManager->installFieldStorageDefinition('new_base_field', 'entity_test_update', 'entity_test', $definitions['new_base_field']);
    $this->entityDefinitionUpdateManager->installFieldStorageDefinition('another_base_field', 'entity_test_update', 'entity_test', $definitions['another_base_field']);