Feature: #99053 - Route aspect fallback value handling

Description

Imagine a route like /news/{news_title} that has been filled with an "invalid" value for the news_title part. Often these are outdated, deleted or hidden records. Usually TYPO3 reacts to these "invalid" URL sections at a very early stage with an HTTP status code 404 (resource not found).

The new property fallbackValue = [string|null] can prevent the above scenario in several ways. By specifying an alternative value, a different record, language or other detail can be represented. Specifying null removes the corresponding parameter from the route result. In this way, it is up to the developer to react accordingly.

In the case of Extbase extensions, the developer can define the parameters in their calling controller action as nullable and deliver corresponding flash messages that explain the current scenario better than a 404 HTTP status code.

Examples

routeEnhancers:
  NewsPlugin:
    type: Extbase
    extension: News
    plugin: Pi1
    routes:
      - routePath: '/detail/{news_title}'
        _controller: 'News::detail'
        _arguments:
          news_title: 'news'
    aspects:
      news_title:
        type: PersistedAliasMapper
        tableName: tx_news_domain_model_news
        routeFieldName: path_segment

        # string values lead to parameter `&tx_news_pi1[news]=0`
        fallbackValue: '0'

        # null values lead to parameter `&tx_news_pi1[news]` being removed
        fallbackValue: null

Custom mapper implementations can incorporate this behavior by implementing the new \TYPO3\CMS\Core\Routing\Aspect\UnresolvedValueInterface which is provided by \TYPO3\CMS\Core\Routing\Aspect\UnresolvedValueTrait .

use TYPO3\CMS\Core\Routing\Aspect\MappableAspectInterface;
use TYPO3\CMS\Core\Routing\Aspect\UnresolvedValueInterface;
use TYPO3\CMS\Core\Routing\Aspect\UnresolvedValueTrait;

class MyCustomEnhancer implements MappableAspectInterface, UnresolvedValueInterface
{
    use UnresolvedValueTrait;
    // ...
}