Breaking: #72438 - Remove deprecated code from FlashMessage

Description

The deprecated render() method has been removed.

Impact

Using the render() method directly in any third party extension will result in a fatal error.

Affected Installations

Instances which use calls to the render() method.

Migration

For FlashMessages that are displayed on top of a page you can replace the render() method with code that enqueues the message to the FlashMessageService.

Replace

$flashMessage = GeneralUtility::makeInstance(\TYPO3\CMS\Core\Messaging\FlashMessage::class,
$message,
$title,
\TYPO3\CMS\Core\Messaging\FlashMessage::ERROR);
$content .= $flashMessage->render();

with

$flashMessage = GeneralUtility::makeInstance(\TYPO3\CMS\Core\Messaging\FlashMessage::class,
$message,
$title,
\TYPO3\CMS\Core\Messaging\FlashMessage::ERROR);
$flashMessageService = GeneralUtility::makeInstance(FlashMessageService::class);
$defaultFlashMessageQueue = $flashMessageService->getMessageQueueByIdentifier();
$defaultFlashMessageQueue->enqueue($flashMessage);

FlashMessages that are used as inline notification should be removed and replaced with custom HTML code.

For the core we have defined output and usage for messages:

1) FlashMessages

FlashMessages are designed to inform a user about success or failure of an action, which was triggered by the user. Example: If the user deletes a record, a FlashMessage informs the user about success or failure. This kind of information is not static, it is a temporary and volatile information and triggered by a user action.

Keep in mind that you must not use HTML markup here, since this information might be shown in a context different from HTML, like processing it via Javascript or showing the message on the command line.

2) Callouts (InfoBox-ViewHelper)

Callouts are designed to display permanent information, a very good example is the usage in the Page-Module. If a user opens a system folder with the page module, the callout explains: 'Hey, you try to use the page module on a sysfolder, please switch to the list module'. This ViewHelper can also be used to show some help or instruction how to use a backend module.

3) Any other information

For any other information e.g. a list of files which has changed, must be handled in the action / view of the module or plugin. This is not a use case for a FlashMessage or Callout! Example: Display a list of a hundred files within a FlashMessage or Callout is a bad idea, build custom markup in the view to handle this kind of message.