Flash messages in Extbase

In Extbase, the standard way of issuing flash messages is to add them in the controller. Code from the "examples" extension:

EXT:examples/Classes/Controller/ModuleController.php

$this->addFlashMessage('This is a simple success message');

Warning

You cannot call this function in the constructor of a controller or in an initialize action as it needs some internal data structures to be initialized.

A more elaborate example:

EXT:examples/Classes/Controller/ModuleController.php

// use TYPO3\CMS\Core\Type\ContextualFeedbackSeverity;

$this->addFlashMessage(
   'This message is forced to be NOT stored in the session by setting the fourth argument to FALSE.',
   'Success',
   ContextualFeedbackSeverity::OK,
   false
);

The messages are then displayed by Fluid with the relevant Viewhelper as shown in this excerpt of EXT:examples/Resources/Private/Layouts/Module.html:

<div id="typo3-docbody">
   <div id="typo3-inner-docbody">
      <f:flashMessages />
      <f:render section="main" />
   </div>
</div>

Where to display the flash messages in an Extbase-based backend module is as simple as moving the ViewHelper around.