Which method creates a flash message in an Extbase controller?

Flash messages are short feedback messages such as "The entry has been saved." that are shown to the user once. In an Extbase controller you create them with $this->addFlashMessage('Saved', 'Success', ContextualFeedbackSeverity::OK);; by default they are stored in the session and therefore also survive a redirect. In the Fluid template, the ViewHelper <f:flashMessages /> outputs all pending messages and then removes them from the queue. Passing a variable with $this->view->assign() would only work for the current view and would be lost after a redirect.