Right now, we have to write code like
public function getBodyMessage() { $msg = $this->msg( 'notification-body-flow-new-topic' ); list( $formattedName, $genderName ) = $this->getAgentForOutput(); $msg->params( $formattedName, $genderName ); $msg->params( other stuff ); // ...
whereas for getHeaderMessage, the parent method does this for us:
public function getHeaderMessage() { $msg = parent::getHeaderMessage(); $msg->params( other stuff ); // ...
See https://gerrit.wikimedia.org/r/#/c/251512/7/includes/Notifications/NewTopicPresentationModel.php for example.
There should be a way that we don't have to duplicate this code, either through parent method support in getBodyMessage or a helper that can be called by both getBodyMessage and getHeaderMessage.