public function once(string $event, callable $handler): void
    {
        $this->on($event, $handler, true);
    }
public function once(string $event, callable $handler): void
    {
        $this->on($event, $handler, true);
    }
public function once ( string $ event , callable $ handler ) : void { $ this -> on ( $ event , $ handler , true ) ; }
register a once event handler
@param          $event
@param callable $handler
register a once event handler