How slim v3 makes the app available in the callback

slim.php

  public function add(\Slim\Middleware $newMiddleware)
    {
        if(in_array($newMiddleware, $this->middleware)) {
            $middleware_class = get_class($newMiddleware);
            throw new \RuntimeException("Circular Middleware setup detected. Tried to queue the same Middleware instance ({$middleware_class}) twice.");
        }
        $newMiddleware->setApplication($this);
        $newMiddleware->setNextMiddleware($this->middleware[0]);
        array_unshift($this->middleware, $newMiddleware);
    }