centerViewController = UIStoryboard.userMapViewController()
    centerViewController.delegate = self

    // wrap the centerViewController in a navigation controller, so we can push views to it
    // and display bar button items in the navigation bar
    centerNavigationController = UINavigationController(rootViewController: centerViewController)
    view.addSubview(centerNavigationController.view)
    addChild(centerNavigationController)

    centerNavigationController.didMove(toParent: self)

    let panGestureRecognizer = UIPanGestureRecognizer(target: self, action: #selector(handlePanGesture(_:)))
    centerNavigationController.view.addGestureRecognizer(panGestureRecognizer)
}

if notAlreadyExpanded {
        addLeftPanelViewController()
    }

    animateLeftPanel(shouldExpand: notAlreadyExpanded)
}

func addLeftPanelViewController() {
    guard leftViewController == nil else { return }

    if let vc = UIStoryboard.leftViewController() {
        addChildSidePanelController(vc)
        leftViewController = vc
    }
}

func animateLeftPanel(shouldExpand: Bool) {
    if shouldExpand {
        currentState = .leftPanelExpanded
        animateCenterPanelXPosition(
            targetPosition: centerNavigationController.view.frame.width - centerPanelExpandedOffset)
    } else {
        animateCenterPanelXPosition(targetPosition: 0) { _ in
            self.currentState = .leftPanelCollapsed
            self.leftViewController?.view.removeFromSuperview()
            self.leftViewController = nil
        }
    }
}