swift addchildviewcontroller

func tapAtLabel() {
        guard let naviController = navigationController,
            let naviView = naviController.view else { return }
        let controller = FollowUserViewController()
        
        controller.view.translatesAutoresizingMaskIntoConstraints = false
        
        naviView.addSubview(controller.view)
        NSLayoutConstraint.activate([
            controller.view.leftAnchor.constraint(equalTo: naviView.leftAnchor),
            controller.view.rightAnchor.constraint(equalTo: naviView.rightAnchor),
            controller.view.topAnchor.constraint(equalTo: naviView.topAnchor),
            controller.view.bottomAnchor.constraint(equalTo: naviView.bottomAnchor)
        ])
        controller.view.setNeedsLayout()
        controller.view.layoutIfNeeded()
        
        naviController.addChild(controller)
        controller.didMove(toParent: naviController)
    }

class FollowUserViewController: UIViewController {
    
    let followView = FollowUserView()
    
    var constraint: NSLayoutConstraint?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = UIColor.black.withAlphaComponent(0.4)
        
        let minHeight: CGFloat = 80
        var height: CGFloat = UIScreen.main.bounds.height * 0.7
        if FDDevice.hasTopNotch {
            height += 34
        }
        
        followView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(followView)
        NSLayoutConstraint.activate([
            followView.leftAnchor.constraint(equalTo: view.leftAnchor),
            followView.rightAnchor.constraint(equalTo: view.rightAnchor),
            followView.heightAnchor.constraint(equalToConstant: height),
        ])
        
        constraint = followView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: height)
        constraint?.isActive = true
                
        followView.offsetYCallBack = { [weak self] constant in
            guard let `self` = self else { return }
            if constant <= 0 {
                self.constraint?.constant = 0
            } else if constant >= height - minHeight {
                self.constraint?.constant = height - minHeight
            } else {
                self.constraint?.constant = constant
            }
            
        }
        followView.panEndCallBack = { [weak self] offsetY in
            guard let `self` = self else { return }
            if offsetY <= height / 2 {
                self.doShowUpAnimation()
            } else {
                self.leave()
            }
        }
    }
    
    override func didMove(toParent parent: UIViewController?) {
        super.didMove(toParent: parent)
        doShowUpAnimation()
    }
    
    func doShowUpAnimation() {
        constraint?.constant = 0
        UIView.animate(withDuration: 0.15) {
            self.view.setNeedsLayout()
            self.view.layoutIfNeeded()
        }
    }
    
    func leave() {
        view.removeFromSuperview()
        willMove(toParent: nil)
        removeFromParent()
    }
}