how to remove all subviews using swfit

I think the problem is that after line 8, view's superview has been removed, so you cannot access its subviews anymore.

let backgroundView = sender.view as UIView?
        if let view = backgroundView{
            UIView.animate(withDuration: 0.3,
                           animations:{ () in
            },
                           completion: {(finished:Bool) in
                            view.alpha = 0
                            view.superview?.removeFromSuperview()
                            if view.subviews.count > 0
                            {
                                view.subviews.forEach({ $0.removeFromSuperview()})
                            }
                         
            })
        }

Try

let backgroundView = sender.view as UIView?
        if let view = backgroundView{
            UIView.animate(withDuration: 0.3,
                           animations:{ () in
            },
                           completion: {(finished:Bool) in
                            view.alpha = 0
                            if view.subviews.count > 0
                            {
                                view.subviews.forEach({ $0.removeFromSuperview()})
                            }
                            view.superview?.removeFromSuperview()

            })
        }