A pannable view class to quickly apply pan gestures onto a UIView to slide vertically, reset to original position and close the view.

CustomPannableView.swift

class CustomPannableView: UIView {
    
    // MARK: Properties
    
    private typealias BottomSheetStyle = StyleConstants.Views.BottomSheet
    private var minimumVelocityToHide = BottomSheetStyle.minimumPanningVelocityToAutoHide
    private var minimumScreenRatioToHide = BottomSheetStyle.minimumPositionRatioToAutoHide
    private var animationDuration = BottomSheetStyle.animationDuration
    private var parentView: UIView?
    private var completion: (() -> ())?
    
    // MARK: Gesture
    
    func setupPanGesture(within parent: UIView, completion: (() -> ())?) {
        self.parentView = parent
        self.completion = completion
        let panGesture  = UIPanGestureRecognizer(target: self, action: #selector(onPan(_:)))
        addGestureRecognizer(panGesture)
    }
    
    private func slideViewVertically(to y: CGFloat, animated: Bool = false, withCompletion: Bool = false) {
        if animated {
            UIView.animate(withDuration: animationDuration, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: 1, options: .curveEaseInOut, animations: {
                self.parentView?.frame.origin = CGPoint(x: 0, y: y)
            }, completion: { (isCompleted) in
                if withCompletion && isCompleted { self.completion?() }
            })
            return
        }
        else {
            self.parentView?.frame.origin = CGPoint(x: 0, y: y)
            if withCompletion { self.completion?() }
        }
    }
    
    @objc private func onPan(_ panGesture: UIPanGestureRecognizer) {
        guard let parent = self.parentView else { return }
        
        switch panGesture.state {
        
        // slide view to follow touch on panning begin/continuation
        case .began, .changed:
            let translation = panGesture.translation(in: parent)
            if translation.y >= 0 {
                slideViewVertically(to: translation.y)
            }

        // close view if should close,
        // reset view if otherwise
        case .ended:
            let translation = panGesture.translation(in: parent)
            let velocity    = panGesture.velocity(in: parent)
            
            // determine based on final touch position or velocity
            let shouldClose =
                (translation.y > parent.frame.size.height * minimumScreenRatioToHide) ||
                (velocity.y > minimumVelocityToHide)
            
            if shouldClose {
                self.slideViewVertically(to: parent.frame.size.height, animated: true, withCompletion: true)
            } else {
                self.slideViewVertically(to: 0, animated: true, withCompletion: false)
            }

        // reset view for undefined pan gesture state
        default:
            self.slideViewVertically(to: 0, animated: true, withCompletion: false)
        }
    }
}