Handling scroll views with (custom, interactive) view controller presentation and dismissal

Question

I have been experimenting with custom interactive view controller presentation and dismissal (using a combination of UIPresentationController, UIPercentDr

Answer 1

For me, this little bit of code answered a lot of my issues and greatly helped my custom transitions in scrollviews, it will hold a negative scrollview offset from moving while trying to start a transition or showing an activity indicator on the top. My guess is that this will solve at least some of your transition/animation hiccups:

func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {

    if scrollView.contentOffset.y < -75 {

        scrollView.contentInset.top = -scrollView.contentOffset.y

    }
    // Do animation or transition
}

Answer 2

Solution

• Make scrollView stop scrolling after it reached top by using UIScrollView's bounces property and scrollViewDidScroll(_:) method.

  func scrollViewDidScroll(_ scrollView: UIScrollView) {
      scrollView.bounces = (scrollView.contentOffset.y > 10);
  }
  

  Don't forget to set scrollView.delegate = self

• Only handle panGestureRecognizer when scrollView reached top - It means when scrollView.contentOffset.y == 0 by using a protocol.

  protocol PanelAnimationControllerDelegate {
      func shouldHandlePanelInteractionGesture() -> Bool
  }
  

  ViewController

  func shouldHandlePanelInteractionGesture() -> Bool {
      return (scrollView.contentOffset.y == 0);
  }
  

  PanelInteractionController

  class PanelInteractionController: ... {
  
    var startY:CGFloat = 0
  
    private weak var viewController: (UIViewController & PanelAnimationControllerDelegate)?
  
    @objc func handlePanGestureRecognizer(_ gestureRecognizer: UIPanGestureRecognizer) {
      switch gestureRecognizer.state {
      case .began:
        break
      case .changed:
        let translation    = gestureRecognizer.translation(in: gestureRecognizer.view!.superview!)
        let velocity    = gestureRecognizer.velocity(in: gestureRecognizer.view!.superview)
        let state      = gestureRecognizer.state
  
        // Don't do anything when |scrollView| is scrolling
        if !(viewController?.shouldHandlePanelInteractionGesture())! && percentComplete == 0 {
          return;
        }
  
        var rawProgress    = CGFloat(0.0)
  
        rawProgress    = ((translation.y - startTransitionY) / gestureRecognizer.view!.bounds.size.height)
  
        let progress    = CGFloat(fminf(fmaxf(Float(rawProgress), 0.0), 1.0))
  
        if abs(velocity.x) > abs(velocity.y) && state == .began {
          // If the user attempts a pan and it looks like it's going to be mostly horizontal, bail - we don't want it... - JAC
          return
        }
  
        if !self.interactionInProgress {
          // Start to pan |viewController| down
          self.interactionInProgress = true
          startTransitionY = translation.y;
          self.viewController?.dismiss(animated: true, completion: nil)
        } else {
          // If the user gets to a certain point within the dismissal and releases the panel, allow the dismissal to complete... - JAC
          self.shouldCompleteTransition = progress > 0.2
  
          update(progress)
        }
      case .cancelled:
        self.interactionInProgress = false
        startTransitionY = 0
  
        cancel()
      case .ended:
        self.interactionInProgress = false
        startTransitionY = 0
  
        if self.shouldCompleteTransition == false {
          cancel()
        } else {
          finish()
        }
      case .failed:
        self.interactionInProgress = false
        startTransitionY = 0
  
        cancel()
      default:
        break;
      }
    }
  }
  

Result

For more detail, you can take a look at my sample project