UIScrollView Detection

This guide explains various DeckTransitionʼs UIScrollView detection heuristic, some edge cases, and how these can be worked around.

How It Works

In keeping with the style used in Apple Musicʼs iOS app, DeckTransition has a fluid swipe-to-dismiss gesture.

In order to activate this, you just need to swipe down on the card. If there is a UIScrollView within the card, the gesture is activated as soon as the scroll view is scrolled to the very top. This is a fluid gesture: The same pan used to scroll the scroll view to the top can be continued to dismiss the modal without starting a new touch, so it’s important for DeckTransition to detect the main scroll view of your view hierarchy.

DeckTransition’s internal methodology to detect which UIScrollView has two parts:

1. Detecting the UIViewController instance which should contain the UIScrollView.
2. Detecting the UIScrollView instance within it.

Detecting The UIViewController

The view controller presented using DeckTransition itself may not contain the UIScrollView instance which needs to be tracked for a dismiss gesture.

You may be using containment to better organise your code, in which case you can conform your view controllers to DeckTransitionViewControllerProtocol and implement the childViewControllerForDeck variable to return the child view controller which contains the UIScrollView instance to be tracked. This works with nested containment as well.

DeckTransition will traverse your view hierarchy until it reaches a UIViewController which either doesn’t conform to the protocol, or returns nil. The view controller reached at the end of this search process will be further searched for the UIScrollView instance to track.

Detecting the UIScrollView

Once the view controller is correctly identified, DeckTransition searches its view’s top-level subviews (i.e. viewController.view.subviews), and by default uses the lowermost UIScrollView instance, if one is found.

However this may not always find the UIScrollView instance you may want it to, in which case you can conform your view controllers to DeckTransitionViewControllerProtocol and implement the scrollViewForDeck variable to return UIScrollView instance to be tracked.