Disable gesture to pull down form/page sheet modal presentation

Question

In iOS 13 modal presentations using the form and page sheet style can be dismissed with a pan down gesture. This is problematic in one of my form sheets because the user draws into this box which interferes with the gesture. It pulls the screen down instead of drawing a vertical line.

How can you disable the vertical swipe to dismiss gesture in a modal view controller presented as a sheet?

Setting isModalInPresentation = true still allows the sheet to be pulled down, it just won't dismiss.

Answer 1

For future internet travelers, in general, one shouldn't disable the swipe to dismiss functionality. But if you have a scenario where your gesture or touch handling is impacted by this feature, I did receive some advice from an Apple engineer on how to accomplish this.

If you can prevent the system's pan gesture recognizer from beginning, this will prevent the gestural dismissal. A few ways to do this:

1. If your canvas drawing is done with a gesture recognizer, such as your own UIGestureRecognizer subclass, enter then began phase before the sheet’s dismiss gesture does. If you recognize as quickly as UIPanGestureRecognizer, you will win, and the sheet’s dismiss gesture will be subverted.

2. If your canvas drawing is done with a gesture recognizer, setup a dynamic failure requirement with -shouldBeRequiredToFailByGestureRecognizer: (or the related delegate method), where you return NO if the passed in gesture recognizer is a UIPanGestureRecognizer.

3. If your canvas drawing is done with manual touch handling (e.g. touchesBegan:), override -gestureRecognizerShouldBegin on your touch handling view, and return NO if the passed in gesture recognizer is a UIPanGestureRecognizer.

With my setup #3 proved to work very well. This allows the user to swipe down anywhere outside of the drawing canvas to dismiss (like the nav bar), while allowing the user to draw without moving the sheet, just as one would expect.

I cannot recommend trying to find the gesture to disable it, as it seems to be rather dynamic and can reenable itself when switching between different size classes for example, and this could change in future releases.

Answer 2

This gesture can be found in the modal view controller's presentedView property. As I debugged, the gestureRecognizers array of this property has only one item and printing it resulted in something like this:

UIPanGestureRecognizer: 0x7fd3b8401aa0 (_UISheetInteractionBackgroundDismissRecognizer);

So to disable this gesture you can do like below:

let vc = UIViewController()

self.present(vc, animated: true, completion: {
  vc.presentationController?.presentedView?.gestureRecognizers?[0].isEnabled = false
})

To re-enable it simply set isEnabled back to true:

vc.presentationController?.presentedView?.gestureRecognizers?[0].isEnabled = true

Note that iOS 13 is still in beta so a simpler approach might be added in an upcoming release.

Answer 3

For navigation Controller, to avoid swipe interaction for presented view we can use:

if #available(iOS 13.0, *) {navController.isModalInPresentation = true}

Answer 4

In the presented ViewController use this in viewDidLoad:

if #available(iOS 13.0, *) {
    self.isModalInPresentation = true
}