UIDynamicAnimator items(in:) crashes in iOS 11

Question

In iOS 11 (Xcode 9 beta 5), I'm calling UIDynamicAnimator's items(in:) method and immediately crashing:

NSArray element failed to match the Swift Array Element type

What's going on?

Answer 1

You've found a bug. (Filed with Apple, radar 33979954.) Hopefully it will be fixed soon, but until then, here's the fix:

extension UIDynamicAnimator {
    func views(in rect: CGRect) -> [UIView] {
        let nsitems = self.items(in: rect) as NSArray
        return nsitems.flatMap{$0 as? UIView}
    }
}

Now call view(in:) instead of items(in:), and all will be well.

The problem is that spurious objects are being put into the array returned from items(in:). Because of these spurious objects, the array cannot cross the bridge from Objective-C to Swift; the returned array is typed in Swift as [UIDynamicItem], but the array contains things that are not UIDynamicItem objects.

The extension works around this by not crossing the bridge. We stay in the NSArray Objective-C world, filter out the spurious objects, and then cross the bridge.