Fix scrollview deceleration reporting

[janek]

Type of change: Bug fix

Motivation (current vs expected behavior)

When comparing behaviour with iOS, I realized that the isDecelerating value for scrollView is not being reported the same.

The scenario was: one "fast" pan that starts a velocity scroll, then another touch that stops it, maybe pans around a bit, and releases.

As soon as the first pan is released and the velocity scroll starts, isDecelerating is true. Then when the second touch starts, the visible deceleration stops and we are reporting isDecelerating == false. This seems reasonable, but it's not what iOS does. On iOS, the scrollView is still marked as decelerating, until the touch is released.

This PR aligns behavior with iOS and adds a test that covers this case.

Warning

I realized that the difference in behaviour could come from the LongPressRecognizer that's there in iOS (but not yet in UIKit-crossplatform). If that's the case, this is likely not the right fix - please keep that in mind and let me know.

Please check if the PR fulfills these requirements

• [ ] Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• [x] The commit messages are clean and understandable
• [x] Tests for the changes have been added (for bug fixes / features)

[codecov[bot]]

Codecov Report

Merging #307 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
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  Coverage    52.7%   52.7%           
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  Files          87      87           
  Lines        3161    3161           
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  Hits         1666    1666           
  Misses       1495    1495

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[janek]

I looked into doing this as a UI test on both iOS and Mac, and it won't be viable for this test, as it won't let us check isDecelerating value during a touch (after touch down and before touch up). I also don't see a way of mocking touches for this as a unit test in iOSTestTarget - certainly the way it's mocked now doesn't work there.

Generally, I see your point, I'll do some more manual testing now to see if another way of proving this comes to mind.

[janek]

This is problematic to cover with tests because we can't evaluate conditions during a touch on iOS (after touch down and before touch up). I got here by debugging with prints but I'm having trouble reproducing that now and I'm not 100% sure of this fix.

I'll close this unmerged for now.

[janek]

If we ever reopen this topic, here are the print logs that should illustrate the problem:

scrollViewDidScroll; is decelerating: false
scrollViewDidScroll; is decelerating: false
(…)
scrollViewDidScroll; is decelerating: false
scrollViewDidScroll; is decelerating: false
< We let go of the touch >
scrollViewDidEndDragging; is Decelerating? false, willDecelerate? true
< The scrollView is still in a velocity scroll, so it continues scrolling >
scrollViewDidScroll; is decelerating: true
scrollViewDidScroll; is decelerating: true
(…)
scrollViewDidScroll; is decelerating: true
scrollViewDidScroll; is decelerating: true
< We touch again to stop the scroll>
< We lift our touch >
scrollViewDidEndDragging; is Decelerating? true, willDecelerate? false

Those are from iOS, so ground truth. The bug would be that isDecelerating is false on Android in the last line - but as I said, I had some trouble recreating that.

The code snippet to put in DemoApp to be able to recreate the logging setup:

class ViewController: UIViewController, UIScrollViewDelegate {
    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        return
    }

    let label = UILabel()

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        let scrollView = UIScrollView(frame: CGRect(x: 200, y: 100, width: 300, height: 150))
        scrollView.contentSize = CGSize(width: 6000, height: 300)
        scrollView.backgroundColor = .white

        scrollView.delegate = self
        view.backgroundColor = UIColor(red: 0 / 255, green: 206 / 255, blue: 201 / 255, alpha: 1)
        view.addSubview(scrollView)
    }

    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        print("scrollViewDidEndDragging; is Decelerating? \(scrollView.isDecelerating), willDecelerate? \(decelerate)")
    }

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        print("scrollViewDidScroll; is decelerating: \(scrollView.isDecelerating)")
    }
}