Pan gesture recogniser in UICollectionView interferes with drag

[fatuhoku]

Thanks for implementing the LongPressGesture recogniser some time ago.

I've been using it in the app I'm developing. I have a list of items arranged in a line. Scrolling along the view works really well. However I have found that when wanting to drag it's not robust enough.

Synopsis

User pans around on the CollectionView.
Some of the taps are registered as a long-press, because of the low duration.
Drag begins; drag view is created on some window layer.
Collection view continues with fixed inertia; stops panning; hijacks touch
Drag is NEVER COMPLETED; drag view stays on screen. Looks like a bug.

Reproduction

This occurs only sometimes, but often enough for the user to notice. The easiest way to bump up the minimmumPressDuration to something really low like 0.05f. A low duration is very important to ensure that the whole operation feels natural for the user.

        _longPressDragRecognizer = [[DNDLongPressDragRecognizer alloc] init];
        _longPressDragRecognizer.minimumPressDuration = 0.05f;
        // ...

I'll try to prepare a demo project to illustrate this problem.

[frenetisch-applaudierend]

Thanks for reporting this. It sure sounds like a bug. I'll try to take a look at it sometime next week.

[frenetisch-applaudierend]

After some investigation it seems that the UIScrollView's panGestureRecognizer somehow hijacks those touches, as you suspected.

The drag recognizer only receives a -touchesBegan: event, then the reporting stops (which is why the drag is never completed). Unfortunately, I don't see how the library could prevent this.

However there is a workaround: require the UIScrollView.panGestureRecognizer to wait until the drag recognizer failed like this:

UIScrollView *scrollView = …; // Or a sublass like UITableView or UICollectionView for that matter
UIGestureRecognizer *dragRecognizer = …;

[scrollView.panGestureRecognizer requireGestureRecognizerToFail:dragRecognizer];

I updated the example code to include this workaround.

[fatuhoku]

Nice one, I'll try it now!