I'm developing an iPhone/iPad app that supports dragging items between table views. Since all the tables don't fit on screen, I've written a custom UIScrollView that lays them out horizontally, and supports paging.
While I've gotten the primary drag and drop together, there are a few remaining issues I can't get past.
After the user has selected an item to drag, and is dragging, they cannot scroll the UIScrollView to find the destination UITableView.
Sometimes the user will want to drag the item within the same table view. But once the drag has begun, the table view no longer recognizes the scroll gesture.
I've tried a variety of different options, including implementing a UIGestureRecognizerDelegate and allowing multiple gesture recognizers to recognize gestures simultaneously.
The problem, as I see it stems from this description from the Event Handling Guide: "iOS recognizes one or more fingers touching the screen as part of a multitouch sequence. This sequence begins when the first finger touches down on the screen and ends when the last finger is lifted from the screen."
UIGestureRecognizer instances always match against the entire sequence. In my case, I want to split a single sequence down into discrete gestures -- some touches recognize a dragging of an item, while different touches within the same sequence should be recognized as a swipe or scroll gesture. Effectively, I want my gesture recognizers to recognize simultaneously, but only different touches. Once one recognizes a touch as part of a gesture, that touch should be ignored by the others.
I haven't found a way to solve all these issues coherently using the default UIGestureRecognizer subclasses, and am now about to write my own custom mutli-part gesture recognizer.
I'd rather not have to though -- is there any more appropriate way to achieve the same result?