Question

I am implementing a view similar to a Table View which contains rows of data. What I am trying to do is that after scrolling, each row snaps to a set of correct positions so the boundaries of the top and bottom row are completely visible - and not trimmed as it normally happens. Is there a way to get the scroll destination before the scrolling starts? This way I will be able to correct the final y-position, for example, in multiples of row height.

Answer 1

I asked the same question a couple of weeks ago.

There is definitely no public API to determine the final resting Y offset of a scroll deceleration. After researching it further, I wasn t able to figure out Apple s formula for how they manage deceleration. I gathered a bunch of data from scrolling events, recording the beginning velocity and how far the deceleration traveled, and from that made some rough estimates of where it was likely to stop.

My goal was to predict well in advance where it would stop, and to convert the deceleration into a specific move to an offset. The problem with this technique is that scrollRectToVisible:animated: always occurs over a set period of time, so instead of the velocity the user expects from a flick gesture, it s either much faster or much slower, depending on the strength of the flick.

Another choice is to observe the deceleration and wait until it slows down to some threshold, then call scrollRectToVisible:animated:, but even this is difficult to get "just right."

A third choice is to wait until the deceleration completes on its own, check to see if it happened to stop at your desired offset multiple, and then adjust if not. I don t care for this personally, as you either coast to a stop and then speed up or coast to a stop and reverse direction.

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