Detect direction of UIScrollView scroll in scrollViewWillBeginDragging

I did google enough, & I did check posts like these ( iphone sdk: finding the direction of scrolling in UIScrollView ) in stackoverflow before posting this. I have a dynamic number of photos in an iPhone App, that am displaying through UIScrollView. At any point in time, I have only 3 photos being displayed in the scroll-view. When I have, say 4 photos, in total: 1st photo : displayed at offset 0.0 2nd photo : displayed at offset 320.0 3rd photo : displayed at offset 640.0

Now, when the user scrolls to the 4th photo, the scroll-view resets to 0.0 offset. If the user tries to scroll 'beyond' the 4th photo, scrolling should stop in the right-direction only (so that user doesn't scroll 'beyond'). But currently, the user 'is able' to scroll beyond the last photo ; however, I detect this programmatically & reset the offset. But it doesn't look neat, as the user sees the black background momentarily. I want to detect that the user has started scrolling 'right' (remember, scrolling 'left' i.e. to the 'previous' photo is okay) in scrollViewWillBeginDragging, so that I can stop any further scrolling to the right.

What I tried:

  1. Trying using self.scrollView.panGestureRecognizer's translationInView isn't working, because there is no panGestureRecognizer instance returned in the first place (!), though the UIScrollView API claims so.
  2. Detecting this in scrollViewDidEndDecelerating is possible, though it'll not serve my purpose.

Answers


I had no issues determining direction in scrollViewWillBeginDragging when checking the scroll view's panGestureRecognizer:

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{ 
    CGPoint translation = [scrollView.panGestureRecognizer translationInView:scrollView.superview];

    if(translation.y > 0)
    {
        // react to dragging down
    } else
    {
        // react to dragging up
    }
}

I found it very useful in canceling out of a scroll at the very first drag move when the user is dragging in a forbidden direction.


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