UIScrollView: paging horizontally, scrolling vertically?

Question

How can I force a UIScrollView in which paging and scrolling are on to only move vertically or horizontally at a given moment?

My understanding is that

Answer 1

I used following method to solve this problem, hope it helps you.

First define following variables in your controllers header file.

CGPoint startPos;
int     scrollDirection;

startPos will keep the contentOffset value when your delegate receives scrollViewWillBeginDragging message. So in this method we do this;

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView{
    startPos = scrollView.contentOffset;
    scrollDirection=0;
}

then we use these values to determine users intended scroll direction in scrollViewDidScroll message.

- (void)scrollViewDidScroll:(UIScrollView *)scrollView{

   if (scrollDirection==0){//we need to determine direction
       //use the difference between positions to determine the direction.
       if (abs(startPos.x-scrollView.contentOffset.x)<abs(startPos.y-scrollView.contentOffset.y)){          
          NSLog(@"Vertical Scrolling");
          scrollDirection=1;
       } else {
          NSLog(@"Horitonzal Scrolling");
          scrollDirection=2;
       }
    }
//Update scroll position of the scrollview according to detected direction.     
    if (scrollDirection==1) {
       [scrollView setContentOffset:CGPointMake(startPos.x,scrollView.contentOffset.y) animated:NO];
    } else if (scrollDirection==2){
       [scrollView setContentOffset:CGPointMake(scrollView.contentOffset.x,startPos.y) animated:NO];
    }
 }

finally we have to stop all update operations when user end dragging;

 - (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{
    if (decelerate) {
       scrollDirection=3;
    }
 }

Answer 2

I managed to solve this by implementing scrollViewDidBeginDragging(_:) and looking at the velocity on the underlying UIPanGestureRecognizer.

NB: With this solution, every pan that would have been diagonal will be ignored by the scrollView.

override func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
    super.scrollViewWillBeginDragging(scrollView)
    let velocity = scrollView.panGestureRecognizer.velocity(in: scrollView)
    if velocity.x != 0 && velocity.y != 0 {
        scrollView.panGestureRecognizer.isEnabled = false
        scrollView.panGestureRecognizer.isEnabled = true
    }
}

Answer 3

Need to reset _isHorizontalScroll to NO in touchesEnded and touchesCancelled.

Answer 4

If you are having a big scrollview... and you want to restrict the diagonal scrolling http://chandanshetty01.blogspot.in/2012/07/restricting-diagonal-scrolling-in.html

Answer 5

For those looking in Swift at @AndreyTarantsov second solution its like this in code:

    var positionScroll:CGFloat = 0

    func scrollViewWillBeginDragging(scrollView: UIScrollView) {
        positionScroll = self.theScroll.contentOffset.x
    }

    func scrollViewDidScroll(scrollView: UIScrollView) {
        if self.theScroll.contentOffset.x > self.positionScroll || self.theScroll.contentOffset.x < self.positionScroll{
             self.theScroll.pagingEnabled = true
        }else{
            self.theScroll.pagingEnabled = false
        }
    }

what we are doing here is to keep the current position just before scrollview gets dragged and then check if either x has increased or decreased compared to the current position of x. If yes, then set the pagingEnabled to true, if no (y has increased/decreased) then set to pagingEnabled to false

Hope that is useful to new comers!

Answer 6

Thanks Tonetel. I slightly modified your approach, but it was exactly what I needed to prevent horizontal scrolling.

self.scrollView.contentSize = CGSizeMake(self.scrollView.contentSize.width, 1);