How to find out if scrollView is about to be scrolled up or down - objective-c

Question

I would like to find out if a scrollView is scrolled up or down. Ideally, I'd like to have only one call if the scrollView is scrolled up or down. I tried this but it will obviously not tell me anything about the direction:
-(void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
NSLog(#"%.2f", scrollView.contentOffset.y);
}
contentOffset will always be 0 - it doesn't matter whether I scrolled up or down. Now I could simply check in -(void)scrollViewDidScroll: if the offset is positive or negative, but this is called constantly. scrollViewWillBeginDragging has the advantage of being called only once and this is what I need. Is there something like scrollViewDidBeginDragging? I didn't find anything in the docs. Any smart workaround?

Answer 1

Store the initial content offset in scrollViewWillBeginDragging:
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
self.initialContentOffset = scrollView.contentOffset.y;
self.previousContentDelta = 0.f;
}
And check it on each scrollViewDidScroll:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
CGFloat prevDelta = self.previousContentDelta;
CGFloat delta = scrollView.contentOffset.y - self.initialContentOffset;
if (delta > 0.f && prevDelta <= 0.f) {
// started scrolling positively
} else if (delta < 0.f && prevDelta >= 0.f) {
// started scrolling negatively
}
self.previousContentDelta = delta;
}

Answer 2

It IS possible to perform this check in scrollViewWillBeginDragging before any scrolling is registered. (IOS 5+). By examining the scroll view's built-in pan gesture recognizer you can check gesture direction.
- (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.

Answer 3

Create a declared property to let us know that the tableview is starting to scroll. Let's use a BOOL called scrollViewJustStartedScrolling.
In scrollViewWillBeginDragging set it to true:
self.scrollViewJustStartedScrolling = YES;
In scrollViewDidScroll do something like:
if (self.scrollViewJustStartedScrolling) {
// check contentOffset and do what you need to do.
self.scrollViewJustStartedScrolling = NO;
}

Answer 4

-(void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset{
if (velocity.y > 0){
NSLog(#"up");
} else {
NSLog(#"down");
}
}