iPhone UIScrollView Speed Check

Question

I know how to get the contentOffset on movement for a UIScrollView, can someone explain to me how I can get an actual number that represents the current speed of a UIScrollView while it is tracking, or decelerating?

Answer 1

Have these properties on your UIScrollViewDelegate

CGPoint lastOffset;
NSTimeInterval lastOffsetCapture;
BOOL isScrollingFast;

Then have this code for your scrollViewDidScroll:

- (void) scrollViewDidScroll:(UIScrollView *)scrollView {    
    CGPoint currentOffset = scrollView.contentOffset;
    NSTimeInterval currentTime = [NSDate timeIntervalSinceReferenceDate];

    NSTimeInterval timeDiff = currentTime - lastOffsetCapture;
    if(timeDiff > 0.1) {
        CGFloat distance = currentOffset.y - lastOffset.y;
        //The multiply by 10, / 1000 isn't really necessary.......
        CGFloat scrollSpeedNotAbs = (distance * 10) / 1000; //in pixels per millisecond

        CGFloat scrollSpeed = fabsf(scrollSpeedNotAbs);
        if (scrollSpeed > 0.5) {
            isScrollingFast = YES;
            NSLog(@"Fast");
        } else {
            isScrollingFast = NO;
            NSLog(@"Slow");
        }        

        lastOffset = currentOffset;
        lastOffsetCapture = currentTime;
    }
}

And from this i'm getting pixels per millisecond, which if is greater than 0.5, i've logged as fast, and anything below is logged as slow.

I use this for loading some cells on a table view animated. It doesn't scroll so well if I load them when the user is scrolling fast.