I'm using scrollViewWillEndDragging:withVelocity:targetContentOffset: to snap my scrollview to pages. I customized the behavior based on the velocity. It works fine except that the scrollview decelerates too early with large offsets. The contentOffset ends up at the correct place but when the scrolling is about to end, the velocity is becoming so low that it takes about 3-4s to reach the target. The problem becomes more obvious when trying to scroll by 3-4 full screen pages. The code below is from a standalone project (Create new Single View project, add a fullscreen scrollview in the storyboard, setup iPad only, landscape mode).
FYI, changing decelerateRate doesn't solve the issue (it just makes scrolling slower/faster).
#import "ViewController.h"
@implementation ViewController
@synthesize scrollView;
- (void)viewDidLoad
{
[super viewDidLoad];
pages_ = 30;
UIView* arr[pages_];
for (int i = 0; i < pages_; ++i) {
CGRect frame = CGRectMake(1024 * i, 0, 1024, 768);
arr[i] = [[UIView alloc] initWithFrame:frame];
[scrollView addSubview:arr[i]];
arr[i].backgroundColor = (i%2) ? [UIColor redColor] : [UIColor greenColor];
}
scrollView.contentSize = CGSizeMake(1024 * pages_, 768);
scrollView.delegate = self;
scrollView.decelerationRate = UIScrollViewDecelerationRateFast;
}
- (void)scrollViewWillEndDragging:(UIScrollView *)sv
withVelocity:(CGPoint)velocity
targetContentOffset:(inout CGPoint *)targetContentOffset {
CGFloat pageWidth = scrollView.frame.size.width;
unsigned int page = floor((scrollView.contentOffset.x - pageWidth / 2) / pageWidth) + 1;
int scrollToPage = page + velocity.x*1.5;
scrollToPage = MAX(0, MIN(scrollToPage, (int)pages_ - 1));
NSLog(@"Scrolling by %d pages", abs(page-scrollToPage));
targetContentOffset->x = scrollView.frame.size.width * scrollToPage;
targetContentOffset->y = 0;
NSLog(@"Scrolling to offset %f", targetContentOffset->x);
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)sv {
NSLog(@"scrollViewDidEndDecelerating");
}
@end
