Infinite UIScrollView w/Paging for UIImageViews - determining new page to load

UIScrollView Infinite Paging

-(void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        CGFloat xOffset = _scrollView.contentOffset.x;
        CGFloat newStart = 0;
        BOOL done = NO;
        NSInteger newIndex = 1;
        while (done == NO) {
            if (newStart + 320 < xOffset) {
                newStart += 320;
                newIndex++;
            }else{
                done = YES;
            }
        }
        if (velocity.x < 0) {
            newIndex--;
            newIndex--;
        }else{
            newIndex++;
        }
        if (newIndex >= [_metadata count] || newIndex < 0)
            return;
        id obj = _imageViewArray[newIndex];
        if ([obj isKindOfClass:[UIImageView class]])
            return;
        
        AfterpartyPhotoInfo *currentInfo = _metadata[newIndex];
        CGRect currentFrame = CGRectMake(0, 0, currentInfo.size.width, currentInfo.size.height);
        UIImageView *newImageView = [[UIImageView alloc] initWithFrame:currentFrame];
        [_imageViewArray replaceObjectAtIndex:newIndex withObject:newImageView];
        UIActivityIndicatorView *indicator = _downloadIndicators[newIndex];
        dispatch_async(dispatch_get_main_queue(), ^{
            [_scrollView insertSubview:newImageView belowSubview:indicator];
            newImageView.frame = [self frameForImageViewAtIndex:newIndex];
            newImageView.center = [self centerForImageViewAtIndex:newIndex];
            [self getImageForIndex:newIndex];
        });
    });
}