在UIScrollView的delegate方法判断滚动快慢

    // 这里做预加载
    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) / 1700; //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;
    }