记录

//用户有两种动作，1.拖动到指定位置后。执行scrollViewDidEndDragging willDecelerate，2.滑动后，任其滚动后停止。先执行DidEndDragging willDecelerate，而滚动停止后执行scrollViewDidEndDecelerating

//1只对应 scrollViewDidEndDragging decelerate为NO

//2对应两种方法，(但scrollViewDidEndDragging的decelerate为YES)所以要判断，防止重复执行操作

官方解释

To determine when scrolling is complete you must implement two delegate methods: scrollViewDidEndDragging:willDecelerate: and scrollViewDidEndDecelerating:. Scrolling is completed either when the delegate receives the scrollViewDidEndDragging:willDecelerate: message with NO as the decelerate parameter, or when your delegate receives the scrollViewDidEndDecelerating: method. In either case, scrolling is complete.

//停止拖拽的时候开始执行
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    if (!decelerate)
    {
        [self loadImagesForOnscreenRows];
    }
    NSLog(@"%d,%@",decelerate,NSStringFromSelector(_cmd));
}

//减速停止
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    [self loadImagesForOnscreenRows];
    NSLog(@"%@",NSStringFromSelector(_cmd));
}