单向滚动
scrollview的contentview的宽高值中的某一项应该小于或者等于scrollview的对应值
约束其滚动方向
scroollview.pagingEnabled=NO;
ScrollviewContent* content=[ScrollviewContent SC_init];
CGSize scrollview_size=self.st_Scroollview.bounds.size;
content.frame=CGRectMake(0, 0, scrollview_size.width/2, 2000);
self.st_Scroollview.contentSize=content.bounds.size;
[self.st_Scroollview addSubview:content];
UIScrollView的contentInset - 简书
Using UIScrollView with Auto Layout in iOS