关于UIScrollView的属性

设置按页滚动时，注意设置pagingEnabled属性，只有这样，滚动才会一页页滚动。
1.bounds属性：origin为当前内容偏移量
2.frame属性：scrollview本身的大小
3.contentsize属性：包含内容的大小
4.contentoffset属性：内容偏移量
5.center属性：scrollview本身的中心
如下示例

(lldb) po (CGRect)[self.contentScrollView bounds]
(origin = (x = 1280, y = 0), size = (width = 320, height = 379))
 (origin = (x = 1280, y = 0), size = (width = 320, height = 379))
(lldb) po (CGRect)[self.contentScrollView frame]
(origin = (x = 0, y = 52), size = (width = 320, height = 379))
 (origin = (x = 0, y = 52), size = (width = 320, height = 379))
(lldb) po (CGSize)[self.contentScrollView contentSize]
(width = 1920, height = 0)
 (width = 1920, height = 0)
(lldb) po (CGPoint)[self.contentScrollView contentOffset]
(x = 1280, y = 0)
 (x = 1280, y = 0)
(lldb) po (CGPoint)[self.contentScrollView center]
(x = 160, y = 241.5)
 (x = 160, y = 241.5)