ScrollView三个基本的要理解的属性；
contentSize
The size of the content view.所以很好理解，contentSize也就是scrollView可滚动的区域.
contentOffset
The point at which the origin of the content view is offset from the origin of the scroll view.
这里标注的是point，是当前显示的区域的origin相对于整个scrollView的origin的位置.
contentInset
The distance that the content view is inset from the enclosing scroll view.
这里说得是distance，四个值分别是上下左右, 默认值是 UIEdgeInsetsZero，也就是四个值都为0.
_|←_cW_→_|_↓_ | | --------------- |content| ↑ ↑ |content| contentInset.top cH |content| ↓ |content| contentInset.bottom |content| ↓ --------------- _|_______|___ ↑ (cH = contentSize.height; cW = contentSize.width)
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView; //返回要缩放的图片(必须在代理类里实现)
图片缩放保持居中，实现代码：
（在viewWillLayoutSubviews也要调用）
func recenterImage() { let scrollViewSize = scrollView.bounds.size let imageSize = imageView.frame.size let horizontalSpace = imageSize.width < scrollViewSize.width ? (scrollViewSize.width - imageSize.width) / 2 : 0 let verticalSpace = imageSize.height < scrollViewSize.height ? (scrollViewSize.height - imageSize.height) / 2 : 0 scrollView.contentInset = UIEdgeInsets(top: verticalSpace, left: horizontalSpace, bottom: verticalSpace, right: horizontalSpace) }
原文：http://www.cnblogs.com/liuliuliu/p/4498058.html