UIScrollView放大缩小图片偏移的问题

添加UIScrollView代理方法：

- (void)scrollViewDidZoom:(UIScrollView *)aScrollView
{
    CGFloat offsetX = (scrollView.bounds.size.width > scrollView.contentSize.width)?
    (scrollView.bounds.size.width - scrollView.contentSize.width) * 0.5 : 0.0;
    CGFloat offsetY = (scrollView.bounds.size.height > scrollView.contentSize.height)?
    (scrollView.bounds.size.height - scrollView.contentSize.height) * 0.5 : 0.0;
    imgView.center = CGPointMake(scrollView.contentSize.width * 0.5 + offsetX,
                                   scrollView.contentSize.height * 0.5 + offsetY);
}