UIScrollView zoom in/out center

UIScrollView在放大的时候可以居中，但是缩小的时候总是跑到左上角，在网上找了好久，经过验证，以下是最完美的解决方案：

#pragma mark - UIScrollViewDelegate
-(UIView*)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    for(UIView *view in scrollView.subviews)
    {
        if([view isMemberOfClass:[UIImageView class]])
        {
            return view;
        }
    }
    return nil;
}
// 使居中
-(void)scrollViewDidZoom:(UIScrollView *)scrollView
{
    CGFloat top = 0, left = 0;
    if (self.scrollView.contentSize.width < self.scrollView.bounds.size.width) {
        left = (self.scrollView.bounds.size.width-self.scrollView.contentSize.width) * 0.5f;
    }
    if (self.scrollView.contentSize.height < self.scrollView.bounds.size.height) {
        top = (self.scrollView.bounds.size.height-self.scrollView.contentSize.height) * 0.5f;
    }
    self.scrollView.contentInset = UIEdgeInsetsMake(top, left, top, left);
}