列表头部放大效果的图

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
CGFloat y = scrollView.contentOffset.y;
CGRect rect = _bgImage.frame;
if (y < 0) {
rect.origin.y = scrollView.contentOffset.y;

if (_ud.loggedIn) {
rect.size.height = 206 - y;
} else {
rect.size.height = 150 - y;
}
_bgImage.frame = rect;
}
}