320為headView本身高度

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    
    CGFloat width = [UIScreen mainScreen].bounds.size.width;
    CGFloat yOffset = scrollView.contentOffset.y;

    if (yOffset < 0) {
        CGFloat totalOffset = 320 + ABS(yOffset);
        CGFloat f = totalOffset / 320;
        
        _headImageView.frame = CGRectMake(- (width * f - width) / 2, yOffset, width * f, totalOffset);
        
    }
    
}