iOS-tableView下拉头部图片变大

#define oriOfftY -244
#define oriHeight 200
@property (weak, nonatomic) IBOutlet NSLayoutConstraint *heightConstr;
self.tableView.contentInset = UIEdgeInsetsMake(244, 0, 0, 0);
-(void)scrollViewDidScroll:(UIScrollView *)scrollView{
    
    //求偏移量
    //当前点 - 最原始的点
    NSLog(@"最原始的点.y%f",scrollView.contentOffset.y);
    CGFloat offset = scrollView.contentOffset.y - oriOfftY;
    NSLog(@"offset======%f",offset);
    
    CGFloat h = oriHeight - offset;
    if (h < 64) {
        h = 64;
    }
    self.heightConstr.constant = h;
    
   
    //根据透明度来生成图片
    //找最大值/这里最大值是1
    CGFloat alpha = offset * 1 / 136.0;
    if (alpha >= 1) {
        alpha = 0.99;
    }
    
    //拿到标题
    UILabel *titleL = (UILabel *)self.navigationItem.titleView;
    titleL.textColor = [UIColor colorWithWhite:0 alpha:alpha];
    
    //把颜色生成图片
    UIColor *alphaColor = [UIColor colorWithWhite:1 alpha:alpha];
    //把颜色生成图片
    UIImage *alphaImage = [UIImage imageWithColor:alphaColor];
    //修改导航条背景图片
    [self.navigationController.navigationBar setBackgroundImage:alphaImage forBarMetrics:UIBarMetricsDefault];

}