iOS 给UINavicationBar设置背景图片坐标便宜

[self.navigationController.navigationBar setBackgroundImage:[UIImage imageNamed:@"navbar_p_bg"] forBarMetrics:UIBarMetricsDefault];

 为什么这个方法执行后坐标会改变啊

   //给navigationBar设置背景图片  
        if ([self.navigationController.navigationBar respondsToSelector:@selector(setBackgroundImage:forBarMetrics:)]) {  
            [self.navigationController.navigationBar setBackgroundImage:[UIImage imageNamed:@"nav_bg.png"] forBarMetrics:UIBarMetricsDefault];  
        }  
        self.navigationController.navigationBar.layer.contents = (id)[UIImage imageNamed:@"nav_bg.png"].CGImage;   
        [homeScrollView setContentSize:CGSizeMake(320, 404)];