//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;

解决方案 ：

[self.navigationController.navigationBar.layer setMasksToBounds:YES];       // 剪切掉多余的背景

解决方案 ：

self.navigationController.navigationBar.clipsToBounds = YES;

解决方案 ：

CustomNavBar *navBar = (CustomNavBar *)self.navigationController.navigationBar;

        [navBar setBackgroundImage:[UIImage imageNamed:@"navigation_bar_gray.png"] forBarMetrics:UIBarMetricsDefault];

        navBar.shadowImage = [[UIImage alloc]init]; // this is what acctually removed the shadow under navigation bar