掩藏导航条底部的黑线（shadowImage）

stretchableImageWithLeftCapWidth:5 topCapHeight:6] forState:UIControlStateNormal];
    [self.navigationController.navigationBar setBackgroundImage:[[UIImage alloc] init] forBarMetrics:UIBarMetricsDefault];
    self.navigationController.navigationBar.shadowImage = [[UIImage alloc] init];