目的:自定義黑線。這里是

 UIImage *colorImage = [self imageWithColor:[UIColor clearColor] size:CGSizeMake(self.view.frame.size.width, 0.5)];

[self.navigationController.navigationBar setBackgroundImage:colorImage forBarMetrics:UIBarMetricsDefault];
[self.navigationController.navigationBar setShadowImage:[self imageWithColor:UIColorWithRGB16Radix(0xECECEB) size:CGSizeMake(self.view.frame.size.width, 0.5)]];
[self.tabBarController.tabBar setShadowImage:[self imageWithColor:UIColorWithRGB16Radix(0xECECEB) size:CGSizeMake(SCREEN_WIDTH,0.5)]];
[self.tabBarController.tabBar setBackgroundImage:[self imageWithColor:[UIColor clearColor] size:CGSizeMake(SCREEN_WIDTH, 0.5)]];

- (UIImage *)imageWithColor:(UIColor *)color size:(CGSize)size {
    if (!color || size.width <=0 || size.height <=0) return nil;
    CGRect rect = CGRectMake(0.0f, 0.0f, size.width, size.height);
    UIGraphicsBeginImageContextWithOptions(rect.size,NO, 0);
    CGContextRef context =UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, color.CGColor);
    CGContextFillRect(context, rect);
    UIImage *image =UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}