Customize UINavigationBar and remove drop shadow at the bottom of UINavigationBar

UIImage *navigationBarImg = [UIImage imageNamed:@"navigationBar"];

//You must specified the BarMetrics for UINavigationBar, Otherwise it reports an exception. 

[[UINavigationBar appearance] setBackgroundImage:navigationBarImg forBarMetrics:UIBarMetricsDefault]; 

//This is the key to eliminate drop shadow of UINavigationBar

[[UINavigationBar appearance] setShadowImage:[[UIImage alloc]init]];