iOS设置UINavigationBar的背景颜色或背景图片

在AppDelegate中写入：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    [[UINavigationBar appearance] setBackgroundImage:[UIImage imageNamed:@"xx.png"] forBarMetrics:UIBarMetricsDefault];
    return YES;
}