早期的方法，现在不一定好使，记下方便参阅。

if ([self.navigationController.navigationBarrespondsToSelector:@selector(setBackgroundImage:forBarMetrics:)]){

    NSArray *list = self.navigationController.navigationBar.subviews;

    for (id obj in list) {

       if ([obj isKindOfClass:[UIImageViewclass]]) {

           UIImageView *imageView=(UIImageView *)obj;

           NSArray *list2=imageView.subviews;

           imageView.hidden = YES;

           for (id obj2 in list2) {

               if ([obj2 isKindOfClass:[UIImageViewclass]]) {

                   UIImageView *imageView2=(UIImageView *)obj2;

                   imageView2.hidden=YES;

               }

           }

       }

   }

}