//从当前视图push到目标视图，如果视图已经存在，则不影响堆栈
[self.navigationController pushViewController:commentListVC animated:YES];
//返回上一控制器
[self.navigationController popViewControllerAnimated:YES];
//返回某一控制器
[self.navigationController popToViewController:[[UIViewController alloc] init] animated:YES];
//返回到根视图
[self.navigationController popToRootViewControllerAnimated:YES];
//B pop 返回A后，B是否释放要看之后的代码是否再次使用了B，如果后面的代码还使用到了B，则B所占的内存一直保留，有一种情况例外，在内存极度不足的情况下，IOS会遵从一定的策略有可能把B释放，B再次被使用的时候再重新创建
另外，即使pop后后面的代码不再使用B了，那么B的内存只是有可能被释放，释放的时机并不一定，这取决于垃圾回收器的回收策略
//去掉返回按钮的文字显示
self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"" style:UIBarButtonItemStyleBordered target:nil action:NULL];
//自定义左右导航项一
UIButton *btn1 = [UIButton buttonWithType:UIButtonTypeCustom];
UIButton *btn2 = [UIButton buttonWithType:UIButtonTypeCustom];
UIBarButtonItem *bar1 = [[UIBarButtonItem alloc] initWithCustomView:btn1];
UIBarButtonItem *bar2 = [[UIBarButtonItem alloc] initWithCustomView:btn2];
//导航栏的左右导航项，可以自定义button
self.navigationItem.leftBarButtonItem = bar1;
self.navigationItem.rightBarButtonItem = bar1;
//也可以添加多个，后面跟数组，排列顺序是依次排列
self.navigationItem.leftBarButtonItems = @[bar1,bar2];
self.navigationItem.rightBarButtonItems = @[bar1,bar2];
//自定义左右导航项二
viewController.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"navItem_course"] style:UIBarButtonItemStylePlain target:self action:@selector(showCourse:)];
viewController.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"注销" style:UIBarButtonItemStylePlain target:self action:@selector(logOut:)];
//设置导航栏的标题视图，可以自定义一个view，label都可以
self.navigationItem.titleView
// back 按钮背景为白色
[self.navigationBar setTintColor:[UIColor whiteColor]];
[self.navigationBar setTitleTextAttributes:[NSDictionary dictionaryWithObjectsAndKeys:COLOR_WITH_RGB(230, 230, 230), UITextAttributeTextColor,[UIFont boldSystemFontOfSize:18.0f],UITextAttributeFont, nil]];
//设置导航栏颜色
[self.navigationBar setBackgroundImage:[UIImage imageWithColor:COLOR_WITH_RGB(70, 70, 70) size:CGSizeMake(SCREEN_WIDTH, KNav_Height)] forBarMetrics:UIBarMetricsDefault];
//隐藏navigationBar
self.navigationController.navigationBarHidden = YES;
