iOS开发viewWillAppear失效

你可曾遇到过viewWillAppear没有被调用到的情况
产生原因是用了UINavigationController. 将UINavigationController的view作为subview添加到了其他viewController的view中。或者把UINavigationController添加到UITabbarController中了。此时，NavigationController的stack里面的viewController就收不到-(void)viewWillAppear:(BOOL)animated；等4个方法的调用。

原因呢

Apple Docs state:

Warning: If the view belonging to a view controller is added to a view hierarchy directly, the view controller will not receive this message. If you insert or add a view to the view hierarchy, and it has a view controller, you should send the associated view controller this message directly. Failing to send the view controller this message will prevent any associated animation from being displayed.
不过后面的到4.0的文档就没有发现这样的文字描述了，但是还是没能够调用的到这样
只是添加的更复杂的文档，头晕晕看不下去了。

解决方法两种：

1. 在导航控制器上层controller的viewWillAppear中显式调用viewWillAppear方法。

-(void)viewWillAppear:(BOOL)animated{
[super viewWillAppear:animated];
[subNavCntlr viewWillAppear:animated];
}

• (void)viewWillDisappear:(BOOL)animated{
  [super viewWillDisappear:animated];
  [subNavCntlr viewWillDisappear:animated];
  }

2.  把导航控制器上层controller设为UINavigationController的delegate
nav.delegate = self;

@interface RootViewController : UIViewController
{ UINavigationController
*navController;}

• (void)navigationController:(UINavigationController
  *)navigationController willShowViewController:(UIViewController
  *)viewController animated:(BOOL)animated {
  [viewController viewWillAppear:animated];
  }
• (void)navigationController:(UINavigationController
  *)navigationController didShowViewController:(UIViewController
  *)viewController animated:(BOOL)animated {
  [viewController viewDidAppear:animated];
  }