让viewDidAppear/viewWillAppear 在页面加载前始终调用

viewDidAppear、viewWillAppear in UINavigationController not be called

When you push or pop a view controller on/off a navigaction controller's stack, the usual viewWillAppear / viewDidappear methods aren't called. If you want to ensure they're always called, just add the UINavigationControllerDelegate protocol to your root view controller:

@interface RootViewController : UIViewController <UINavigationControllerDelegate> {
UINavigationController *navController;
}

Then implement these two methods:
- (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];
}

Be sure to set the root view controller as the delegate for the nav controller. Now viewWillAppear / viewDidAappear will be called whenever a controller is pushed/popped from the stack.

If you want to call the viewWillDisappear/viewDidDisappear methods, your view controller still has to do that manually before popping itself off the nav stack.