Traversing the ViewController hierarchy properly?

I'm having trouble referencing one view controller from another. The code works but I get warnings which makes me think I'm going about it wrong. I'm trying to reload the data in a tableView whose controller is in a NavigationController.

What's wrong with a message like this:

From the AppDelegate:

[self.tabBarController.selectedViewController.topViewController.tableView reloadData];

Although this works, I get the warning request for member 'topViewController' in something not a structure or union because Xcode doesn't know that the selectedViewController will return a navigationController. So I could do the following:

UINavigationController *myNavigationController = self.tabBarController.selectedViewController;
[myNavigationController.topViewController.tableView reloadData];

But then I get this warning: incompatible Objective-C types initializing 'struct UIViewController *', expected 'struct UINavigationController *'

How far do I have to go with this? The first line works. To get to the "right way" is it gonna开发者_JS百科 take 8 lines of code?

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A major code smell here, IMO. You're trying to do action at a (great) distance. It's not exactly clear what you're trying to accomplish, nor why you need to do this action from the app delegate. I have seen some developers treat the app delegate like a giant catch-all global lump of mud, and I think this is an anti-pattern that should be eliminated from iOS development.

Back to your question: you're trying to force a table view controller, inside a tab view controller, to reload its data. I'm assuming this is in response to something happening. Why not have the view controller in charge of that table watching for that event instead of the app delegate? That way, the thing that owns the table view is directly controlling it -- which is the entire point of the MVC pattern. This is a much better approach than having the app delegate drill down through a hierarchy to find a table view... in terms of complexity, readability, and brittleness.

If, for some reason, you can't or won't have that view controller observing for the event directly (hard to fathom why offhand), you could always have the app delegate post an NSNotification and let the view controller in charge of the table register as an observer for it. Not as good as direct observation, but definitely better than your current approach.

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You can't use dot-notation unless the compiler knows what type of object you are using it on, and that that object type can receive a message with that name.

You can use dot-notation with a bunch of type-casts (which in this case, is hideously ugly):

[((UITableViewController *) ((UINavigationController *) self.tabBarController.selectedViewController).topViewController).tableView reloadData];

Or you can break it up into discrete steps:

UINavigationController *navController = (UINavigationController *) self.tabBarController.selectedViewController;
UITableViewController *tableViewController = (UITableViewController *) navController.topViewController;
[tableViewController.tableView reloadData];

Note that I'm assuming that your top VC is a sub-class of UITableViewController.

You really shouldn't be accessing the .tableView property externally - you should encapsulate that behaviour with a reloadData method on the View Controller itself. Even if all it does is call reloadData on its .tableView, you should encapsulate it. This will make your code more modular (which makes it easier to understand for you and others), and make it easier to expand on and add complexity to your View Controller down the track.

Without knowing exactly how this app is structured, I would guess that you're probably better off using notifications or observers to get your VC to reload its data. If you have some global event that requires a UI refresh, an NSNotification is a good way to make the UI layer get the message while keeping your code nice and modular.