ViewController Hierarchy getting 'lost'

I've got a view hierarchy which is setup (programmatically) as follows:

Window.root = TabBarController-->UINavigationControllers-->UIViewControllers

I presume that's rather standard. Here's开发者_高级运维 my problem:

I'm on Tab A. I want to navigate to Tab B, and call a method on the visibleViewController on Tab B.

// View Changes OK
[AppDelegate.tabBarController setSelectedIndex:tabB];

// nav  = 0x387ABF i.e. Valid Address
UINavigationController *nav = (UINavigationController*)[AppDelegate.tabBarController selectedViewController];

// The problem:
nav.viewControllers; // this is nil
nav.topViewController; // as is this
nav.visibleViewContorller; // this too.

Even if I put the calls to nav.viewControllers in a separate method which is called from the Main Thread, I still get 0x0/nil.

What am I doing wrong?

A follow-up question is: How can I pass information from one ViewController to another when changing tabs? (If I can't call methods on VC's from tabA to tabB)

I have a feeling it is related to my question here.

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You should store the information in a common place, either a singleton or as you are a beginner just make a class and pass it down.

sharedDataObject = [[MySharedDataObject alloc] init];

firstViewController.myDataObject = sharedDataObject;
secondViewController.myDataObject = sharedDataObject;