- (void)keyboardWasShown not called when switching to another UITextField

I'm having a strange problem that I don't understand. I have a UIScrollView with several UITextField objects. When I switch to the view, I set the first UITextField as firstresponder, and the keyboardWasShown method gets called due to the UIKeyboardDidShowNotification that the view is registered for. The weird thing is, when I touch the next UITextField, the keyboardWasShown method is not called. I don't understand this, since Apple's documentation says "If your interface has multiple text fields, the user can tap between them to edit the开发者_如何学Go values in each one. When that happens, however, the keyboard does not disappear but the system does still generate UIKeyboardDidShowNotification notifications each time editing begins in a new text field." My code I've copied directly from Apple's documentation as well, and it works properly, but it only gets called the first time. What am I missing?

- (void)registerForKeyboardNotifications {
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWasShown:) name:UIKeyboardDidShowNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWasHidden:) name:UIKeyboardDidHideNotification object:nil];
}

- (void)keyboardWasShown:(NSNotification *)aNotification {
//if (keyboardShown) return;
NSDictionary* info = [aNotification userInfo];
CGSize kbSize = [[info objectForKey:UIKeyboardBoundsUserInfoKey] CGRectValue].size;
CGRect bkgndRect = activeField.superview.frame; 
bkgndRect.size.height += kbSize.height; 
[activeField.superview setFrame:bkgndRect]; 
[scrollView setContentOffset:CGPointMake(0.0, activeField.frame.origin.y) animated:YES]; 
keyboardShown = YES;

UIBarButtonItem *doneButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemDone target:self action:@selector(done)];
self.navigationItem.rightBarButtonItem = doneButton;
[doneButton release];
}

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This notification is not posted every time you change first responders. For that, you must implement the delegate protocols for either UITextField or UITextView (depending on what you are using) and deal with these changes from there. You only get the notification that the keyboard was displayed if the keyboard was previously hidden.