UIWindow have global notifications for keyboard in iOS devices. How it works i show in this.Read UIWindow class description for study all events. (ReactiveCocoa update)

ManageKeyboard.m

[[[[[NSNotificationCenter defaultCenter] rac_addObserverForName:UIKeyboardWillChangeFrameNotification
                                                             object:nil] takeUntil:[self rac_willDeallocSignal]] deliverOnMainThread] subscribeNext:^(NSNotification *notification) {
        CGRect  keyboardFrame   = [[notification userInfo][UIKeyboardFrameEndUserInfoKey] CGRectValue];
        CGRect  frameOfScreen   = [UIScreen mainScreen].bounds;
}];