I have a picker View declared in the ViewControllerA (in Xib file). This Xib is loaded as a custom cell in ViewControllerB's tableView. If I change the value of Picker View, How can I access this changed value in ViewControllerB.
You can access the value of the UIPickerView by exposing a property on ViewControllerA... If there are set circumstances when viewControllerB needs to know the value, under the control of viewControllerB, then viewControllerB can check the property on viewControllerA at those times...
However, perhaps you're asking a more general question about communication - with the specific example of ViewControllerB needing to know about a change to the UIPickerView that's inside ViewControllerA...
This article on Communication Patterns is worth reading. It covers KVO, Notifications, Delegation, Blocks and Target/Action.
There's a flow-chart near the middle of the article that can help to evaluate what communication strategy to use in a given situation.
From what you've written, it sounds like you could use either KVO (key-value observing) or delegation.
I tend to use delegation in scenarios when one UIViewContoller wants to know about changes made in another UIViewController, e.g. when viewControllerC presents viewControllerD - and wants to know about changes made in viewControllerD.
In your case, you might use a delegate method along the lines of:
- (void)pickerViewValueDidChange:(NSString*)newValue;
That delegate method would be part of an @protocol. Something like:
@protocol ABCPickerViewDelegate
- (void)pickerViewValueDidChange:(NSString*)newValue;
@end
See Working with Protocols if this is new to you...
viewControllerB would conform to that protocol. viewControllerA would have a property that conforms to the protocol, something like:
@property (weak, nonatomic) id <ABCPickerViewDelegate> pickerDelegate;
Then, when the UIPickerView value changes within viewControllerA - it can call the delegate method... And, viewControllerB would then know that the change has occurred and what the value is.