iOS model layer notifying Controller Object
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https://developer.apple.com/library/mac/#documentation/General/Conceptual/DevPedia-CocoaCore/MVC.html

At the above link, I see the following in regards to how the model layer should communicate with the ViewController.

"When a model object changes (for example, new data is received over a network connection), it notifies a controller object, which updates the appropriate view objects."

**My question is, how does the model object notify the Controller object?** What are various ways this can be done? My model layer gets the user's location and then calls a web service. How should I notify the controller object when that data is downloaded?

Thanks