Consider an app that displays realtime data such as the current price of a financial stock. The app would probably use some form of stock price web service to continuously update the data model within the ViewModel with the latest information. Obviously, this realtime data is of little use unless it is displayed to the user in a timely manner. There are only two ways that the UI controller can ensure that the latest data is displayed in the user interface. One option is for the controller to continuously check with the ViewModel to find out if the data has changed since it was last displayed. The problem with this approach, however, is that it is inefficient. To maintain the realtime nature of the data feed, the UI controller would have to run on a loop, continuously checking for the data to change.
A better solution would be for the UI controller to receive a notification when a specific data item within a ViewModel changes. This is made possible by...
