The base class for the protocols that must be implemented by all Open UI field extension classes.
This is an "abstract" protocol, in that you need to implement one of its child protocols so that there is an initWithXxxModel method. The client host will create an instance of the specified class, and call its initWithXxxModel method. When the extension control is to be displayed, the viewForFrame: method will be called and the returned view will be added as a subview of the Agentry Screen. Each adapter will have a host view and view controller that will determine the space dedicated for the adapter.