796: Mainline

The model: message accomplishes two key objectives:

• The model becomes the assignment of the model instance variable residing in the viewer instance. This assignment allows the model to be obtained from the viewer instance via model, a getter defined for all dependent parts, including, of course, all dependent viewers.
• The viewer instance becomes part of a list of dependent parts attached to the model instance. The model instance uses this list to send update messages to all dependent parts, including, of course, all viewers.