858: Mainline

You define your own displayOn: method to draw, because VisualWorks provides mechanisms that send displayOn: to a view, with a graphics-context argument, thePen, whenever one of the following occurs:

• The operating system notes that a change has occurred that calls for redrawing, such as uncovering or moving a window.
• The view receives an update: message from a model, suggesting that a value has changed that may require a change in what is displayed.