GenView: 1.1 GenView Overview: GenView Model

The GenView is simply an object that maintains a window for your geode's use. It must be instructed in how to display your data, although it defaults to a certain display scheme. It receives and sends out messages that define how and when your data is drawn; you must provide an object that can receive the view's messages and respond correctly.

The object that communicates with the view on the application's side is known as the content object. The content may be one of three different classes (see below) depending on the type of data you wish to display. Essentially, what type of object you designate as your content is irrelevant to the view; the view sends and receives the same messages regardless.

There are three basic models you may employ based on the type of data you have to display:

Graphic document
If you are just displaying standard graphics, you will want the content to be an object of ProcessClass . This means that your application's Process object will receive all the messages sent by the view (including MSG_META_EXPOSED , described later) and must know how to draw the graphics document when notified. This model is common.
Visible object tree
If you have a tree of visible objects, you will want to set the view's content as an object of VisContentClass . This object understands how to respond to several of the view's messages and how to draw a visible tree when notified. This approach is also common.
Generic object tree
Though seldom used, it is possible to display generic gadgets in a scrolling view by setting the view's content to be a GenContent object. The GenContent is analogous to the VisContent but works with generic object trees rather than visible object trees. The GenContent is used most often as the superclass of GenDocumentClass (see the Documents chapter).

As described in the Graphics Environment chapter, GEOS uses two different coordinate systems, both of which are based on points (1/72 inch). The default system uses 16-bit coordinates and allows documents up to 113 inches on a side. The large model uses 32-bit coordinates, resulting in documents up to 2,147,483,648 points on a side, or 940 miles square. The GenView uses the large model (32-bit coordinates) by default. This means that all coordinates related to the view will be 32 bits, though you can still use 16 bit coordinates for your documents.

Because the graphic document model described above is the simplest, this chapter will focus on having a ProcessClass object as your view's content.

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