Canvas Items and the GTK+ Object System

Canvas items are GTK+ objects derived from an abstract GnomeCanvasItem class, which defines several methods that all items must implement. These methods are used to perform drawing, hit testing, and updating of items when their attributes change.

Using the GTK+ object system for canvas items provides several advantages:

• No extra work is involved in wrapping them for different language bindings, since GTK+ objects and their attributes are wrapped automatically by most bindings.

• Canvas items use the usual signal/slot mechanism to emit events, making it easy for the programmer to define behavior for the items.

• One can associate arbitrary data items to canvas items, via the GTK+ dataset mechanism.

All the attributes of canvas items, like color, position, and line style, are configured and queried using the GTK+ object argument system. Canvas items may have many configurable attributes, so using the argument system allows us to minimize the number of API entry points, and also makes it easy to write language bindings for the canvas and its items -- all canvas item attributes can be configured using a single function call.