To help combine nodes from various sources into a single graph, the graph evaluation API provides a primitive form of reflection in the AcDbEvalConnectable class. This class derives from and enriches the AcDbEvalExpr class, and thus represents a node type. Applications may derive from AcDbEvalConnectable rather than AcDbEvalExpr to gain extra functionality.
An AcDbEvalConnectable object is a node that knows how to connect to other connectable nodes. To implement this intelligence, the AcDbEvalConnectable class exposes functions for elaborating connectable properties by name, and also for connecting properties from different objects.
AcDbEvalConnectable objects provide metadata that describes connections between themselves and other connectable objects. Such connections are represented by edges in a graph. Objects derived from AcDbEvalConnectable implement the connectTo() and disconnectFrom() virtual functions by calling upon the graph to add and remove the desired edges.
Applying this approach to the circle grip example, the authoring elements resemble “meta” nodes that expose connectable properties to dependent objects.