Demand Loading and Xref Performance

ActiveX and VBA Developer's Guide > Advanced Drawing and Organizational Techniques > Use External References >

Through a combination of demand loading and saving drawings with indexes, you can increase the performance of drawings with external references. Demand loading works in conjunction with the XLOADCTL and INDEXCTL system variables. When you turn on demand loading, if indexes have been saved in the referenced drawings, AutoCAD loads into memory only the data from the reference drawing that is necessary to regenerate the current drawing. In other words, referenced material is read in “on demand.”

To realize the maximum benefits of demand loading, you need to save the referenced drawings with layer and spatial indexes. The performance benefits of demand loading are most noticeable when you

Clip the xref to display a small fraction of it, and a spatial index is saved in the externally referenced drawing.
Freeze several layers of the xref, and the externally referenced drawing is saved with a layer index.

To turn on demand loading, use the XRefDemandLoad property. If you turn on demand loading with the acDemandLoadEnabledWithCopy option, AutoCAD makes a temporary copy of the externally referenced file and demand loads the temporary file. You can then demand load the xref while allowing the original reference drawing to be available for modification. When you disable demand loading, AutoCAD reads in the entire reference drawing regardless of layer visibility or clip instances.

To turn on layer and spatial indexes, set the INDEXCTL system variable using the SetVariable method. The following settings apply to the INDEXCTL s-ystem variable:

0 = no indexes created
1 = layer index created
2 = spatial index created
3 = both spatial and layer indexes created

By default, INDEXCTL is set to 0 when you create a new AutoCAD drawing.

For more information on demand loading and xrefs, see “Increase Performance with Large Xrefs” in the User's Guide.