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data from the reference drawing that is necessary to regenerate the current
drawing is loaded into memory. In other words, referenced data is read in "on
demand."
Demand loading works in conjunction with the INDEXCTL, XLOADCTL, and
XLOADPATH system variables.
Unload Xrefs in Large Drawings
When a referenced drawing (xref) is unloaded from the current drawing, the
drawing opens much faster and uses less memory.
The xref definition is unloaded from the drawing file, but the internal pointer
to the referenced drawing remains. The xref is not displayed, and nongraphical
object information does not appear in the drawing. However, you can restore
all the information by reloading the xref. If XLOADCTL (demand loading) is
set to 1, unloading the drawing unlocks the original file.
You should unload a reference file if it is not needed in the current drawing
session but may be used later for plotting. You can maintain a working list of
unloaded xrefs in the drawing file that you can load as needed.
Work with Demand Loading in Large Drawings
With demand loading, only the data from the referenced drawing that is
necessary to regenerate the current drawing is loaded into memory.
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 do one of the following:
Clip the xref with the program to display a small fraction of it. A spatial
index is saved in the externally referenced drawing.
Freeze several layers of the xref. The externally referenced drawing is saved
with a layer index.
If demand loading is turned on, and you have clipped xrefs that were saved
with spatial indexes, objects in the referenced drawing database that are
contained within the clip volume comprise the majority of the objects read
into the drawing. If the clip volume is modified, more objects are loaded as
required from the reference drawing. Similarly, if you have xrefs with many
layers frozen that were saved with layer indexes, only the objects on those
thawed layers are read into the current drawing. If those xref-dependent layers
478 | Chapter 11 Share Data Between Files
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