150 Chapter 7: Creating an Operation
Operations and the Undo/Redo System iTool Developer’s Guide
Operations and the Undo/Redo System
The iTools system provides users with the ability to interactively undo and redo
actions performed on visualizations or data items. As an iTool developer, you will
need to provide some code to support the undo/redo feature; the amount of code
required depends largely on the type of operation your operation class performs. The
main dividing line is between data-centric operations that act directly on the data that
underlies a visualization, and operations that act in a more generalized way, changing
some value that may not be directly related to a data item. In most cases, operations
that act directly on data are based on the IDLitDataOperation class, whereas
operations that are more generalized are based on the IDLitOperation class.
Data-Centric Operations
Undo/redo functionality is handled automatically for data-centric operations based on
the IDLitDataOperation class. The following things happen when the user requests an
operation:
• For each selected item, data that matches the type supported by the operation is
extracted and passed to the operation’s Execute method. The Execute method
modifies the data in place. When the data changes, all visualizations that
observe the data item are notified, and update accordingly.
• If the user undoes the operation, the original data values are restored. By
default, the original values are cached before the Execute method is called, and
undoing the operation simply retrieves the data values from the cache. If the
REVERSIBLE_OPERATION property of the IDLitDataOperation object is
set, however, the original values are not cached, and the UndoExecute method
is called when the user undoes the operation. The UndoExecute method must
exist and must reverse the action performed by the Execute method, restoring
the data items to their original values. Using the REVERSIBLE_OPERATION
property allows you to avoid caching the data set (which may be large) when
the operation performed on the data is easily reversed by computation.
• If the user redoes the operation, the data values computed by the Execute
method are restored. By default, the Execute method is simply called again. If
the EXPENSIVE_COMPUTATION property of the IDLitDataOperation
object is set, however, the computed values are cached after the Execute
method is called, and redoing the operation simply restores the cached data
values. Using the EXPENSIVE_COMPUTATION property allows you to
avoid having to recompute a computationally-intensive operation each time
the user undoes and then redoes the operation.
