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205

Using Manipulator Public Instance Data

The 

nSelectionList

 field contains the number of currently selected items in the 

window associated with the current manipulator. This corresponds to the number of 
visualizations contained within the 

pSelectionList

 pointer, described in the 

following section. If no visualizations have been selected, the 

nSelectionList

 

value equals 0 and the 

pSelectionList

 will contain an undefined IDL variable. 

The 

nSelectionList

 can be used to ensure the user has made a selection. For 

example, in an OnMouseDown method, you may use a statement similar to the 
following to ensure a selection has been made:

; If nothing selected we are done.

IF (self.nSelectionList EQ 0) THEN $

RETURN

The 

nSelectionList

 field value can also be used to loop through the collection of 

selected visualizations as shown in the following section.

The 

pSelectionList

 field is a pointer to an array of visualizations currently 

selected in the window. Use the 

nSelectionList

 value to cycle through this array. 

If a manipulator only acts upon visualizations of a certain type you can verify the 
type of each selected item in 

pSelectionList

 before attempting to modify the 

visualization. The 

nSelectionList

 and 

pSelectionList

 public instance data 

fields are available from any manipulator object’s predefined or custom methods. 

; Loop through all selected visualizations.

FOR i=0, self.nSelectionList-1 DO BEGIN

oVis = (*self.pSelectionList)[i]

; Verify type of visualization or manipulate it.

; ...

ENDFOR

The 

pSelectionList

 field is a pointer. You must use IDL pointer syntax to 

access items in the field.

See 

 on page 226 for a complete example that 

uses these public instance data fields.