Obtaining a Pointer to Model Space

When you add entities through ActiveX functions, you need to identify the model space or paper space in which the entity is to be inserted. (In ActiveX terminology, entities are objects, but this tutorial will continue using the term entity.) To tell AutoCAD which space the new entities should occupy, you need to obtain a pointer to that space. Unfortunately, obtaining a pointer to model space is not a simple, single-shot function. The following code fragment shows how the operation needs to be set up:

(vla-get-ModelSpace (vla-get-ActiveDocument
  (vlax-get-Acad-Object)))

Working from the inside out, the vlax-get-Acad-Object function retrieves a pointer to AutoCAD. This pointer is passed to the vla-get-ActiveDocument function, which retrieves a pointer to the active drawing (document) within AutoCAD. The Active Document pointer is then passed to the vla-get-ModelSpace function that retrieves a pointer to the model space of the current drawing.

This is not the kind of expression you want to type over and over. For example, look at how much more complicated the code for adding a polyline using ActiveX appears when the entire model space expression is used:

(setq pline (vla-addLightweightPolyline
  (vla-get-ModelSpace
    (vla-get-ActiveDocument
      (vlax-get-Acad-Object)
    )
  )
  VLADataPts)
)
(vla-put-closed pline T)

The function is definitely less understandable. Not only that, but within every expression within your program where an entity is created, you repeat the same set of nested functions. This demonstrates one of the few excellent uses for global variables. The garden path application can add a lot of entities to model space (think of all the tiles in the path), so, set up a global variable to store the pointer to the model space, as in the following code:

(setq *ModelSpace* (vla-get-ModelSpace (vla-get-ActiveDocument 
                 (vlax-get-Acad-Object))))

You can use the variable *ModelSpace* anytime you call an ActiveX entity creation function. The only tricky thing with this scheme is the *ModelSpace* variable must be ready to go before you start drawing. For this reason, the setq establishing this variable will be called at the time the application is loaded, immediately after the call to vl-load-com. These calls will be placed before any defun in the program file. As a result, they are executed as soon as the file is loaded.