AfraLISP - Learn AutoLISP for AutoCAD productivity

Properties & Methods - Part 3

by Kenny Ramage

Determining Whether an Object is Available for Updating

If other applications are working with any AutoCAD Objects at the same time as your program, those Objects may not be accessible. This is especially important to lookout for if your application includes reactors, because reactors execute code segments in response to external events that cannot be predicted in advance. Even a simple thing such as a locked Layer can prevent you from changing an Objects Properties.

Visual Lisp provides the following functions to test the accessibility of an Object before trying to use the Object :

  • vlax-read-enabled-p Tests whether you can read an Object.
  • vlax-write-enabled-p Determines whether you can modify an Objects Properties.
  • vlax-erased-p Checks to see if an Object has been erased. Erased Objects may still exist in the drawing database.

All these test functions return T if true, and nil if false.

Let's test them out. Draw a line anywhere in AutoCAD then enter this at the Console prompt :

_$ (vl-load-com)
_$ (setq ent (entsel))
(<Entity name: 17e39f8> (434.601 389.588 0.0))

_$ (setq myLine (vlax-Ename->Vla-Object (car ent)))
#<VLA-OBJECT IAcadLine 01e81f84>

Determine whether the line is readable :

_$ (vlax-read-enabled-p myLine)

Determine whether the line is modifiable :

_$ (vlax-write-enabled-p myLine)

See if the line has been erased :

_$ (vlax-erased-p myLine)

Erase the line :

_$ (vla-delete myLine)

See if the line is still readable :

_$ (vlax-read-enabled-p myLine)

Check to confirm that the object has been deleted :

_$ (vlax-erased-p myLine)

Determining If a Method or Property Applies to an Object

Trying to use a Method that does not apply to a specified Object will result in an error. The same goes for trying to reference a Property that does not apply to an Object. This will also result in an error. In instances where you are not sure what applies, use the vlax-method-applicable-p and the vlax-property-available-p functions.

These functions return T if the Method or Property is available for the Object, and nil if not.

The syntax for vlax-method-applicable-p is :

(vlax-method-applicable-p object method)

The following command checks to see if the "Copy" Method can be applied to an Object :

_$ (vlax-method-applicable-p myLine "Copy")

The following command determines if the "addBox" Method can be applied to an Object :

_$ (vlax-method-applicable-p myLine "addBox")

For vlax-property-available-p the syntax is :

(vlax-property-available-p object property [T])

The following commands determine if Color and Center are properties of the myLine Object :

(vlax-property-available-p myLine "Color")

(vlax-property-available-p myLine "Center")

Supplying the optional T argument to vlax-property-available-p changes the meaning of the test. If you supply this argument, the function returns T only if the Object has the Property AND the Property can be modified. If the Object has no such Property or the Property is Read-Only, vlax-property-available-p returns nil.

For example, a Circle contains an Area Property, but you cannot update it. If you check the Property without specifying the optional argument the result is T.

(vlax-property-available-p myCircle "area")

If you supply the optional argument, the result is nil :

(vlax-property-available-p myCircle "area" T)

Well, that's it with Methods and Properties. Time for me to pour myself a nice long, cold beer and retire to the garden and sit in the sun. The advantages of living in Africa. Eat your heart out…

Oh, by the way, my Mum has got a great Method of cooking Tripe and Onion's if anybody is interested!