Custom Properties

[Richard Gaskin]

Mark Wieder wrote:

> What this lets me do is subclass from existing objects 
> (in this case a field) to create a new ledger field
> object, and then duplicate and reuse it.

A word of caution for newcomer jumping into getProp and setProp, to save 
them some gray hairs I earned a couple weeks ago:

The docs note that if you handle a property setting for an object, and 
in that setProp also set a property of the same name, the call to set 
the property from within the setProp handler won't trigger another 
setProp, e.g.:

   setProp MyProp pMyVal
      set the MyProp of me to pMyVal
   end MyProp

This prevents recursion, since if the setProp did trigger another 
setProp you'd be stuck in an infinite loop.

However, setting other properties other than the one being handled will 
trigger a setProp for that other property:

   setProp MyProp pMyVal
       set the MyOtherProp of me to pMyVal --triggers setProp MyOtherProp
   end MyProp


Now here's the tricky part:  the no-recursion feature only kicks in if 
the handler is in the target object itself.  I learned this the hard way 
  by moving my getProps and setProps to a library and then wondering why 
I was getting recursion errors.

So if you want to centralize your getProp and setProp handlers, you can 
do so safely if the true name of any property you set isn't the same and 
the property which triggered the setProp:

   -- Library version of first example:
   on setProp MyProp pMyVal
     set the _MyProp of the target to pMyVal
   end MyProp


Hope that was clear enough to be helpful....


--
  Richard Gaskin
  Managing Editor, revJournal
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