Best practice for using the aparent class property

The 'APARENT' class property gives access to the instance that references the class you are working on.

If a child class is used by different parent classes, code writing that refers to the parent class in the script associated with the child class can be risky. You can test this.APARENT.objecttype, but the result will be a fragile code that depends on the parent class evolution.

Example

Suppose you have an "ORDER" class, a "DELIVERY" class, and a "LINE" class.

• In your "ORDER" header, you have the properties CODE, NAME, DATEORD, and LINES().
• In your "DELIVERY" header, you have the properties CODE, NAME, DATEDEL, and LINES().

The "LINES" structure is the same in both cases as it carries the properties DATELINE, QUANTITY, and ITEM.

If you wanted to default the "DATELINE" property with the date found on the header, the incorrect method to use would be to write it in the script associated with the lines.

$PROPERTIES
 Case CURPRO
   When "DATELINE" : Gosub DATELINE
   ...
 Endcase
Return

$DATELINE
 Case ACTION
   When "INIT" : Gosub INIT_DATELINE
   ...
 Endcase
Return

$INIT_DATELINE
 Case this.APARENT.objecttype :
  When "ORDER" : this.DATELINE=this.APARENT.DATEORD
  When "DELIVERY" : this.DATELINE=this.APARENT.DATEDEL
 Endcase
Return

• The code written handles the logic attached to the 'order' and 'delivery' documents. How can a code reader understand that the logic of the 'sales order' and the 'delivery' is mixed in the 'line' code?
• If for any reason a new document needs to use this type of line, you will have to add a corresponding case. Delivering a new document type requires to re-deliver the line code, while it should be independent if your design is smart.
• If there is a change on "ORDER" or "DELIVERY", it might once again have an impact on the code delivered for the 'line'.

In summary, THIS CODE IS FRAGILE, DIFFICULT TO READ AND THEREFORE MUST BE AVOIDED.

The correct method to use is as follows:

On the script associated with the "ORDER" class, we will find the following lines:

$PROPERTIES
 Case CURPRO
   When "LINES.DATELINE" : Gosub DATELINE
   ...
 Endcase
Return

$DATELINE
 Case ACTION
   When "INIT" : Gosub INIT_DATELINE
   ...
 Endcase
Return

$INIT_DATELINE
  this.DATELINE=this.APARENT.DATEORD
Return

$PROPERTIES
 Case CURPRO
   When "LINES.DATELINE" : Gosub DATELINE
   ...
 Endcase
Return

$DATELINE
 Case ACTION
   When "INIT" : Gosub INIT_DATELINE
   ...
 Endcase
Return

$INIT_DATELINE
  this.DATELINE=this.APARENT.DATEDEL
Return

Summarized, the rule can be expressed as follows:

If the control or the initialization of a property in a child instance does not depend on the parent, place the code at the child class level; otherwise, place it at the parent class level.