Accessor Events

Accessor events are automatic and syntactically transparent accessor methods. For example, in VisualFoxPro 6, if a method named property_Assign() exists, this method fires automatically when property is modified. If a method named property_Access() is defined, this method fires automatically when property is read. The property_Assign() and property_Access() methods, if they exist, are responsible for completing the transaction and circularities are preempted by the class internals.

Moreover, if a method named THIS_Access() is defined, this method fires for every property read, every property write, and every method call. This_Access() should return an object instance (usually THIS) that will handle the message. Think of this as a pre-hook for all inbound messages destined to an instance.

An example, in VisualFoxPro 6 code:

  x= CREATE("Foo")
  x.Caption= "this is lowercase"
  ?x.Caption  && echoes "The Caption is: THIS IS LOWERCASE"
  ?x.Tag      && echoes "Instance B"

  DEFINE CLASS Foo AS Label
    Caption= ""
    Tag= "Instance A"

    FUNCTION Caption_Assign( tcPassed)
      *-- Force caption to uppercase
      THIS.Caption= UPPER( tcPassed)

    FUNCTION Caption_Access
       RETURN "The Caption is: " + THIS.Caption

    FUNCTION THIS.Access()
      LOCAL oBar
      oBar= CREATE( "Foo")
      oBar.Tag= "Instance B"
      RETURN oBar

  ENDDEFINE

Is this cool, or what? StevenBlack

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Yes. I believe SatherLanguage (and CecilLanguage and probably others) are similar, and LispLanguage variants (CommonLispObjectSystem) go way back. Another approach is to use a MetaObjectProtocol to change what assignment etc mean for particular objects. -- DaveHarris

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CategoryJargon CategoryEvents