TObject AfterConstruction / Before Destruction callback methods.
lazarus / 64-FPC development

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damieiro · 2021-09-22 10:34

Hi!

I would like that some kind of Method like
- TObject.SetAfterconstructionCallback (MyCallback: Procedure with
self parameter) could be set in the object and be called if not
null/enabled.
Same for Before Destruction (like TObject.SetBeforeDestructionCallback
(MyCallback: Procedure with self parameter) )
And their pairs of disabling it.

Why?
Because for debugging needs it is really interesting to have the
possibility of a Callback here, because All Objects inherits from
TObject in historical code.
The Self information gives a good entry point for debugging routines
and high level routines to show actual system consumption/order of
resource compsuption. And not a fixed photo.

TVirtualMethod interceptors seems weirder https://docwiki.embarcadero.
com/Libraries/Rio/en/System.Rtti.TVirtualMethodInterceptor and not
implemented.
Intercepting the memory manager can be done, but you only get when a
heap memory is needed (and their quantity), not the class info.

How is actually done?
One can derive a Class like TDebugObject=TObject, and then descend
from these, but it's useless for existing code. So the Derive first
paradigm is of no application/use.

Other ways?

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i     program
Project1;    {$mode objfpc}{$H+}    uses classes;         procedure
MyAfterConstruction(self: TObject);    begin     
 writeln('AfterConstruction: ', self.ClassName);    end;         var
o: TObject;      ovmt: PVmt;      p: TPersistent;      l: TList;   
begin      ovmt:=PVmt(TObject);      ovmt^.vAfterConstruction :=
@MyAfterConstruction;      ovmt:=PVmt(TPersistent);     
ovmt^.vAfterConstruction := @MyAfterConstruction;     
ovmt:=PVmt(TList);      ovmt^.vAfterConstruction :=
@MyAfterConstruction;      o := TObject.create;      p :=
TPersistent.Create;      l := TList.Create;    end.              
But this is weird, doesn't work in windows, and bad programming style
(hacking). (well, works if one calls windows to unprotect memory). And
this is not a portable way of doing things.

A method for callback INMHO its the right way OR a controlled (by
method/procedure in FPC) to redirect de AfterConstruction / Before
construction calls...
It could even be enforced with compiler directives if needed (i think
it is desirable)

And for object oriented programming gives a powerful tool for
bookeeping/accounting classes and teaching if all it's not done ok.
As a compiler implementation, i ask devs if this is possible and if
could be done. I really think it's very useful with current TObject
implementation.

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damieiro · 2021-09-23 17:38

To mods: Please, move this to suggestions: I think it's the right
forum. thx in advance.