Re: [DUG] How do you determine the subclasses of a given class at runtime?

[john rae]

Thankyou Ben for the explanation/example.
That is an interesting piece of code.
A few observations/questions:
1.
f.GetClasses(CallbackClass);
I cannot find information about TClassFinder or TClassFinder.GetClasses 
anywhere.
Where would one find this information? (I assume the return type for the 
function getClasses is TClassList)

Following the code in the debugger, the line above appears to be a pipe-like 
construct that serializes the output from getClasses. Is this a correct 
interpretation?
(coming from a Jade background I would have expected the call syntax to be 
callbackClass(f.GetClasses) with a for loop in callbackClass method to 
iterate the passed collection).

2.
It is perhaps unfortunate that Delphi requires class registration? This 
appears to negate the purpose- addition of new subclasses will require added 
code to register them (for a small number of classes it would be simpler to 
manually add them to TSubclassList).

thanks again, John.


From: Ben Taylor <[EMAIL PROTECTED]>
Reply-To: NZ Borland Developers Group - Delphi List <[EMAIL PROTECTED]>
To: NZ Borland Developers Group - Delphi List <[EMAIL PROTECTED]>
Subject: Re: [DUG] How do you determine the subclasses of a given class at 
runtime?
Date: Fri, 23 Jan 2004 14:19:11 -0800 (PST)

hi,

this should help :-)

  TSubclassList = class(TClassList)
  private
    FParentClass:TPersistentClass;
    procedure CallbackClass(AClass: TPersistentClass);
  public
    procedure Execute(const aClass:TPersistentClass);
  end;
procedure TSubclassList.CallbackClass(AClass: TPersistentClass);
begin
 if AClass.InheritsFrom(FParentClass) and (AClass<>FParentClass) then
  begin
  Self.Add(AClass);
  end;
end;
procedure TSubclassList.Execute(const aClass: TPersistentClass);
var
 f:TClassFinder;
begin
 FParentClass:=aClass;
 f:=TClassFinder.Create(nil);
 try
 f.GetClasses(CallbackClass);
 finally
 FreeAndNil(f);
 end;
end;
-------------------------------------

example use:

  TFruit = class(TPersistent);
  TOrange = class(TFruit);
  TBigOrange = class(TOrange);
  TApple = class(TFruit);
procedure TForm1.FormCreate(Sender: TObject);
var
l:TSubclassList;
i:Integer;
begin
 l:=TSubclassList.Create;
 l.Execute(TFruit);
 for i:=0 to l.Count-1 do
 begin
 ListBox1.Items.Add(l[i].ClassName);
 end;
 freeandnil(l);
end;
initialization
RegisterClass(TFruit);
RegisterClass(TOrange);
RegisterClass(TBigOrange);
RegisterClass(TApple);
cya,
ben
--- john rae <[EMAIL PROTECTED]> wrote:
> Hi. I would like to be able to generate class references to the 
subclasses
> of a given class so that I can populate a list with sub class class 
specific
> information and then upon user selection, use a soft coded factory 
method to
> instantiate the corresponding subclass. This means new subclasses can be
> added and everything will still work...
> Function TObject.parent supports upward navigation, how do I navigate 
down?
>
> thanks, John.

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