Giovanni
The 'correct' way would be to implement an interface
1/ Declare the interface
type
IPresentation = interface(IInterface)
procedure SetColor(Value: TColor);
end
2/ Implement the IF on your classes
type
TColorableFrom = class(TForm, IPresentation)
you then have the option of implementuing the interface or delegating it
to an 'helper' object
then to use it
var
PesI: IPresentation;
if Supports(Instance,IPresentation, PresI) then Pesi.SetColor(Color)
HTH
Neven
----- Original Message -----
From: "Moretti, Giovanni" <[EMAIL PROTECTED]>
To: "Multiple recipients of list delphi" <[EMAIL PROTECTED]>
Sent: Wednesday, October 23, 2002 6:23 PM
Subject: [DUG]: Polymorphic assignment to Color - How? (sometime it's a
Property, sometimes not)
> Hi
>
> I'm building a list of objects that can be coloured and want to be able to
get/set their colour.
>
> The object properties are all going to be stored in a Tlist so the types
will probably need to be stored along with them. I'm happy to store the
Object type (eg Tform ...) along with the reference to the object but I
can't see a general way to do this and get/set the color.
>
> ========= ATTEMPT #1 ===========================================
>
> In the olden days, before properties, I'd have just passed a pointer to
the Tcolor value:
>
> procedure setObjectColour (Color : ^TColor ;
> newColour : TColor);
> begin
> Color^ := newColour;
> end;
>
> setObjectColour(@Label3.Color, clRed);
>
> I tried this, but it's unpredictable as sometimes an object's color is
just a TColor (a glorified integer) but other times it's a property with
get/set methods.
>
> ========= ATTEMPT #2 ===========================================
>
> Figuring the run-time type information should be able to handle this, I
tried:
>
> Type TControlClass = Class of Tcontrol;
>
> procedure setObjectColour ( obj : TObject;
> whatClass : TControlClass;
> newColour : TColor);
> begin
> (obj as whatClass).color := newColour;
> ^-------------- ERROR - COLOR not declared
> // OR
>
> whatClass(object).color := newColour;
> ^----------------------------Missing Operator or ;
> end;
>
> setObjectColour(Label3, Tlabel, clRed); // To Recolour something
>
> But neither will compile without errors.
>
> ========= FINALLY ===========================================
>
> It's not elegant but has the great virtue that it actually works:
>
> procedure setObjectColour ( obj : TObject;
> newColour : TColor);
> begin
> if (obj is TForm) then (obj as TForm) .color := newColour
> else if (obj is TLabel) then (obj as TLabel).color := newColour
> else if (obj is TFont ) then (obj as TFont) .color := newColour
> else if (obj is TBrush) then (obj as TBrush).color := newColour
> else if (obj is TPen) then (obj as TPen) .color := newColour
> else showMessage('setObjectColour: Don''t know how');
> end;
>
> setObjectColour(Form1, clRed);
>
> Except this means that adding new types of things will require adding of
extra "if .." statements which with polymorphism should be necessary.
Unfortunately you can't do the more general:
>
> (obj as Tcontrol).color := newColour;
>
> As Tcontrol's "color" is protected (and can't be seen this won't compile).
Only Tcontrol's descendents expose some way of getting/setting color.
>
> I want to be able to recolour anything that has a colour but there must be
a better way than tests for explicit types/classes ...
>
> Any Ideas?
>
> Thanks
> Giovanni
> ========================================================================
> Giovanni Moretti | Institute of Information Sciences and Technology
> Senior Lecturer | Massey University, Palmerston North, New Zealand
> Computer Science | Ph 64-6-3505799x2474 == Fax 64-6-3502259 == ZL2BOI
> ------------------------------------------------------------------------
> http://www-ist.massey.ac.nz/moretti mailto:G.Moretti@;massey.ac.nz
> --------------------------------------------------------------------------
-
> New Zealand Delphi Users group - Delphi List - [EMAIL PROTECTED]
> Website: http://www.delphi.org.nz
> To UnSub, send email to: [EMAIL PROTECTED]
> with body of "unsubscribe delphi"
> Web Archive at: http://www.mail-archive.com/delphi%40delphi.org.nz/
>
---------------------------------------------------------------------------
New Zealand Delphi Users group - Delphi List - [EMAIL PROTECTED]
Website: http://www.delphi.org.nz
To UnSub, send email to: [EMAIL PROTECTED]
with body of "unsubscribe delphi"
Web Archive at: http://www.mail-archive.com/delphi%40delphi.org.nz/
