Delphi: Proper time to subclass, and restore, a control

delphisubclasssubclassingthemes

What is the correct place/time to start subclassing a control?

What is the proper time to restore the original window proc?

Right now i subclass during form creation:

procedure TForm1.FormCreate(Sender: TObject);
begin
   FOldPanel1WindowProc := Panel1.WindowProc;
   Panel1.WindowProc := Panel1WindowProc;
end;

and i restore the subclassing during form destruction:

procedure TForm1.FormDestroy(Sender: TObject);
begin
   Panel1.WindowProc := FOldPanel1WindowProc;
end;

This works fine, as long as i don't use ThemeManager, which subclasses controls itself. As soon as a try to subclass a control that ThemeManager also sub-classes, i get very nasty errors.

i assume it's becuase i should not start the subclass during Create and restore it during Destroy. So what is the properly documented time to subclass and unsubclass a control in Delphi?

Best Answer

It's not clear from your question but I assume you get the errors when you're trying to restore the old window procedure in the form's OnDestroy event handler.

ThemeManager reverts its subclassing when processing WM_DESTROY for a control. Therefore you probably have to do the same: watch for WM_DESTROY in your new window procedure and revert your subclassing first, then call the old window procedure (and let ThemeManager do the same thing).

I haven't tested this but I think it should work.

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I would manually activate the 'owning' window with an api call just before one of the forms close:

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action := caFree;
  SetForegroundWindow(GetWindowLong(Handle, GWL_HWNDPARENT));
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This won't be a problem with the OS (i.e. no flashing task bar button) because our application is already in the foreground.

If MainFormOnTaskBar is set, the owning window will be our main form, if not it will be the hidden application window. In either case the application will stay in the foreground.

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Best Answer

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