I want to create or change an event on a form, dynamically, how can i do that?

Question

i'm trying to write a taskbar for my program, and i need to add one line of code in 2 events, OnClose and OnActivate, but my program has over 100 forms, so i'd like to do it dynamically. Is there an way to do it?

The language is Delphi 7.

Answer 1

As TLama and David Hefferman say, inheritance is right way to do it, but sometimes pragmatism wins out over the right way. So here is a horrible Kludge that will do the job.

unit Unit1;

interface

uses
  VCL.Forms,
  System.Classes,
  System.Generics.Collections;

type
  TKludge = class
  private
    fForm: TForm;
    fOnClose: TCloseEvent;
    fOnActivate: TNotifyEvent;
    procedure fNewOnActivate( Sender : TObject );
    procedure fNewOnClose( Sender : TObject; var Action : TCloseAction );
  public
    property Form : TForm
             read fForm;
    property OnClose : TCloseEvent
             read fOnClose;
    property OnActivate : TNotifyEvent
             read fOnActivate;
    constructor Create( pForm : TForm );

  end;

  TKludges = class( TObjectList<TKludge> )
  private
    fApplication: TApplication;
    procedure SetApplication(const Value: TApplication);
  public
    property Application : TApplication
             read fApplication
             write SetApplication;
  end;


implementation

{ TKludge }

constructor TKludge.Create(pForm: TForm);
begin
  fForm := pForm;
  fOnClose := pForm.OnClose;
  pForm.OnClose := fNewOnClose;
  fOnActivate := pForm.OnActivate;
  pForm.OnActivate := fOnActivate;
end;

procedure TKludge.fNewOnActivate(Sender: TObject);
begin
  if assigned( fOnActivate ) then fOnActivate( Sender );
  // my extra line
end;

procedure TKludge.fNewOnClose(Sender: TObject; var Action: TCloseAction);
begin
  if assigned fOnClose then fOnClose( Sender, Action );
  // my extra line
end;

{ TKludges }

procedure TKludges.SetApplication(const Value: TApplication);
var
  i: Integer;
begin
  fApplication := Value;
  for i := 0 to fApplication.ComponentCount do
  begin
    if fApplication.Components[ i ] is TForm then
    begin
      Add( TKludge.Create( fApplication.Components[ i ] as TForm ));
    end;
  end;
end;

end.

Create an instance of the TKludges class and pass the Application to it. For every form that it finds it will replace the events with new ones that call the original if it exists and put your extra lines in (just comments at the minute).

What makes it particularly horrible is the EVERY form will be affected, including ones you might not expect. But if you are sure...

Use at your own risk!