Save and restore event handlers

Question

My class contains dataset (TDataSet). Users of my class can assign event handlers for this dataset:

ds.FieldByName('ID').OnChange := @ID_OnChange;

Then I have to reopen dataset:

ds.Close;
ds.Open;

After this all event handlers are gone:

if Assigned(ds.FieldByName('ID').OnChange) //returns false

So, I need to save handlers and restore them after reopen. I use TStringList for it:

var
  EventHandlers: TStringList;
...
  //I do this for every event of every field
  if Assigned(ds.FieldByName('ID').OnChange) then
    EventHandlers.AddObject('ID', @ds.FieldByName('ID').OnChange);

The problem is how to restore handlers:

ds.FieldByName('ID').OnChange := TFieldNotifyEvent(ObjToInt(EventHandlers.Objects[0]));//Invalid typecast error

How can I assign stored address to event handler? Thanks.

Answer 1

If you really want to save the events, you can use TMethod Record:

unit Unit6;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm6 = class(TForm)
    btn1: TButton;
    btn2: TButton;
    procedure btn1Click(Sender: TObject);
    procedure btn2Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    MTD : TMethod;
  end;

var
  Form6: TForm6;

implementation

{$R *.dfm}

procedure TForm6.btn1Click(Sender: TObject);
begin
  ShowMessage('Hello World!');
end;

procedure TForm6.btn2Click(Sender: TObject);
begin
  ShowMessage('I am copy cat!');
  MTD := TMethod(btn1.OnClick);
  btn2.OnClick := TNotifyEvent(MTD);
end;

end.

The First Click on Btn2 will show "I am copy cat!" and The 2nd one will show Hello World.

Edit : Improve assign event to MTD(TMethod). More simple and allow events from other objects.