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procedure TForm1.GridMouseMove(Sender:TObject; X,Y :integer);
begin
  lblTest.Caption := Format('X: %d, Y: %d',[X,Y]);
end;					

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procedure TMeinObjekt.MeinMouseMove(Sender:TObject; X,Y : integer);
begin
  self.X := X;
  self.Y := Y;
end;					

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procedure TMeinObjekt.AddGrid(aGrid: TGrid);
begin

  aGrid.OnMouseMove := self.MeinMouseMove;
end;					

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  var
    a: TNotifyEvent;

procedure TForm1.Button1Click(Sender: TObject);
begin
  showmessage('Hallo wie gehts');
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  if assigned(a) then a(sender);
  Showmessage('Wat wer bist du denn');
end;

procedure TForm1.Button3Click(Sender: TObject);
begin
  a := Button1.onClick;
  button1.OnClick := button2click;
end;					

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type
  TMyEvent = procedure (Sender:TObject; Shift:TShiftState; X, Y : integer) of object;
  ...

  TMyObject = class(TObject)
  protected
    FMyOnMouseMove : TMyEvent;
   ...
   ...
  
procedure TMyObject.AddGrid(aGrid:TGrid);
begin
  self.FMyOnMouseMove := aGrid.OnMouseMove;
  aGrid.OnMouseMove := self.GridMouseMove;
end;

procedure TMyObject.GridMouseMove(...);
begin
  if Assigned(self.FMyOnMouseMove) then self.FMyOnMouseMove(...);
end;