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TTastenfeld = class(TCustomPanel)
    private
      ButtonList: TList;
      FOnLeftButtonClick: TLeftButtonClickEvent;
      FOnRightButtonClick: TRightButtonClickEvent;
      procedure LeftButtonClick(Sender: TObject);
      procedure RightButtonClick(Sender: TObject);
    published
      property OnLeftButtonClick: TLeftButtonClickEvent read FOnLeftButtonClick write FOnLeftButtonClick;
      property OnRightButtonClick: TRightButtonClickEvent read FOnRightButtonClick write FOnRightButtonClick;
  end;

implementation

constructor TTastenfeld.Create(AOwner: TComponent);
var
  vButton: TTastenfeldButton;
  i: Integer;
begin
  inherited;
  ButtonList := TList.Create;

  for i := ord(Low(TButtonPos)) to ord(High(TButtonPos)) do
  begin
    vButton := TTastenfeldButton.Create(self); 
    vButton.Caption := GetInitialButtonCaption(TButtonPos(i));
    if vButton.Caption = CaptionLeft then
      vButton.OnClick := OnLeftButtonClick
    else
    if vButton.Caption = CaptionRight then
      vButton.OnClick := OnRightButtonClick
    end;
    ButtonList.Add(vButton);
  end;
end;

procedure TTastenfeld.LeftButtonClick(Sender: TObject);
begin
  inherited OnClick(Sender);
  if assigned(OnLeftButtonClick) then
    OnLeftButtonClick(Sender);
end;

procedure TTastenfeld.RightButtonClick(Sender: TObject);
begin
  inherited OnClick(Sender);
  if assigned(OnRightButtonClick) then
    OnRightButtonClick(Sender);
end;					

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type
  TMyPanel = class(TCustomPanel)
  private
    FOnButtonLeftClick: TNotifyEvent;
  protected
    procedure HandleButtonLeftClick(Sender: TObject);
  public
    constructor Create(AOwner: TComponent); override;
  published
    property ButtonLeftClick: TNotifyEvent read FOnButtonLeftClick write FOnButtonLeftClick;   

...

constructor TMyPanel.Create(AOwner: TComponent); 
begin
  inherited Create(AOwner);

  ButtonLeft.OnClick := HandleButtonLeftClick;
end;

procedure TMyPanel.HandleButtonLeftClick(Sender: TObject);
begin

  // evtl. noch was machen

  if Assigned(FOnButtonLeftClick) then
    OnButtonLeftClick(Sender);

end;