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// Deklaration auch bei TForm1.
// Erst bei public, dann dort, wo alle anderen Event-Handler deklariert werden
// ändert nichts am Fehler
procedure TForm1.WidgetMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
...
end;

SetLength(Buttons, Length(Buttons) + 1);
Buttons[Length(Buttons) - 1] := TButtonWidget.Create(TabSheet6);
with Buttons[Length(Buttons) - 1] do
begin
  Parent := TabSheet6;
  Name := 'wxButton' + IntToStr(Length(Buttons));
  Caption := Name;
  OnMouseDown := WidgetMouseDown;
  OnClick     := WidgetClick;
end;					

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main.pas(234,39) Error: Wrong number of parameters specified for call to "WidgetMouseDown"
main.pas(185,18) Hint: Found declaration: TForm1.WidgetMouseDown(TObject, TMouseButton, TShiftState, LongInt, LongInt);
main.pas(235,35) Error: Wrong number of parameters specified for call to "WidgetClick"
main.pas(148,15) Hint: Found declaration: TForm1.WidgetClick(TObject);					

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  TButtonWidget = class(TToggleBox)
    constructor Create(AOwner: TComponent); override;
  public
    IsSelected: Boolean;
  end;

...

{          TButtonWidget         }
constructor TButtonWidget.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  Caption := 'Button';
  IsSelected := False;
  AllowGrayed := True;
  State := cbGrayed;
  Width := 75;
  Height := 25;
end;

{ aus den StdCtrls: }
{ nur als Referenz: Deklaration von TToggleBox }
  TToggleBox = class(TCustomCheckBox)
  private
  public
    constructor Create(TheOwner: TComponent); override;
  published
    property AllowGrayed;
    property Anchors;
    property AutoSize default false;
    property BorderSpacing;
    property Caption;
    property Checked;
    property DragCursor;
    property DragKind;
    property DragMode;
    property Enabled;
    property Hint;
    property OnChange;
    property OnClick;
    property OnDragDrop;
    property OnDragOver;
    property OnEndDrag;
    property OnEnter;
    property OnExit;
    property OnMouseDown;
    property OnMouseMove;
    property OnMouseUp;
    property OnStartDrag;
    property ParentShowHint;
    property PopupMenu;
    property ShowHint;
    property State;
    property TabOrder;
    property TabStop;
    property Visible;
  end;					

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Buttons[Length(Buttons) - 1].OnMouseDown := PaintBox1.OnMouseDown;