IServerState = interface( IInterface )
    ['{C5D0851E-E9CD-422B-8E34-BA2C8E53E729}']
    procedure ServerStatusChanged( _state: Boolean );
  end;

  //TFormMain = class( TForm, IServerState ) -> Funktioniert
  TFormMain = class( TFormBase, IServerState )
    procedure FormCreate( _sender: TObject );
    procedure FormClose( _sender: TObject; var _action: TCloseAction );
    procedure ServerStatusChanged( _state: Boolean );
  end;

{------------------------------------------------ TFormMain }
procedure TFormMain.FormCreate( _sender: TObject );
begin
  Inherited;
  GetSource.RegisterObserver( Self ); //Registriere sich selbst. GetSource ist Singleton
end {FormCreate};

procedure TFormMain.FormClose( _sender: TObject; var _action: TCloseAction );
begin
  GetSource.RemoveObserver( Self );
  inherited;
end;

procedure TFormMain.ServerStatusChanged( _state: Boolean );
begin
  //Wird benachrichtigt und tut was
end;

{------------------------------------------------ TFormBase }
  TFormBase = class( TForm )
    BevelTop: TBevel;
    ImageBackground: TImage;
    ImageHead: TImage;
    LabelTitel: TLabel;
    ImageListEdit: TImageList;
    procedure FormCreate( _sender: TObject );
    procedure FormClose( _sender: TObject; var _action: TCloseAction );
  end;
(*
  In FormCreate und FormClose werden Registryeinträge getätigt. Inherited wird bei beiden aufgerufen
*)

{------------------------------------------------ TSource }
  TSource = class( TInterfacedObject )
  private
    FObservers: TInterfaceList;
  public
    procedure RegisterObserver( _obs: IServerState );
    procedure RemoveObserver( _obs: IServerState );
  end;

procedure TSource.RegisterObserver( _obs: IServerState );
begin
  if not Assigned( FObservers ) then begin
    FObservers:= TInterfaceList.Create;
  end;
  FObservers.Add( _obs );
end {RegisterObserver};

procedure TSource.RemoveObserver( _obs: IServerState );
begin
  if Assigned( FObservers ) then begin
    FObservers.Remove( _obs );
    if FObservers.Count = 0 then begin
      FObservers:= nil;
    end;
  end;
end {RemoveObserver};