在cxTreeList的右上角加上一个红色小三角

• 结果如图
  
• 方法

1. 需要去重写cxTreeList的重绘事件，也就是OnCustomDrawDataCell事件。

procedure TForm13.lst1CustomDrawDataCell(Sender: TcxCustomTreeList; ACanvas:
    TcxCanvas; AViewInfo: TcxTreeListEditCellViewInfo; var ADone: Boolean);
begin
  AViewInfo.EditViewInfo.OnDrawBackground := OnDrawBackground;
end;

procedure TForm13.OnDrawBackground(Sender: TcxCustomEditViewInfo;
  ACanvas: TcxCanvas; var AHandled: Boolean);
var
  x, y: Integer;
  PointA, PointB, PointC: TPoint;
begin
  ACanvas.Pen.Color := clRed;
  x := Sender.InnerEditRect.Right;
  y := Sender.InnerEditRect.Top;

  PointA := Point(x, y);
  PointB := Point(x - 6, y);
  PointC := Point(x, y + 6);

  ACanvas.Brush.Color := clRed;
  ACanvas.Polygon([PointA, PointB, PointC]);
end;