fwclienDataset 控件！求助。。

Question

fCDS中的SUM 字段 。   sum:=price*quantity;如何能给SUM 字段的值 赋值成price*quantity相乘的合？  并且显示在fcds中！下面是我的代码-----------------------------form2窗体，负责添加数据。-----------
unit Unit2;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs,  StdCtrls, ExtCtrls,T_fwDataDriver;type
  TForm2 = class(TForm)
    pnl1: TPanel;
    edt1: TEdit;
    edt2: TEdit;
    edt3: TEdit;
    edt4: TEdit;
    btn1: TButton;
    btn2: TButton;
    procedure btn1Click(Sender: TObject);
    procedure btn2Click(Sender: TObject);
  private
    acds : TfwClientDataset;
    { Private declarations }
  public
    class procedure createSelf(fcds : TfwClientDataset);
    { Public declarations }
  end;var
  Form2: TForm2;implementation
uses
  Unit1;{$R *.dfm}procedure TForm2.btn1Click(Sender: TObject);
begin
     with acds do
     begin
       append;
       FieldByName('unit_code').AsString:=edt1.Text;
       FieldByName('parts_code').AsString:=edt2.Text;
       FieldByName('price').AsFloat:=StrToFloat(edt3.Text);
       FieldByName('quantity').AsFloat:=StrToFloat(edt4.Text);
       FieldByName('sum').AsFloat:=StrToFloat(edt3.Text)*StrToFloat(edt4.Text);
       post;
     end;
end;procedure TForm2.btn2Click(Sender: TObject);
begin
  Close;
end;
class procedure TForm2.createSelf(fcds: TfwClientDataset);
var aform : TForm2;
begin
  aform := tform2.create(nil);
  aform.acds := fcds;
  if aform.acds.active = False then aform.acds.open;  aform.ShowModal;
  FreeAndNil(aform);end;
end.
------------------------------------------------------------
-----------------------form1窗体主窗体-----------------如何进行 SUM合的计算？
unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, cxStyles, cxCustomData, cxGraphics, cxFilter, cxData,
  cxDataStorage, cxEdit, DB, cxDBData, StdCtrls, cxGridCustomTableView,
  cxGridTableView, cxGridDBTableView, cxGridLevel, cxClasses, cxControls,
  cxGridCustomView, cxGrid, Grids, DBGrids, T_fwDataDriver, ExtCtrls,
  cxDBLookupComboBox;type
  TForm1 = class(TForm)
    pnl1: TPanel;
    dbgrd1: TDBGrid;
    btn1: TButton;
    btn2: TButton;
    btn3: TButton;
    fcds1: TfwClientDataset;
    strngfldfcds1unit_code: TStringField;
    strngfldfcds1parts_code: TStringField;
    ds1: TDataSource;
    pnl2: TPanel;
    ds2: TDataSource;
    fcds2: TfwClientDataset;
    fcds1price: TFloatField;
    fcds1quantity: TFloatField;
    strngfldfcds2unit_code: TStringField;
    fcds1sum: TFloatField;
    strngfldfcds2parts_code: TStringField;
    strngfldfcds2clienSite: TStringField;
    strngfldfcds2clienName: TStringField;
    cxgrdbtblvwGrid1DBTableView1: TcxGridDBTableView;
    cxgrdlvlGrid1Level1: TcxGridLevel;
    lv1: TcxGrid;
    cxgrdbtblvwGrid1DBTableView1RECORD_ID: TcxGridDBColumn;
    cxgrdbtblvwGrid1DBTableView1unit_code: TcxGridDBColumn;
    cxgrdbtblvwGrid1DBTableView1clienName: TcxGridDBColumn;
    cxgrdbtblvwGrid1DBTableView1clienSite: TcxGridDBColumn;
    cxgrdbtblvwGrid1DBTableView1parts_code: TcxGridDBColumn;
    btn4: TButton;    procedure FormCreate(Sender: TObject);
    procedure btn1Click(Sender: TObject);
    procedure btn2Click(Sender: TObject);
    procedure btn3Click(Sender: TObject);
    procedure cxgrdbtblvwGrid1DBTableView1EditChanged(
      Sender: TcxCustomGridTableView; AItem: TcxCustomGridTableItem);
    procedure btn4Click(Sender: TObject);  private
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementation
uses
  Unit2;
{$R *.dfm}procedure TForm1.FormCreate(Sender: TObject);
begin
  fcds1.LoadFromTextFile('E:\作业\习题5\数据.txt');
  fcds1.FieldByName('sum').AsFloat:=
end;procedure TForm1.btn1Click(Sender: TObject);
var
  Form2:TForm2;
begin  Form2.createSelf(fcds1);
end;procedure TForm1.btn2Click(Sender: TObject);
begin
  if fcds1.RecordCount>0 then
  begin
    fcds1.Delete;
  end;  
end;procedure TForm1.btn3Click(Sender: TObject);
begin
  fcds1.SaveToTextFile('E:\作业\习题5\数据.txt');
end;procedure TForm1.cxgrdbtblvwGrid1DBTableView1EditChanged(
  Sender: TcxCustomGridTableView; AItem: TcxCustomGridTableItem);
begin
  fcds2.Append;
  fcds2.FieldByName('unit_code').AsString:='ss';
  fcds2.FieldByName('clienName').AsString:='sss黑';
  fcds2.FieldByName('clienSite').AsString:='天上';
  fcds2.FieldByName('parts_code').AsString:='code2';
  fcds2.Post;
end;procedure TForm1.btn4Click(Sender: TObject);
begin
  Form1.fcds1.LoadFromTextFile('E:\作业\习题5\数据.txt');
end;end.