如何改变输入框中的光栅？

CreateCaret()
showcaret()
destroycaret();
HideCaret();
***********
unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, ExtCtrls;
const
  wm_newcaret=wm_user+1;
type
  TForm1 = class(TForm)
    Edit1: TEdit;
    Edit2: TEdit;
    Image1: TImage;
    procedure Edit1Enter(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    procedure newcaret(var msg:Tmessage);message wm_newcaret;
  end;var
  Form1: TForm1;implementation{$R *.DFM}procedure TForm1.Edit1Enter(Sender: TObject);
begin
  postmessage(handle,wm_newcaret,0,0);
end;procedure TForm1.newcaret(var msg: Tmessage);
begin
  edit1.setfocus;
  CreateCaret(edit1.Handle,image1.Picture.Bitmap.Handle,edit1.width,edit1.height);
  showcaret(edit1.handle);
end;end.
//利用onenter事件，用postmessage发出一个自定义消息
//在自定义消息中,用sendmessage强制setfocus后，再createcaret就可以了
Edit收到焦点时，光标是变了，但一旦我键入字符，光标又变回原样。我想知道如何在编辑文字时，光标一直不变（自定义的）
只要响应application.onactivate事件可以了
unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, ExtCtrls, AppEvnts;
const
  wm_newcaret=wm_user+1;
type
  TForm1 = class(TForm)
    Edit1: TEdit;
    Edit2: TEdit;
    Image1: TImage;
    ApplicationEvents1: TApplicationEvents;
    procedure Edit1Enter(Sender: TObject);
    procedure ApplicationEvents1Activate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    procedure newcaret(var msg:Tmessage);message wm_newcaret;
  end;var
  Form1: TForm1;implementation{$R *.DFM}procedure TForm1.Edit1Enter(Sender: TObject);
begin
  postmessage(handle,wm_newcaret,0,0);
end;procedure TForm1.newcaret(var msg: Tmessage);
begin
  edit1.setfocus;
  CreateCaret(edit1.Handle,image1.Picture.Bitmap.Handle,edit1.width,edit1.height);
  showcaret(edit1.handle);
end;procedure TForm1.ApplicationEvents1Activate(Sender: TObject);
begin
  CreateCaret(edit1.Handle,image1.Picture.Bitmap.Handle,edit1.width,edit1.height);
  showcaret(edit1.handle);
end;end.