TStrings.BeginUpdate Method

Enables the TStrings object to track when the list of strings is changing.

Pascal

procedure BeginUpdate;

C++

__fastcall BeginUpdate();

Description

BeginUpdate is called automatically by any property or method that changes the list of strings. Once the changes are complete, the property or method calls EndUpdate. Call BeginUpdate before directly modifying the strings in the list, and EndUpdate after. When implementing properties or methods that change the list in descendants of TStrings, call BeginUpdate before the changes are made, and EndUpdate when the changes are complete.

TStrings simply keeps track of when the list of strings is being changed. Some descendants of TStrings use this information to perform certain actions, such as telling a control to repaint, when updates are complete.

C++ Examples:

Copy Code

/*
This example fills the list of a combo box with the set of
open forms when the user drops down the list. The list is
regenerated every time the list is dropped down, so if the
application opens or closes forms, the combo box stays current.
*/
#include "Unit2.h"
#include "Unit3.h"

void __fastcall TForm1::ComboBox1DropDown(TObject *Sender)
{
  ComboBox1->Items->BeginUpdate(); // prevent repaints until done
  ComboBox1->Items->Clear(); // empty the list of any old values
  for (int i = 0; i < Screen->CustomFormCount; i++)
  {
    if ((reinterpret_cast<TForm1 *>(Screen->CustomForms[i]))->Visible)
      ComboBox1->Items->Add(Screen->CustomForms[i]->Caption);  // add form names
  }
  ComboBox1->Items->EndUpdate(); //reenable painting
}

void __fastcall TForm1::FormCreate(TObject *Sender)
{
  ComboBox1->Sorted = True; // make sure the form names are sorted
}

void __fastcall TForm1::Button1Click(TObject *Sender)
{
  Form2->Show();
}

void __fastcall TForm1::Button2Click(TObject *Sender)
{
  Form3->Show();
}

Copy Code

/*
This example requires that a memo and a TApplicationEvent be
added to the form.  Change the Windows Locale by using the
Regional and Language Options in the Windows Control Panel.
*/
void __fastcall TForm1::ApplicationEvents1SettingChange(TObject *Sender, int Flag, const AnsiString Section, int &Result)
{
  Memo1->Lines->BeginUpdate();
  try
  {
//    char buffer[64];
    AnsiString buffer;
    buffer.sprintf("Section = %s", Section.c_str());
    Memo1->Lines->Add(buffer);
    buffer.sprintf("Flag = %8x", Flag);
    Memo1->Lines->Add(buffer);
    if (strcmp(Section.c_str(), "intl") == 0)
    {
      buffer.sprintf("DefaultLCID = %8x", SysLocale.DefaultLCID);
      Memo1->Lines->Add(buffer);
      buffer.sprintf("PriLangID = %8x", SysLocale.PriLangID);
      Memo1->Lines->Add(buffer);
      buffer.sprintf("SubLangID = %8x", SysLocale.SubLangID);
      Memo1->Lines->Add(buffer);
      buffer.sprintf("FarEast = %s", BoolToStr(SysLocale.FarEast, true));
      Memo1->Lines->Add(buffer);
      buffer.sprintf("MiddleEast = %s", BoolToStr(SysLocale.MiddleEast, true));
      Memo1->Lines->Add(buffer);
      Memo1->Lines->Add("");
    }
  }
  __finally
  {
    Memo1->Lines->EndUpdate();
  }
}

Delphi Examples:

Copy Code

{
This example fills the list of a combo box with the set of
open forms when the user drops down the list. The list is
regenerated every time the list is dropped down, so if the
application opens or closes forms, the combo box stays current.
}
procedure TForm1.ComboBox1DropDown(Sender: TObject);
var
  I: Integer;
begin
  with ComboBox1 do
  begin
    Items.BeginUpdate; { prevent repaints until done }
    Items.Clear; { empty the list of any old values }
    for I := 0 to Screen.CustomFormCount - 1 do
    begin
      if (Screen.CustomForms[I].Visible) then
        Items.Add(Screen.CustomForms[I].Caption); { add form name }
    end;
    Items.EndUpdate; {reenable painting }
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  ComboBox1.Sorted := True; { make sure the form names are sorted }
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Form2.Show;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  Form3.Show;
end;

Copy Code

{
This example requires that a memo and a TApplicationEvent be
added to the form.  Change the Windows Locale by using the
Regional and Language Options in the Windows Control Panel.
}
procedure TForm1.ApplicationEvents1SettingChange(Sender: TObject; Flag: Integer; const Section: string; var Result: Integer);
begin
  Memo1.Lines.BeginUpdate;
  try
    Memo1.Lines.Add(Format('Section  = %s', [Section]));
    Memo1.Lines.Add(Format('Flags  = %.8x', [Flag]));
    if AnsiSameStr(Section, 'intl') then
    with SysLocale do
    begin
      Memo1.Lines.Add(Format('DefaultLCID  = %.8x', [DefaultLCID]));
      Memo1.Lines.Add(Format('PriLangID  = %.8x', [PriLangID]));
      Memo1.Lines.Add(Format('SubLangID  = %.8x', [SubLangID]));
      Memo1.Lines.Add(Format('FarEast  = %s', [BoolToStr(FarEast, True)]));
      Memo1.Lines.Add(Format('MiddleEast  = %s', [BoolToStr(MiddleEast, True)]));
    end;
    Memo1.Lines.Add('');
  finally
    Memo1.Lines.EndUpdate;
  end;
//  Result:= 0;
end;