TComboBox.OnChange Event

Occurs when the user changes the text displayed in the edit region.

Pascal

property OnChange: TNotifyEvent;

C++

__property TNotifyEvent OnChange;

Description

Write an OnChange event handler to take specific action immediately after the user edits the text in the edit region or selects an item from the list. The Text property gives the new value in the edit region.

Note: OnChange only occurs in response to user actions. Changing the Text property programmatically does not trigger an OnChange event.

C++ Examples:

Copy Code

/*
This example requires a TPageControl already on the form.
Also, you must add pages to the TPageControl by right
clicking and selecting New Page.
The example code will allow the user to select the
ActivePage property through selection of a ComboBox item.
During the form create, the Combo Box control is loaded with
the names of each of the tabs, as well as the instance
pointers to the corresponding tab.  When the user selects
the Combo Box item, the associated TTabSheet object
contained in the Combo Box Objects array is used to set the
ActivePage property.  Select the Lines property of the 
TRichEdit to enter a string.  Select the string before 
justifying.
*/
void __fastcall TForm1::ComboBox1Change(TObject *Sender)
{
  TComboBox *pCB = dynamic_cast<TComboBox *>(Sender);
  if (pCB)
    PageControl1->ActivePage = dynamic_cast<TTabSheet *>(pCB->Items->Objects[pCB->ItemIndex]);
}

void __fastcall TForm1::FormCreate(TObject *Sender)
{
  const TColor colorPalette[12] = {
    clRed, clGreen, clYellow, clBlue, clWhite, clFuchsia,
    clTeal, clNavy, clMaroon, clLime, clOlive, clPurple};
  for (int i = 0; i < PageControl1->PageCount; i++)
  {
    PageControl1->Pages[i]->Brush->Color = colorPalette[i];
    ComboBox1->Items->AddObject(PageControl1->Pages[i]->Name, PageControl1->Pages[i]);
  }
  ComboBox1->ItemIndex = 0;
  PageControl1->ActivePage = dynamic_cast<TTabSheet *>(ComboBox1->Items->Objects[ComboBox1->ItemIndex]);
}

Copy Code

/*
The following example assumes that a drive combo box, a file
list box, and a directory list box are on a form. Add this
code as the OnChange event handler for the drive combo box
and the OnChange event handler for the directory list box.
When the user changes the drive using the combo box, the
directory list box and file list box will update to reflect
the new drive and the current directory on that drive.  When
the user double clicks on a directory in the file list box
will update to reflect the new directory.
*/
void __fastcall TForm1::DriveComboBox1Change(TObject *Sender)
{
  DirectoryListBox1->Drive = DriveComboBox1->Drive;
  FileListBox1->Drive = DriveComboBox1->Drive;
  FileListBox1->Directory = DirectoryListBox1->Directory;
}

void __fastcall TForm1::DirectoryListBox1Change(TObject *Sender)
{
  FileListBox1->Directory = DirectoryListBox1->Directory;
}

Delphi Examples:

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{
This example requires a TPageControl already on the form.
Also, you must add pages to the TPageControl by right
clicking and selecting New Page.
The example code will allow the user to select the
ActivePage property through selection of a ComboBox item.
During the form create, the Combo Box control is loaded with
the names of each of the tabs, as well as the instance
pointers to the corresponding tab.  When the user selects
the Combo Box item, the associated TTabSheet object
contained in the Combo Box Objects array is used to set the
ActivePage property.  Select the Lines property of the 
TRichEdit to enter a string.  Select the string before 
justifying.
} 
procedure TForm1.FormCreate(Sender: TObject);
var
  i: Integer;
const
  colorarray : Array[0..4] of TColor = (
    clYellow, clGreen, clBlue, clLime, clFuchsia);
begin
  for i := 0 to PageControl1.PageCount - 1 do
    begin
    PageControl1.Pages[i].Brush.Color := colorarray[i];
    ComboBox1.Items.AddObject(PageControl1.Pages[i].Name,
      PageControl1.Pages[i]);
    end;
  ComboBox1.ItemIndex := 0;
  PageControl1.ActivePage := TTabSheet(ComboBox1.Items.Objects[ComboBox1.ItemIndex]);
end;

procedure TForm1.ComboBox1Change(Sender: TObject);
begin
  if (Sender is TComboBox) then
    with (Sender as TComboBox) do
      PageControl1.ActivePage := TTabSheet(Items.Objects[ItemIndex]);
end;

Copy Code

{
The following example assumes that a drive combo box, a file
list box, and a directory list box are on a form. Add this
code as the OnChange event handler for the drive combo box
and the OnChange event handler for the directory list box.
When the user changes the drive using the combo box, the
directory list box and file list box will update to reflect
the new drive and the current directory on that drive.  When
the user double clicks on a directory in the file list box
will update to reflect the new directory.
}
procedure TForm1.DirectoryListBox1Change(Sender: TObject);
begin
  FileListBox1.Directory := DirectoryListBox1.Directory;
end;

procedure TForm1.DriveComboBox1Change(Sender: TObject);
begin
  DirectoryListBox1.Drive := DriveComboBox1.Drive;
  FileListBox1.Drive := DriveComboBox1.Drive;
  FileListBox1.Directory := DirectoryListBox1.Directory;
end;