TTabControl.OnChange Event

Occurs after a new tab is selected.

Pascal

property OnChange: TNotifyEvent;

C++

__property TNotifyEvent OnChange;

Description

Write an OnChange event handler to take specific action immediately after the selected tab changes. Use the TabIndex property to determine which tab is now selected. This is the opportunity to make any changes to the control that reflect the new state implied by the selected tag.

Before the value of TabIndex changes, an OnChanging event occurs.

C++ Examples:

Copy Code

/*
This project requires a Page Control.  You will need to
populate the Page Control by right clicking and selecting
New Page from the context menu. As the tabs are selected by
the user, the Page Control OnChange event fires, which
causes the form caption to display the captions of each of
the TTabSheets attached to the Page Control.
Note the use of the Form’s OnShow event handler to set the
caption when the form first appears.  This occurs before
any changes trigger the OnChange event of the Page Control.
*/
void __fastcall TForm1::PageControl1Change(TObject *Sender)
{
  Caption = AnsiString("Now working on tab: " + PageControl1->ActivePage->Caption);
}

void __fastcall TForm1::FormShow(TObject *Sender)
{
  PageControl1Change(Sender);
}

Copy Code

/*
This example uses a tab control to display the contents of
several files. To run the example, place a tab control on a
form and add a memo control that fits into its client area.
Be sure to leave enough room for the tabs when they appear.
Then add an OpenDialog and a button to the form.  The
"Add a file" button adds a single file to the tabcontrol.
The "Assign files" button removes previous files and can be
used to assign multiple files.  To assign multiple files,
use CNTL Select or SHIFT Select to select files in the
OpenDialog.
*/
void __fastcall TForm1::Add_a_fileClick(TObject *Sender)
{
  OpenDialog1->Options << ofAllowMultiSelect << ofFileMustExist << ofHideReadOnly;
  if (OpenDialog1->Execute())
  {
    int index = TabControl1->Tabs->Add(OpenDialog1->FileName);
    Memo1->Lines->LoadFromFile(TabControl1->Tabs->Strings[index]);
    TabControl1Change(Sender);
  }
}

void __fastcall TForm1::Assign_filesClick(TObject *Sender)
{
  OpenDialog1->Options << ofAllowMultiSelect << ofFileMustExist << ofHideReadOnly;
  if (OpenDialog1->Execute())
  {
    TabControl1->Tabs->Assign(OpenDialog1->Files);
    Memo1->Lines->LoadFromFile(TabControl1->Tabs->Strings[TabControl1->TabIndex]);
  }
}
/*
Place the following code in the tab control’s OnChange event
handler:
*/
void __fastcall TForm1::TabControl1Change(TObject *Sender)
{
    Memo1->Lines->LoadFromFile(
      TabControl1->Tabs->Strings[TabControl1->TabIndex]);
}

Copy Code

/*
This example requires a new TPageControl, with no new pages
created at design time.  The form OnCreate event handler
adds several new TabSheet controls to the Page Control.  The
Page Control’s OnChange event handler displays a message
dialog when the user changes tabs.  The message dialog
contains the captions for the tabs immediately before and
after the active tab.
*/
void __fastcall TForm1::FormCreate(TObject *Sender)
{
  for (int i = 0; i < 10; i++)
  {
    TTabSheet *pPage = new TTabSheet(PageControl1); // These tabsheets will be cleaned up by their owner (PageControl1).
    pPage->PageControl = PageControl1;
    pPage->Caption = AnsiString("Page") + IntToStr(i);
    pPage->Name = AnsiString("ts") + pPage->Caption;
  }
}

void __fastcall TForm1::PageControl1Change(TObject *Sender)
{
  AnsiString PrevCaption, NextCaption;
  TPageControl *pPC = dynamic_cast<TPageControl *>(Sender);
  PrevCaption = pPC->FindNextPage(pPC->ActivePage, false, false)->Caption;
  NextCaption = pPC->FindNextPage(pPC->ActivePage, true, false)->Caption;
  ShowMessage(AnsiString("Previous tab caption: '") + PrevCaption +
    AnsiString("'  Next tab Caption: '") + NextCaption + AnsiString("'"));
}

Delphi Examples:

Copy Code

{
This project requires a Page Control.  You will need to
populate the Page Control by right clicking and selecting
New Page from the context menu. As the tabs are selected by
the user, the Page Control OnChange event fires, which
causes the form caption to display the captions of each of
the TTabSheets attached to the Page Control.
Note the use of the Form’s OnShow event handler to set the
caption when the form first appears.  This occurs before
any changes trigger the OnChange event of the Page Control.
} 
procedure TForm1.PageControl1Change(Sender: TObject);
begin
  Caption := ' Now working on tab:  ' + PageControl1.ActivePage.Caption;
end;

procedure TForm1.FormShow(Sender: TObject);
begin
  PageControl1Change(Sender);
end;

Copy Code

{
This example uses a tab control to display the contents of
several files. To run the example, place a tab control on a
form and add a memo control that fits into its client area.
Be sure to leave enough room for the tabs when they appear.
Then add an OpenDialog and a button to the form.  The
"Add a file" button adds a single file to the tabcontrol.
The "Assign files" button removes previous files and can be
used to assign multiple files.  To assign multiple files,
use CNTL Select or SHIFT Select to select files in the
OpenDialog.
}
procedure TForm1.Add_a_fileClick(Sender: TObject);
var index : integer;
begin
  OpenDialog1.Options :=
    [ofAllowMultiSelect, ofFileMustExist, ofHideReadOnly];
  if OpenDialog1.Execute then
  begin
    index:= TabControl1.Tabs.Add(OpenDialog1.FileName);
    Memo1.Lines.LoadFromFile(TabControl1.Tabs[index]);
    TabControl1Change(Sender);
  end;
end;

procedure TForm1.Assign_filesClick(Sender: TObject);
var index : integer;
begin
  OpenDialog1.Options :=
    [ofAllowMultiSelect, ofFileMustExist, ofHideReadOnly];
  if OpenDialog1.Execute then
  begin
    TabControl1.Tabs.Assign(OpenDialog1.Files);
    Memo1.Lines.LoadFromFile(
      TabControl1.Tabs[TabControl1.TabIndex]);
  end;
end;

{
Place the following code in the tab control’s OnChange event
handler:
}
procedure TForm1.TabControl1Change(Sender: TObject);
begin
  with TabControl1 do
    Memo1.Lines.LoadFromFile(Tabs[TabIndex]);
end;

Copy Code

{
This example requires a new TPageControl, with no new pages
created at design time.  The form OnCreate event handler
adds several new TabSheet controls to the Page Control.  The
Page Control’s OnChange event handler displays a message
dialog when the user changes tabs.  The message dialog
contains the captions for the tabs immediately before and
after the active tab.
}
procedure TForm1.FormCreate(Sender: TObject);
var
  i: Integer;
begin
  for i := 0 to 9 do
    with TTabSheet.Create(Self) do
    begin
      PageControl := PageControl1;
      Caption := 'TabSheet #' + IntToStr(i);
    end;
end;

procedure TForm1.PageControl1Change(Sender: TObject);
var
  PrevCaption, NextCaption: ShortString;
begin
  with (Sender as TPageControl) do
  begin
    PrevCaption :=
      FindNextPage(ActivePage, False, False).Caption;
    NextCaption :=
      FindNextPage(ActivePage, True, False).Caption;
  end;
  ShowMessage('Previous tab caption: "' + PrevCaption +
    '"    Next tab Caption: "' + NextCaption + '"');
end;