TWinControl.OnKeyDown Event

Occurs when a user presses any key while the control has focus.

Pascal

property OnKeyDown: TKeyEvent;

C++

__property TKeyEvent OnKeyDown;

Description

Use the OnKeyDown event handler to specify special processing to occur when a key is pressed. The OnKeyDown handler can respond to keyboard keys, including function keys and keys combined with the Shift, Alt, and Ctrl keys, and pressed mouse buttons.

Note: TCheckBox does not fire OnKeyDown events for arrow keys.

The TKeyEvent type points to a method that handles keyboard events.

The Key parameter is the key on the keyboard. For non-alphanumeric keys, use virtual key codes to determine the key pressed. For more information, see Virtual Key codes.

The Shift parameter indicates whether the Shift, Alt, or Ctrl keys are combined with the keystroke.

An application gets Windows WM_KEYDOWN messages for all keys when the user presses a key. These messages indirectly fire the OnKeyDown event. If you set the Key to 0 there you prevent only any further processing of this message. But for keys that generate characters Windows also produces WM_CHAR. At the time your OnKeyDown event fires, the WM_CHAR message for the key will already be in the message queue. Setting Key to 0 does not stop it from being delivered, so it fires the OnKeyPress event. You must set the Key to #0 to prevent the control from intercepting the WM_CHAR message.

This method of organizing key processing has advantages. Code that only deals with characters, including control characters like #13 for carriage return, #3 for Ctrl-C and so on, should go into the OnKeyPress event. Code that deals with keys that do not generate characters should be put into the OnKeyDown event.

C++ Examples:

Copy Code

/*
This example changes a form’s color to aqua when the user
presses the F1 key, even when a control on the form has the
focus. When the user releases the key, the form returns to
its original color.  Set the KeyPreview on the form to True.
Set the OnKeyDown event to FormKeyDown and set the OnKeyUp
event to FormKeyUp.  Set the KeyPreview on the form to True.
Behavior is dependent on keyboard driver(USB, serial, etc).
*/
void __fastcall TForm1::FormKeyDown(TObject *Sender, WORD &Key,
      TShiftState Shift)
{
  if (Key == VK_F1)
  {
    FormColor = Form1->Color;
    Form1->Color = clAqua;
    if (ComboBox1->Text == "KeyDown")
      ComboBox1->Text = "KeyDown2";
    else
      ComboBox1->Text = "KeyDown";
    Form1->Refresh();
    Memo1->Lines->Add(ComboBox1->Text);
  }
}

void __fastcall TForm1::FormKeyUp(TObject *Sender, WORD &Key, TShiftState Shift)
{
  if (Key == VK_F1)
  {
    Form1->Color = FormColor;
    if (ComboBox1->Text == "KeyUp")
      ComboBox1->Text = "KeyUp2";
    else
      ComboBox1->Text = "KeyUp";
    Form1->Refresh();
    Memo1->Lines->Add(ComboBox1->Text);
  }
}

Copy Code

/*
The following code aborts a print job if the user presses
Esc. Note that you should set KeyPreview to true to ensure
that the OnKeyDown event handler of Form1 is called.
*/
#include <Printers.hpp>

void __fastcall TForm1::FormCreate(TObject *Sender)
{
   KeyPreview = True;

}

void __fastcall TForm1::FormKeyDown(TObject *Sender, WORD &Key, TShiftState Shift)

{
  if (Key == VK_ESCAPE && Printer()->Printing)
  {
    Printer()->Abort();
    MessageDlg("Printing aborted", mtInformation,
               TMsgDlgButtons() << mbOK,0);
  }
}

void __fastcall TForm1::Button1Click(TObject *Sender)
{
  TMemo *Memo1 = new TMemo(Form1); // The parent (Form1) will clean up this object.
  Memo1->Parent = Form1;
  Memo1->Visible = True;
  Memo1->Width = 700;
  if (OpenDialog1->Execute())
  {
    Memo1->Lines->LoadFromFile(OpenDialog1->FileName);

    Printer()->BeginDoc();
    int X = 200;
    int Y = 200;
    for (int I = 0; I <= 10; I++)
      if (!Printer()->Aborted)
      {
        Printer()->Canvas->TextOut(X, Y, Memo1->Lines->Strings[I]);
        Y = Y + 80;
        if (Y > (Printer()->PageHeight - 300))
        {
          Y = 200;
          Printer()->NewPage();
          Sleep(1000);  // to give you time to abort!
        }
      }
    if (!Printer()->Aborted) Printer()->EndDoc();
  }
}

Delphi Examples:

Copy Code

{
This example changes a form’s color to aqua when the user
presses the F1 key, even when a control on the form has the
focus. When the user releases the key, the form returns to
its original color.  Set the KeyPreview on the form to True.
Set the OnKeyDown event to FormKeyDown and set the OnKeyUp
event to FormKeyUp.  Set the KeyPreview on the form to True.
Behavior is dependent on keyboard driver(USB, serial, etc).
}
var
  FormColor: TColor;
procedure TForm1.FormCreate(Sender: TObject);
begin
  KeyPreview := True;
  FormColor := Form1.Color;
end;

procedure TForm1.FormKeyDown(
  Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  if Key = VK_F1 then
  begin
    Form1.Color := clAqua;
    if (ComboBox1.Text = 'KeyDown') then
      ComboBox1.Text := 'KeyDown2'
    else
      ComboBox1.Text := 'KeyDown';
    WriteLn(ComboBox1.Text);
  end;
end;

procedure TForm1.FormKeyUp(
  Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  if Key = VK_F1 then
  begin
    Form1.Color := FormColor;
    if (ComboBox1.Text = 'KeyUp') then
      ComboBox1.Text := 'KeyUp2'
    else
      ComboBox1.Text := 'KeyUp';
    WriteLn(ComboBox1.Text);
  end;
end;

Copy Code

{
The following code aborts a print job if the user presses
Esc. Note that you should set KeyPreview to true to ensure
that the OnKeyDown event handler of Form1 is called.
} 

uses Printers;

procedure TForm1.Button1Click(Sender: TObject);
var
  I, X, Y: Integer;
  Memo1 : TMemo;
  r: TRect;
begin
  Memo1 := TMemo.Create(Form1);
  Memo1.Parent := Form1;
  Memo1.Visible := True;
  Memo1.Width := 700;
  if (OpenDialog1.Execute) then
  begin
    Memo1.Lines.LoadFromFile(OpenDialog1.FileName);

    Printer.BeginDoc;
    X := 200;
    Y := 200;
    for I := 0 to 140 do
      if (not Printer.Aborted) then
      begin
        Printer.Canvas.TextOut(X, Y, Memo1.Lines[I]);
        Y := Y + 80;
        if (Y > (Printer.PageHeight - 300)) then
        begin
          Y := 200;
          Printer.NewPage;
          Sleep(1000);  // to give you time to abort!
        end;
      end;
    if (not Printer.Aborted) then Printer.EndDoc;
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
   KeyPreview := True;
end;

procedure TForm1.FormKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
if (Key=VK_ESCAPE) and Printer.Printing then
  begin
  Printer.Abort;
  MessageDlg('Printing aborted', mtInformation, [mbOK],0);
  end;
end;