TCustomForm.KeyPreview Property

Specifies whether the form should receive keyboard events before the active control.

Pascal

property KeyPreview: Boolean;

C++

__property Boolean KeyPreview;

Description

If KeyPreview is true, keyboard events occur on the form before they occur on the active control. (The active control is specified by the ActiveControl property.)

If KeyPreview is false, keyboard events occur only on the active control.

Navigation keys (Tab, BackTab, the arrow keys, and so on) are unaffected by KeyPreview because they do not generate keyboard events. Similarly, when a button has focus or when its Default property is true, the Enter key is unaffected by KeyPreview because it does not generate a keyboard events.

KeyPreview is false by default.

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;