Problem with Keyboard hook proc

Question

The background: My form has a TWebBrowser. I want to close the form with ESC but the TWebBrowser eats the keystrokes - so I decided to go with a keyboard hook.

The problem is that the Form can be open in multiple instances at the same time.

No matter what I do, in some situations, if there are two instances open of my form, closing one of them closes the other as well.

I've attached some sample code. Any ideas on what causes the issue?

var
  EmailDetailsForm: TEmailDetailsForm;
  KeyboardHook: HHook;

implementation

function KeyboardHookProc(Code: Integer; wParam, lParam: LongInt): LongInt; stdcall;
var
  hWnd: THandle;
  I: Integer;
  F: TForm;
begin
  if Code < 0 then
    Result := CallNextHookEx(KeyboardHook, Code, wParam, lParam)
  else begin
    case wParam of
      VK_ESCAPE:  
        if (lParam and $80000000) <> $00000000 then
        begin
          hWnd := GetForegroundWindow;
          for I := 0 to Screen.FormCount - 1 do
          begin
            F := Screen.Forms[I];
            if F.Handle = hWnd then
              if F is TEmailDetailsForm then
              begin
                PostMessage(hWnd, WM_CLOSE, 0, 0);
                Result := HC_SKIP;
                break;
              end;
          end; //for
        end; //if
      else
        Result := CallNextHookEx(KeyboardHook, Code, wParam, lParam);
    end;  //case
  end;  //if
end;

function TEmailDetailsForm.CheckInstance: Boolean;
var
  I, J: Integer;
  F: TForm;
begin
  Result := false;

  J := 0;

  for I := 0 to Screen.FormCount - 1 do
  begin
    F := Screen.Forms[I];
    if F is TEmailDetailsForm then
    begin
      J := J + 1;
      if J = 2 then
      begin
        Result := true;
        break;
      end;
    end;
  end;
end;

procedure TEmailDetailsForm.FormCreate(Sender: TObject);
begin
    if not CheckInstance then    
      KeyboardHook := SetWindowsHookEx(WH_KEYBOARD, @KeyboardHookProc, 0, GetCurrentThreadId());
end;

procedure TEmailDetailsForm.FormDestroy(Sender: TObject);
begin
    if not CheckInstance then
      UnHookWindowsHookEx(KeyboardHook);
end;

Answer 1

The background: My form has a TWebBrowser. I want to close the form with ESC but the TWebBrowser eats the keystrokes - so I decided to go with a keyboard hook.

There might be a simpler solution. Have you tried setting the form's KeyPreview property to True?

Answer 2

Well, both forms are signed up to receive the keyboard notice, so they both close. You need to put code in there to decide "is this ESC for me?". Maybe by determining if you're the window with focus or not. If it's not your ESCape, then don't close.

But, this all seems rather drastic. There must be a simpler, non-obtrusive way to detect the ESC within THIS APP, without having to monitor the keyboard for the whole system.

Answer 3

You could do this with TApplicationEvents.OnMessage instead. Drop a TApplicationEvents component on your application's main form with this code:

procedure TMainForm.ApplicationEvents1Message(var Msg: tagMSG;
  var Handled: Boolean);
var
  C: TControl;
  H: HWND;
begin
  if (Msg.message = WM_KEYDOWN) and (Msg.wParam = VK_ESCAPE) then begin
    H := Msg.hwnd;
    while GetParent(H) <> 0 do
      H := GetParent(H);
    C := FindControl(H);
    if C is TEmailDetailsForm then begin
      TEmailDetailsForm(C).Close;
      Handled := True;
    end;
  end;
end;

If you want to keep using a keyboard hook instead, you should only hook it once, rather than once for each form, especially since you're overwriting a global variable. Try adding a HookCount global variable, and only hook/unhook if it's the only form.