Key press getting lost in menu loop

Question

I want to build a menu form that acts similar to ribbon keytips - you can

press and hold Alt, then press and release e. g. d, then release Alt to trigger an action or
press and release Alt, then press and release d to trigger the same action

I took inspiration at Hidden Main Menu in a delphi program automatically shown using Alt key and came up with the following demo:

unit Unit1;

interface

uses
  Windows,
  Messages,
  SysUtils,
  Variants,
  Classes,
  Graphics,
  Controls,
  Forms,
  Dialogs,
  ImgList,
  StdCtrls;

type
  TForm1 = class(TForm)
    Label1: TLabel;
    procedure FormKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
  strict private
    FShowKeyTips: Boolean;

    procedure UpdateKeyTipState(AShowKeyTips: Boolean);
    procedure WMExitMenuLoop(var Message: TMessage); message WM_EXITMENULOOP;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

uses
  ShellAPI,
  Menus;

{ TForm1 }

constructor TForm1.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  Label1.Caption := 'Dummy';
end;

destructor TForm1.Destroy;
begin
  inherited Destroy;
end;

procedure TForm1.WMExitMenuLoop(var Message: TMessage);
begin
  UpdateKeyTipState(False);
end;

procedure TForm1.FormKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
const
  MAPVK_VK_TO_CHAR = 2;

  // Adapted from dxBar.pas:
  function IsTextCharForKeyTip(AKey: Word): Boolean;
  var
    ARes: UINT;
  begin
    ARes := MapVirtualKey(AKey, MAPVK_VK_TO_CHAR);
    Result := ((ARes and $FFFF0000) = 0) and (Char(ARes) <> ' ') and (Char(ARes) in [#32..#255]);
  end;

var
  hk: string;
  CheckKeyTips: Boolean;
begin
  if (Key = VK_MENU) or (Key = VK_F10) then
  begin
    UpdateKeyTipState(True);
    Exit;
  end;

  if FShowKeyTips then
    CheckKeyTips := True
  else
    CheckKeyTips := Shift = [ssAlt];
  if CheckKeyTips and IsTextCharForKeyTip(Key) then
  begin
    hk := Char(Key); // TODO: Handle analogouos to TdxBarItemLink.IsAccel?
    if SameText(hk, 'd') then
    begin
      Caption := Caption + '+';
      Key := 0;
      Exit;
    end;
  end;
end;

procedure TForm1.UpdateKeyTipState(AShowKeyTips: Boolean);
begin
  if FShowKeyTips = AShowKeyTips then
    Exit;

  FShowKeyTips := AShowKeyTips;
  if AShowKeyTips then
    Label1.Caption := 'Dummy (d)'
  else
    Label1.Caption := 'Dummy';
end;

end.

(Create a standard VCL app, add Label1 to Form1 and replace the contents of Unit1.pas with the above.)

The first bullet point works (adds a + to the form caption), however I can't make the second one work. I can't find where the d gets handled. I tried WM_(SYS)KEYDOWN, CM_DIALOGCHAR and more to no avail.

Any ideas?

For future consideration, instead of trying random handlers it's easier to override wndproc and dump messages and parameters. — Sertac Akyuz, Mar 23 '18 at 14:51

Sertac Akyuz · Accepted Answer · 2018-03-23T17:23:35.633

As documented the Alt key, when pressed and released alone, "toggles in and out of menu bar mode". This is true even if your form does not have a window menu, the system menu is sufficient for the system to put the window into a modal menu loop. In this mode a non-accelerator will generate a WM_MENUCHAR message:

Sent when a menu is active and the user presses a key that does not correspond to any mnemonic or accelerator key.

This is the message that you're looking for, read the character from the User field. And you don't have to track the Alt key, since the window being in a modal menu loop means the Alt key has been pressed once. Otherwise a key down message is generated instead of a menu character message.

Note that if your form does not have a system menu (in BorderIcons uncheck biSystemMenu) and a window menu, a regular WM_KEYDOWN will be sent which you're already handling.

Key press getting lost in menu loop

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