How to detect modifier key change in a control which doesn't have focus?

Question

Background:

I\'m working on a control derived from TCustomControl class which can get focus and which has some inner elements inside. Those inner elem

Answer 1

I can't tell if it would be less overkill than using a hook, but one option would be to use "raw input". If you register your control accordingly, it will receive input also when it's not active. Sample implementation to decide..:

type
  TMyCustomControl = class(TCustomControl)
    ..
  protected
    ..
    procedure CreateWindowHandle(const Params: TCreateParams); override;
    procedure WMInput(var Message: TMessage); message WM_INPUT;
  ..
  end;

uses
  types;

type
  tagRAWINPUTDEVICE = record
    usUsagePage: USHORT;
    usUsage: USHORT;
    dwFlags: DWORD;
    hwndTarget: HWND;
  end;
  RAWINPUTDEVICE = tagRAWINPUTDEVICE;
  TRawInputDevice = RAWINPUTDEVICE;
  PRawInputDevice = ^TRawInputDevice;
  LPRAWINPUTDEVICE = PRawInputDevice;
  PCRAWINPUTDEVICE = PRawInputDevice;

function RegisterRawInputDevices(
  pRawInputDevices: PCRAWINPUTDEVICE;
  uiNumDevices: UINT;
  cbSize: UINT): BOOL; stdcall; external user32;

const
  GenericDesktopControls: USHORT = 01;
  Keyboard: USHORT = 06;
  RIDEV_INPUTSINK = $00000100;

procedure TMyCustomControl.CreateWindowHandle(const Params: TCreateParams);
var
  RID: TRawInputDevice;
begin
  inherited;

  RID.usUsagePage := GenericDesktopControls;
  RID.usUsage := Keyboard;
  RID.dwFlags := RIDEV_INPUTSINK;
  RID.hwndTarget := Handle;
  Win32Check(RegisterRawInputDevices(@RID, 1, SizeOf(RID)));
end;

type
  HRAWINPUT = THandle;

function GetRawInputData(
  hRawInput: HRAWINPUT;
  uiCommand: UINT;
  pData: LPVOID;
  var pcbSize: UINT;
  cbSizeHeader: UINT): UINT; stdcall; external user32;

type
  tagRAWINPUTHEADER = record
    dwType: DWORD;
    dwSize: DWORD;
    hDevice: THandle;
    wParam: WPARAM;
  end;
  RAWINPUTHEADER = tagRAWINPUTHEADER;
  TRawInputHeader = RAWINPUTHEADER;
  PRawInputHeader = ^TRawInputHeader;

  tagRAWKEYBOARD = record
    MakeCode: USHORT;
    Flags: USHORT;
    Reserved: USHORT;
    VKey: USHORT;
    Message: UINT;
    ExtraInformation: ULONG;
  end;
  RAWKEYBOARD = tagRAWKEYBOARD;
  TRawKeyboard = RAWKEYBOARD;
  PRawKeyboard = ^TRawKeyboard;
  LPRAWKEYBOARD = PRawKeyboard;

//- !!! bogus declaration below, see winuser.h for the correct one
  tagRAWINPUT = record
    header: TRawInputHeader;
    keyboard: TRawKeyboard;
  end;
//-
  RAWINPUT = tagRAWINPUT;
  TRawInput = RAWINPUT;
  PRawInput = ^TRawInput;
  LPRAWINPUT = PRawInput;

const
  RIM_INPUT = 0;
  RIM_INPUTSINK = 1;
  RID_INPUT = $10000003;
  RIM_TYPEKEYBOARD = 1;
  RI_KEY_MAKE = 0;
  RI_KEY_BREAK = 1;

procedure TMyCustomControl.WMInput(var Message: TMessage);
var
  Size: UINT;
  Data: array of Byte;
  RawKeyboard: TRawKeyboard;
begin
  if (Message.WParam and $FF) in [RIM_INPUT, RIM_INPUTSINK] then
    inherited;

  if not Focused and
      (WindowFromPoint(SmallPointToPoint(SmallPoint(GetMessagePos))) = Handle) and
      (GetRawInputData(Message.LParam, RID_INPUT, nil, Size,
      SizeOf(TRawInputHeader)) = 0) then begin
    SetLength(Data, Size);
    if (GetRawInputData(Message.LParam, RID_INPUT, Data, Size,
        SizeOf(TRawInputHeader)) <> UINT(-1)) and
        (PRawInput(Data)^.header.dwType = RIM_TYPEKEYBOARD) then begin
      RawKeyboard := PRawInput(Data)^.keyboard;

      if (RawKeyboard.VKey = VK_CONTROL) then begin
        if RawKeyboard.Flags and RI_KEY_BREAK = RI_KEY_BREAK then
          Cursor := crDefault
        else
          Cursor := crSizeAll; // will call continously until key is released
      end;
      // might opt to reset the cursor regardless of pointer position...


      if (RawKeyboard.VKey = VK_MENU) then begin
        ....
      end;

    end;

  end;
end;