How to write a callback method in delphi dll to a delphi client app

DLL:
------------------
library KBHook;
 
uses
  {ShareMem, }Windows, Classes, Messages, SysUtils;
 
var
  HookHandle: HHOOK;
  uiWM_CapturedKeyboardKey: UINT;
 
 
function KeyBoardProc(Code: integer; wParam: integer; lParam: integer): integer; stdcall;
var
  KeyUp: boolean;
  dwRecipients: DWORD;
begin
  Result := 0;
 
  case Code of
    HC_ACTION:
      begin
        KeyUp := ((lParam AND (1 shl 31)) <> 0);
        if (KeyUp) then begin
          dwRecipients := BSM_APPLICATIONS;
//          if (wParam >= 32) and (wParam <= 126) then begin
             BroadcastSystemMessage(BSF_POSTMESSAGE, @dwRecipients, uiWM_CapturedKeyboardKey, wParam, lParam);
//            KeyList.Text := KeyList.Text + Char(wParam) ;
//            KeyList.SaveToFile('C:\Temp\KeyList.txt') ;
//          end ;
        end;
        Result := 0;
      end;
    HC_NOREMOVE:
      begin
        Result := 0;
        Exit;
      end;
  end;
 
  if (Code < 0) then
    Result := CallNextHookEx(HookHandle, Code, wParam, lParam);
end;
 
procedure StartKeyBoardHook ; stdcall;
begin
  HookHandle := SetWindowsHookEx(WH_KEYBOARD, @KeyBoardProc, hInstance, 0);
end;
 
 
procedure StopKeyBoardHook ; stdcall;
begin
 if (HookHandle <> 0) then
   if UnHookWindowsHookEx(HookHandle) then
     HookHandle := 0;
end;
 
function IsKeyboardHooked: boolean; stdcall;
begin
  Result := HookHandle <> 0;
end;
 
procedure DllEntryPoint(dwReason : DWORD);
begin
  case dwReason of
    Dll_Process_Attach:
      begin
        uiWM_CapturedKeyboardKey := RegisterWindowMessage('KBHook_WM_CapturedKeyboardKey');
//        KeyList := TStringList.Create ;
      end;
    Dll_Process_Detach:
      begin
//        KeyList.Free ;
      end;
  end;
end;
 
exports
 StartKeyBoardHook name 'STARTKEYBOARDHOOK',
 StopKeyBoardHook name 'STOPKEYBOARDHOOK',
 IsKeyboardHooked name 'ISKEYBOARDHOOKED';
 
begin
 DLLProc := @DllEntryPoint;
 DllEntryPoint(Dll_Process_Attach);
end.
 
Application:
------------------
unit UfrmMain;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
 
type
  TStartKeyBoardHook = procedure stdcall;
  TStopKeyBoardHook = procedure stdcall;
  TIsKeyboardHooked = function: boolean stdcall;
 
  TfrmKBHookLoader = class(TForm)
    btnHook: TButton;
    btnUnHook: TButton;
    mmoKeys: TMemo;
    btnClose: TButton;
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure btnHookClick(Sender: TObject);
    procedure btnUnHookClick(Sender: TObject);
    procedure FormShow(Sender: TObject);
    procedure btnCloseClick(Sender: TObject);
  private
    hDLL: THandle;
    StartKeyBoardHook: TStartKeyBoardHook;
    StopKeyBoardHook: TStopKeyBoardHook;
    IsKeyboardHooked: TIsKeyboardHooked;
    { Private declarations }
  protected
    procedure WndProc(var Message: TMessage); override;
  public
    procedure SetEnabledStates;
    { Public declarations }
  end;
 
var
  frmKBHookLoader: TfrmKBHookLoader;
  uiWM_CapturedKeyboardKey: UINT;
 
implementation
 
{$R *.dfm}
 
procedure TfrmKBHookLoader.btnCloseClick(Sender: TObject);
begin
  if Assigned(IsKeyboardHooked) then begin
    if IsKeyboardHooked() then begin
      if Assigned(StopKeyBoardHook) then begin
        StopKeyBoardHook();
      end;
    end;
  end;
  Close;
end;
 
procedure TfrmKBHookLoader.btnHookClick(Sender: TObject);
begin
  if Assigned(StartKeyBoardHook) then
    StartKeyBoardHook();
  SetEnabledStates;
end;
 
procedure TfrmKBHookLoader.btnUnHookClick(Sender: TObject);
begin
  if Assigned(StopKeyBoardHook) then
    StopKeyBoardHook();
  SetEnabledStates;
end;
 
procedure TfrmKBHookLoader.FormCreate(Sender: TObject);
begin
  hDLL := LoadLibrary('KBHook.DLL');
  if (hDLL <> 0) then begin
    @StartKeyBoardHook := GetProcAddress(hDLL, 'STARTKEYBOARDHOOK');
    @StopKeyBoardHook :=  GetProcAddress(hDLL, 'STOPKEYBOARDHOOK');
    @IsKeyboardHooked :=  GetProcAddress(hDLL, 'ISKEYBOARDHOOKED');
  end;
  uiWM_CapturedKeyboardKey := RegisterWindowMessage('KBHook_WM_CapturedKeyboardKey')
end;
 
procedure TfrmKBHookLoader.FormDestroy(Sender: TObject);
begin
  FreeLibrary(hDLL);
end;
 
procedure TfrmKBHookLoader.FormShow(Sender: TObject);
begin
  SetEnabledStates;
end;
 
procedure TfrmKBHookLoader.SetEnabledStates;
var
  blnIsHooked: boolean;
begin
  if Assigned(IsKeyboardHooked) then begin
    blnIsHooked := IsKeyboardHooked();
    btnHook.Enabled := not blnIsHooked;
    btnUnHook.Enabled := blnIsHooked;
  end else begin
    btnHook.Enabled := False;
    btnUnHook.Enabled := False;
  end;
end;
 
procedure TfrmKBHookLoader.WndProc(var Message: TMessage);
begin
  if Message.Msg = uiWM_CapturedKeyboardKey then begin
    if (Message.wParam >= 32) and (Message.wParam <= 126) then begin
      mmoKeys.Lines.Text := mmoKeys.Lines.Text + char(Message.wParam);
    end else if Message.wParam = 13 then begin
      mmoKeys.Lines.Text := mmoKeys.Lines.Text + #13;
    end;
  end else
    inherited;
end;
 
end.
                                              

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