Passing a callback pointer within a class = Exception ?

Hi, i'm using SetWindowsHookEx and i have placed my hook proc under private declaration within my hook class...

when ever i call it like this

SetWindowHookEx( @THookClass.HookProc ) it grants an exception @ run time, why ?

placing it outside the form works well...

unit uHook;

interface

uses
  Windows,
  dialogs,
  Classes,
  Messages;

type
  THookClass = class(TObject)
    private
      TempBuf: string;

      function LLKeybdProc(nCode: Integer; wParam: WPARAM; lParam: LPARAM): LRESULT stdcall;
    public
      property FinalBuf: string read TempBuf;

      procedure Stop;
      constructor Create;
  end;

var
  HookClass: THookClass;

  HookBuf: TStringList; // holds captured keys
  keybdHook: HHOOK; // hook handle

implementation

// converts h/w scan code to key name
Function VKtoStr(AScanCode: Cardinal): String;
var
  Buffer: array [0..255] of Char;
begin
  Result := '';

  if (AScanCode <> 0) then
  begin
    GetKeyNameText(AScanCode shl 16, Buffer, SizeOf(Buffer));
    Result := Buffer;
  end;
end;

function THookClass.LLKeybdProc(nCode: Integer; wParam: WPARAM; lParam: LPARAM): LRESULT stdcall;
type
  PKBDLLHOOKSTRUCT = ^TKBDLLHOOKSTRUCT;
  TKBDLLHOOKSTRUCT = packed record
    vkCode: DWORD;
    scanCode: DWORD;
    flags: DWORD;
    time: DWORD;
    dwExtraInfo: DWORD;
  end;
var
  pkbhs: PKBDLLHOOKSTRUCT;
begin
  Result := CallNextHookEx(keybdHook, nCode, wParam, lParam);

  if (nCode < 0) then // do not process message
    exit;

  pkbhs := PKBDLLHOOKSTRUCT(lParam);

  case wParam of
    WM_KEYUP:
    case pkbhs^.scanCode of
      $39: HookBuf.Append(' '); // Spacebar (VK_BACK)
      else
        HookBuf.Append(VKtoStr(pkbhs^.scanCode));
    end;
  end;
end;

procedure THookClass.Stop;
begin
  if (keybdHook <> INVALID_HANDLE_VALUE) then
    UnhookWindowsHookEx(keybdHook);

  if (HookBuf.Count > 0) then
    TempBuf := HookBuf.Text;

  if (HookBuf <> nil) then
    HookBuf.Free;
end;

constructor THookClass.Create;
begin
  inherited;

  HookBuf := TStringList.Create;
  HookBuf.LineBreak := '';

  keybdHook := SetWindowsHookEx(WH_KEYBOARD_LL, @LLKeybdProc, 0, 0);
end;

end.

Open in new window

LVL 4
rotem156Asked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

ThommyCommented:
Check, if this helps you:

Let's say you want a WH_CALLWNDPROC hook in a class. You define your hook function, a pointer variable (that will be a
pointer to the method) and a hook handle (so you can correctly chain the hook events):

     type
       TMyClass = class (TWhat)
     ....
       private
         ThisHook : hHook;
         HookProc : pointer;
     ....
        function MyHookProc (var HookCall: THookCall);
     ....
       end;

And in the class constructor, you create a pointer to the class method and set the hook running with a call to the API:

       HookProc := MakeHookInstance (MyHookProc);
       ThisHook := SetWindowsHookEx (WH_CALLWNDPROC, HookProc, HInstance, GetCurrentThreadID);

And next you must define your hook function, which is now a method:

     function TMyClass.MyHookProc (var HookCall: THookCall);
     begin
     ....
       with HookCall do
         Result := CallNextHookEx (ThisHook, Code, wParam, lParam);
     ....
     end;

Finally, in the destructor, you clean up:

       UnhookWindowsHookEx (ThisHook);
       FreeHookInstance (HookProc);

So, in conclusion...

No global variables! No stand alone hook function! The hook function can easily reference the class methods!

Open in new window

Above description is extracted from http://www.fortunecity.com/skyscraper/linux/1344/dc000015.html
0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Delphi

From novice to tech pro — start learning today.