[Webinar] Streamline your web hosting managementRegister Today

x
?
Solved

Passing a callback pointer within a class = Exception ?

Posted on 2012-03-21
1
Medium Priority
?
537 Views
Last Modified: 2016-09-29
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

0
Comment
Question by:rotem156
1 Comment
 
LVL 19

Accepted Solution

by:
Thommy earned 2000 total points
ID: 37751573
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

Featured Post

Free Tool: SSL Checker

Scans your site and returns information about your SSL implementation and certificate. Helpful for debugging and validating your SSL configuration.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

The uses clause is one of those things that just tends to grow and grow. Most of the time this is in the main form, as it's from this form that all others are called. If you have a big application (including many forms), the uses clause in the in…
Objective: - This article will help user in how to convert their numeric value become words. How to use 1. You can copy this code in your Unit as function 2. than you can perform your function by type this code The Code   (CODE) The Im…
This video tutorial shows you the steps to go through to set up what I believe to be the best email app on the android platform to read Exchange mail.  Get the app on your phone: The first step is to make sure you have the Samsung Email app on your …
Is your organization moving toward a cloud and mobile-first environment? In this transition, your IT department will encounter many challenges, such as navigating how to: Deploy new applications and services to a growing team Accommodate employee…
Suggested Courses

591 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question