THook Delphi Class with Source Code

Use Windows Hooks in your Delphi applications

Keyboard Hook Delphi example
Keyboard Hook Delphi example.
Code submitted by Jens Borrisholt. Text by Zarko Gajic.

By Jens: Hooks, I’ve seen a lot of people trying to make a clean solution for hooking messages in an application. So I decided some time ago to implement hooks as a class, with nice events and stuff :)

Hook.pas makes it possible to assign a method pointer to a procedure pointer (with some help from assembler).

For example: if you want to trap ALL keystrokes in your application - simply declare an instance of TKeyboardHook, assign an event handler for OnPreExecute or OnPostExecute, or both.

Set you KeyboadHook active (KeyboardHook.Active := True) and you are out and running ..

On Windows Hooks

Here's what the Windows API guide has to say on hooks:

A hook is a point in the system message-handling mechanism where an application can install a subroutine to monitor the message traffic in the system and process certain types of messages before they reach the target window procedure.

Put shortly, a hook is a function you can create as part of a dll or your application to monitor the 'goings on' inside the Windows operating system.

The idea is to write a function that is called every time a certain event in windows occurs - for example when a user presses a key on the keyboard or moves the mouse.

For a more in depth introduction to hooks, take a look at What Windows hooks are and how to use them within a Delphi application.

Hooking mechanism relies on Windows messages and callback functions.

Types of Hooks

Different hook types enable an application to monitor a different aspect of the system's message-handling mechanism.

For example:
You can use the WH_KEYBOARD hook to monitor keyboard input posted to a message queue;
You can use the WH_MOUSE hook to monitor mouse input posted to a message queue;
You can a WH_SHELL hook procedure when the shell application is about to be activated and when a top-level window is created or destroyed.


The hooks.pas unit defines several hook types:
  • TCBTHook - called before activating, creating, destroying, minimizing, maximizing, moving, or sizing a window; before completing a system command; before removing a mouse or keyboard event from the system message queue; before setting the input focus; or before synchronizing with the system message queue.
  • TDebugHook - called before calling hook procedures associated with any other hook in the system
  • TGetMessageHook - enables an application to monitor messages about to be returned by the GetMessage or PeekMessage function
  • TJournalPlaybackHook - enables an application to insert messages into the system message queue.
  • TJournalRecordHook - enables you to monitor and record input events (to record a sequence of mouse and keyboard events to play back later by using the WH_JOURNALPLAYBACK Hook).
  • TKeyboardHook - enables an application to monitor message traffic for WM_KEYDOWN and WM_KEYUP messages.
  • TMouseHook - enables you to monitor mouse messages about to be returned by the GetMessage or PeekMessage function.
  • TLowLevelKeyboardHook - enables you to monitor keyboard input events about to be posted in a thread input queue.
  • TLowLevelMouseHook - enables you to monitor mouse input events about to be posted in a thread input queue.

    TKeyboardHook example

    To show you how to use the hooks.pas, here's a section of the keyboard hook demo application:

    Download hooks.pas + demo application

     uses hooks, ....
      KeyboardHook: TKeyboardHook;
    //MainForm's OnCreate event handler
    procedure TMainForm.FormCreate(Sender: TObject) ;
      KeyboardHook := TKeyboardHook.Create;
      KeyboardHook.OnPreExecute := KeyboardHookPREExecute;
      KeyboardHook.Active := True;
    //handles KeyboardHook's OnPREExecute
    procedure TMainForm.KeyboardHookPREExecute(Hook: THook; var Hookmsg: THookMsg) ;
      Key: Word;
      //Here you can choose if you want to return   //the key stroke to the application or not
      Hookmsg.Result := IfThen(cbEatKeyStrokes.Checked, 1, 0) ;
      Key := Hookmsg.WPARAM;
      Caption := Char(key) ;
    Ready, set, hook :)