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Introduction

The attached source code is for a system-wide global hotkey control written in Visual Basic 6, orginally posted on the Merrion Computing Downloads

Registering a hotkey

There are two API calls concerned with registering a system wide hotkey: RegisterHotkey abd UnregisterHotkey - the declaratiosn are:

Private Declare Function RegisterHotKey Lib "user32" (ByVal hwnd As Long, ByVal id As Long, ByVal fsModifiers As Long, ByVal vk As Long) As Long
Private Declare Function UnregisterHotKey Lib "user32" (ByVal hwnd As Long, ByVal id As Long) As Long

To register the hotkey you need to specify hwnd - the window which will be notified when the hotkey combination is pressed, id - the unique identifier for the hotkey, fsModifiers - the modifiers of the hotkey (i.e. whether ALT, SHIFT, CTRL or WIN keys are pressed) and finally vKey - the virtual key code for the hot key. The modifiers which can be combined together are:

Public Enum enHotkeyModifiers
    MOD_ALT = &H1
    MOD_CONTROL = &H2
    MOD_SHIFT = &H4
    MOD_WIN = &H8
End Enum

Listening out for the hotkey

When a hotkey which has been registered successfully is pressed, the window specified in the hwnd is posted a WM_HOTKEY registered window message. Additionally the lParam parameter holds the modifier and virtual key code in it's high and low words respectively

Public Function VB_WindowProc(ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long

Dim VKey As Long
Dim Modifier As Long

If wMsg = WM_HOTKEY Then
    VKey = HiWord(lParam)
    Modifier = LoWord(lParam)
    '\\ Raise an event here to resppond to the hotkey
End If

If sWindow.OldWndProc = 0 Then
    VB_WindowProc = DefWindowProc(hwnd, wMsg, wParam, lParam)
Else
    VB_WindowProc = CallWindowProc(sWindow.OldWndProc, hwnd, wMsg, wParam, lParam)
End If

End Function

Visual Basic does not handle the WM_HOTKEY message by default which means that you need to subclass the window hwnd in order to respond to it. To do this you use the API call SetWindowLong with the index GWL_WNDPROC to replace the existing window procedure with your one.

'\\ API Calls used in subclassing windows....
Private Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" (ByVal lpPrevWndFunc As Long, ByVal hwnd As Long, ByVal msg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
Private Declare Function DefWindowProc Lib "user32" Alias "DefWindowProcA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
'\\ Window specific information
Private Declare Function GetWindowLongApi Lib "user32" Alias "GetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long) As Long
Private Declare Function SetWindowLongApi Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
Private Const GWL_WNDPROC = (-4)

Using the control in your application

To use this control in a Visual basic application, compile the attached source code and then add an instance of the resulting OCX to your project. Add an instance of the MCLHotkey control to your form and press F4 to set the properties. Then add the code that you want executed when the hotkey is pressed to the HotkeyPressed event:

Private Sub MCLHotkey1_HotkeyPressed()
  Debug.Print "You pressed the hotkey at " & Now
End Sub