How to use GetFocus across process boundaries?

From：http://www.smartphonedn.com/qa/getfocusip.html

By Alexander Shargin, August 20, 2004.

Question

GetFocus is a very useful API function used to find window which has keyboard focus. However, this function has a limitation - focused window must be created by the calling thread or GetFocus will return NULL. On desktop Windows it was possible to walkaround this problem using AttachThreadInput function. However, on CE-based devices this function is not available. So, how can I use GetFocus to find focused window created by another process?

Answer

To make GetFocus work, you need to call it from a thread which created focused window. Usually (though not quite always) this is the same thread which created top-level foreground window (obtained with GetForegroundWindow). The trick is to subclass this window and the send it a special message which will be processed by your window proc. Since your window proc will be called by a thread in another process, you can just call GetFocus and return HWND as LRESULT to your own process. Note that on desktop this trick will not work since each process has its own address space and subclassing between process boundaries is not possible. But on CE it does work.

Sample code

I've written a small function, GetFocusEx, which uses the trick described above. You can use it in your projects the same way you use GetFocus. GetFocusWindowProc is a window proc assigned to foreground window. If it gets special message (registered with RegisterWindowMessage function) it calls GetFocus and returns obtained HWND as LRESULT. Otherwise it just forwards messages to original window proc using CallWindowProc function.

staticWNDPROC g_pOldWndProc = NULL;
staticUINT g_uGetFocusMessage = RegisterWindowMessage(_T("SpecialGetFocusMessage"));

static LRESULT WINAPI GetFocusWindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    if(uMsg == g_uGetFocusMessage)
    {
        return (LRESULT)GetFocus();
    }
    else
    {
        return CallWindowProc(g_pOldWndProc, hWnd, uMsg, wParam, lParam);
    }
}

HWND GetFocusEx()
{
    HWND hWnd = GetForegroundWindow();
    if(!IsWindow(hWnd))
        return NULL;

    g_pOldWndProc = (WNDPROC)GetWindowLong(hWnd, GWL_WNDPROC);
    SetWindowLong(hWnd, GWL_WNDPROC, (LONG)GetFocusWindowProc);

    HWND hResult = (HWND)SendMessage(hWnd, g_uGetFocusMessage, 0, 0);

    SetWindowLong(hWnd, GWL_WNDPROC, (LONG)g_pOldWndProc);
    g_pOldWndProc = NULL;

    return hResult;
}