Question

I have a program with several custom controls. One of these custom controls is a text input control. Since a window does not automatically receive keyboard focus when you click on it, i’ve created a mouse hook in my program that calls SetFocus() on a window when the user clicks in that window. However, there is a problem.

If another program has focus when you click on my program’s window (or any of the controls in that window) SetFocus() fails. I then have to click again for it to succeed. Here’s the code:

LRESULT CALLBACK kbfProc(int nCode, WPARAM wParam, LPARAM lParam) // Keyboard focus switching procedure
{
    switch(nCode)
    {
        case HC_ACTION:
        {
            if(wParam == WM_LBUTTONDOWN || wParam == WM_NCLBUTTONDOWN)
            {
                MOUSEHOOKSTRUCT * mhs = (MOUSEHOOKSTRUCT*) lParam;

                if(SetFocus(mhs->hwnd) == NULL)
                {
                    printf("SetFocus(Hwnd = %.8x) failed. Error code: %lu\n", mhs->hwnd, GetLastError());
                } else {

                    printf("SetFocus(Hwnd = %.8x) returned success.\n", mhs->hwnd);
                }
            }

        }
        break;
    }

    return CallNextHookEx(0, nCode, wParam, lParam);
}

And the output of those printf calls is this:

SetFocus(Hwnd = 00410c06) failed. Error code: 87
SetFocus(Hwnd = 00410c06) returned success.
SetFocus(Hwnd = 01740fc8) failed. Error code: 87
SetFocus(Hwnd = 01740fc8) returned success.

Error code 87 is ERROR_INVALID_PARAMETER, but i’m obviously passing a valid window handle to the function, so why is it failing?

Answer 1

I’ve found a solution. After a lot of googling and trial & error I eventually came across this webpage (backup link). It goes over the behavior of window focus and activation in detail.

I ended up adding some code to the WM_ACTIVATE handler of my main window that searches for the child window that was clicked when the window is activated. Here’s all the code:

Here’s the hook procedure:

LRESULT CALLBACK kbfProc(int nCode, WPARAM wParam, LPARAM lParam)
{
    switch(nCode)
    {
        case HC_ACTION:
        {
            if(wParam == WM_LBUTTONDOWN || wParam == WM_NCLBUTTONDOWN)
            {
                MOUSEHOOKSTRUCT * mhs = (MOUSEHOOKSTRUCT*) lParam;
                
                BringWindowToTop(MainWindow->t_hwnd);
                SetFocus(mhs->hwnd);
            }
            
        }
        break;
    }
    
    return CallNextHookEx(0, nCode, wParam, lParam);
}

Here’s the code i put in the WM_ACTIVATE handler:

    case WM_ACTIVATE:
    {
        unsigned long state = (unsigned long) wParam & 0x0000FFFF;
        unsigned long mState = (unsigned long) wParam & 0xFFFF0000;
        
        if(state != 0)
        {
            ...[some code here]...
            
            FocusChildWindow(hwnd);
        }
        
        ...[some code here]...
    }
    break;

And here’s the FocusChildWindow() function:

void FocusChildWindow(HWND hwnd)
{
    POINT mpos;
    GetCursorPos(&mpos);
    
    RECT wr;
    GetWindowRect(hwnd, &wr);
    
    mpos.x -= wr.left;
    mpos.y -= wr.top;
    
    HWND cw = ChildWindowFromPoint(hwnd, mpos);
    
    if(cw == NULL || cw == hwnd)
    {
        SetFocus(hwnd);
    } else {
        
        GetCursorPos(&mpos);
        HWND cw2;
        
        while(1)
        {
            POINT sc = mpos;
            MapWindowPoints(HWND_DESKTOP, cw, &sc, 1);
            
            cw2 = ChildWindowFromPoint(cw, sc);
            
            if(cw2 == NULL || cw2 == cw)
            {
                SetFocus(cw);
                break;
            } else {
                
                cw = cw2;
            }
            
            
        }
        
    }
    
}