Debugging WIN32 focus bugs

Question

I\'m developing a WIN32/C++ application containing a main window and a lot of child windows, and I\'m looking for tools that could assist me in tracking down focus bugs.

Answer 1

A Spy++ message log will give a complete record of focus changes, but trying to decode what was happening from the log is a chore. Spy++'s nasty UI doesn't help.

Remote debugging is helpful because the debugger won't interfere with your application's activation and focus, but I'm not sure there's a simple way to determine the focused window from the debugger. Here are a couple of articles on configuring it. Hint: if it doesn't work, double-check your DCOM and firewall settings.

Obviously it would be best to find a tool that does exactly what you want but I couldn't and I was bored so I wrote this code. It creates a semi-transparent blue window that sits on top of the focused control. It's transparent to clicks so it shouldn't interfere with using your app.

To initialise it simply create a timer somewhere in your program's initialisation code:

// Check the focus ten times a second
// Change hwndMain to your main application window
// Note that this won't work if you have multiple UI threads
::SetTimer(hwndMain, 1, 100, HighlightTimerProc);

and add the following code:

LRESULT CALLBACK HighlightWndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
    case WM_NCHITTEST:
        return HTTRANSPARENT;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

VOID CALLBACK HighlightTimerProc(HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime)
{
    // static locals are bad
    static bool initialised = false;
    static HWND hwndHighlight = 0;

    if (!initialised)
    {
        HINSTANCE hInstance = 0;

        WNDCLASSEX wcex;

        wcex.cbSize = sizeof(WNDCLASSEX);

        wcex.style          = CS_HREDRAW | CS_VREDRAW;
        wcex.lpfnWndProc    = HighlightWndProc;
        wcex.cbClsExtra     = 0;
        wcex.cbWndExtra     = 0;
        wcex.hInstance      = hInstance;
        wcex.hIcon          = 0;
        wcex.hCursor        = 0;
        wcex.hbrBackground  = (HBRUSH)(COLOR_HIGHLIGHTTEXT);
        wcex.lpszMenuName   = 0;
        wcex.lpszClassName  = L"HighlightWindowClasss";
        wcex.hIconSm        = 0;

        ATOM atomHighlightClass = RegisterClassEx(&wcex);

        hwndHighlight = CreateWindowEx(WS_EX_LAYERED | WS_EX_TOPMOST | WS_EX_NOACTIVATE | WS_EX_TOOLWINDOW,
            (LPCTSTR)atomHighlightClass, L"", WS_POPUP,
            CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

        // Set opacity to 200/255
        SetLayeredWindowAttributes(hwndHighlight, 0, 200, LWA_ALPHA);

        initialised = true;
    }

    static HWND hwndCurrentHighlight = 0;

    HWND hwndFocus = GetFocus();
    if (hwndFocus != hwndCurrentHighlight)
    {
        if (hwndFocus == 0)
        {
            ShowWindow(hwndHighlight, SW_HIDE);
        }
        else
        {
            RECT rect;
            GetWindowRect(hwndFocus, &rect);
            MoveWindow(hwndHighlight, rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top, false);
            ShowWindow(hwndHighlight, SW_SHOW);
        }
        hwndCurrentHighlight = hwndFocus;
    }
}