PostMessage() succeeds but my message processing code never receives the message - c++

Question

In my C++ application's GUI object I have the following in the main window procedure:
case WM_SIZE:
{
OutputDebugString(L"WM_SIZE received.\n");
RECT rect = {0};
GetWindowRect(hwnd, &rect);
if (!PostMessage(0, GUI_MSG_SIZECHANGED, w, MAKELONG(rect.bottom - rect.top, rect.right - rect.left))) {
OutputDebugString(L"PostMessage failed.\n"); // <--- never called
}
}
return 0; // break;
The GUI object also has the following getMessage() method:
int GUI::getMessage(MSG & msg) {
BOOL result = 0;
while ((result = GetMessage(&msg, 0, 0, 0)) > 0) {
if (msg.message > (GUI_MSG_BASE-1) && msg.message < (GUI_MSG_LAST+1)) {
OutputDebugString(L"GUI message received.\n");
break;
}
else {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
return result;
}
The application object calls this method in the following way:
while ((result = _gui.getMessage(msg)) > 0) {
switch (msg.message) {
// TODO: Add gui message handlers
case GUI_MSG_SIZECHANGED:
OutputDebugString(L"GUI_MSG_SIZECHANGED received.\n");
_cfg.setWndWidth(HIWORD(msg.lParam));
_cfg.setWndHeight(LOWORD(msg.lParam));
if (msg.wParam == SIZE_MAXIMIZED)
_cfg.setWndShow(SW_MAXIMIZE);
else if (msg.wParam == SIZE_MINIMIZED)
_cfg.setWndShow(SW_MINIMIZE);
else if (msg.wParam == SIZE_RESTORED)
_cfg.setWndShow(SW_SHOWNORMAL);
break;
}
}
The application object is interested in the window size because it stores this information in a configuration file.
When I run this in Visual Studio's debugger, the output window looks like this after resizing the window:
WM_SIZE received.
GUI message received.
GUI_MSG_SIZECHANGED received.
WM_SIZE received.
WM_SIZE received.
WM_SIZE received.
WM_SIZE received.
...etc...
The PostMessage() function never fails, but seems to only send GUI_MSG_SIZECHANGED (#defined as WM_APP + 0x000d) the first time WM_SIZE is handled, which is right after handling WM_CREATE.
I have no idea what could be causing this. I tried using SendMessage and PostThreadMessage but the result is the same. Also read through MSDN's message handling documentation but couldn't find what's wrong with my code.
Could anyone help?

Answer 1

Hacking a custom message loop is something you'll live to regret some day. You hit it early.
Don't post messages with a NULL window handle, they can only work if you can guarantee that your program only ever pumps your custom message loop. You cannot make such a guarantee. These messages fall into the bit bucket as soon as you start a dialog or Windows decides to pump a message loop itself. Which is the case when the user resizes a window, the resize logic is modal. Windows pumps its own message loop, WM_ENTERSIZEMOVE announces it. This is also the reason that PostThreadMessage is evil if the thread is capable of displaying any window. Even a MessageBox is fatal. DispatchMessage cannot deliver the message.
Create a hidden window that acts as the controller. Now you can detect GUI_MSG_SIZECHANGED in its window procedure and no hacks to the message loop are necessary. That controller is not infrequently the main window of your app btw.