how to handle WM_NCHITTEST in a naked event loop? - c

Question

I'm playing with Windows API trying to understand how it behaves, and I realized that I can remove WNDPROC altogether and handle things with a naked event loop, like this:
#include <Windows.h>
static struct {
HWND desktop;
HWND window;
} global;
int main(int argc, char **argv)
{
/* anonymous scope: register window class */
{
WNDCLASSEXW wcx;
wcx.cbSize = sizeof(wcx);
wcx.style = CS_HREDRAW | CS_VREDRAW;
wcx.lpfnWndProc = DefWindowProc;
wcx.cbClsExtra = sizeof(void *);
wcx.cbWndExtra = sizeof(void *);
wcx.hInstance = (HINSTANCE)GetModuleHandle(NULL);
wcx.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wcx.hCursor = LoadCursor(NULL, IDC_ARROW);
wcx.hbrBackground = (HBRUSH)COLOR_WINDOWFRAME;
wcx.lpszMenuName = NULL;
wcx.lpszClassName = L"MyWindow";
wcx.hIconSm = wcx.hIcon;
RegisterClassExW(&wcx);
}
global.desktop = GetDesktopWindow();
global.window = CreateWindowExW (
0,
L"MyWindow",
NULL,
WS_POPUPWINDOW | WS_VISIBLE,
0,
0,
320,
200,
global.desktop,
NULL,
(HINSTANCE)GetModuleHandle(NULL),
NULL
);
/* anonymous scope, event loop */
{
MSG msg;
while (GetMessage(&msg, NULL, 0, 0) > 0) {
TranslateMessage(&msg);
if(msg.hwnd == global.window) {
if (msg.message == WM_PAINT) {
PAINTSTRUCT ps;
HDC hdc;
hdc = BeginPaint(msg.hwnd, &ps);
SelectObject(hdc, GetStockObject(BLACK_BRUSH));
Rectangle(hdc, 0, 0, 320, 200);
EndPaint(msg.hwnd, &ps);
} else {
DispatchMessage(&msg);
}
} else {
DispatchMessage(&msg);
}
}
}
return 0;
}
I wanted to go one step further and try making this window moveable using this technique, and got confused because I can't "return" from a message loop in the way i'm used to(the statement return hit;) does not make sense in this context.
Here is how I started, and got confused:
#include <Windows.h>
static struct {
HWND desktop;
HWND window;
} global;
int main(int argc, char **argv)
{
/* anonymous scope: register window class */
{
WNDCLASSEXW wcx;
wcx.cbSize = sizeof(wcx);
wcx.style = CS_HREDRAW | CS_VREDRAW;
wcx.lpfnWndProc = DefWindowProc;
wcx.cbClsExtra = sizeof(void *);
wcx.cbWndExtra = sizeof(void *);
wcx.hInstance = (HINSTANCE)GetModuleHandle(NULL);
wcx.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wcx.hCursor = LoadCursor(NULL, IDC_ARROW);
wcx.hbrBackground = (HBRUSH)COLOR_WINDOWFRAME;
wcx.lpszMenuName = NULL;
wcx.lpszClassName = L"MyWindow";
wcx.hIconSm = wcx.hIcon;
RegisterClassExW(&wcx);
}
global.desktop = GetDesktopWindow();
global.window = CreateWindowExW (
0,
L"MyWindow",
NULL,
WS_POPUPWINDOW | WS_VISIBLE,
0,
0,
320,
200,
global.desktop,
NULL,
(HINSTANCE)GetModuleHandle(NULL),
NULL
);
/* anonymous scope, event loop */
{
MSG msg;
while (GetMessage(&msg, NULL, 0, 0) > 0) {
TranslateMessage(&msg);
if(msg.hwnd == global.window) {
if (msg.message == WM_PAINT) {
PAINTSTRUCT ps;
HDC hdc;
hdc = BeginPaint(msg.hwnd, &ps);
SelectObject(hdc, GetStockObject(BLACK_BRUSH));
Rectangle(hdc, 0, 0, 320, 200);
EndPaint(msg.hwnd, &ps);
} else if (msg.message == WM_NCHITTEST) {
LRESULT hit = DefWindowProc(msg.hwnd, msg.message, msg.wParam, msg.lParam);
if (hit == HTCLIENT) {
hit = HTCAPTION;
}
// return hit; // makes no sense here
} else {
DispatchMessage(&msg);
}
} else {
DispatchMessage(&msg);
}
}
}
return 0;
}
How can I simulate returning "hit" from the WM_NCHITTEST condition so that it moves the window like in the solution here: https://stackoverflow.com/a/7773941/2012715 ?
PS: I know that it's better to use a map(like std::unordered_map) rather than a long if/switch for scalability and readability, but I wanted to keep the example more direct.

Answer 1

You can't.
The code you have shown will NEVER work, because (Get|Peek)Message() returns only QUEUED messages. You cannot reply to a queued message, because the sender is not waiting for a reply, it put the message in the queue and moved on to other things. Only NON-QUEUED messages that are sent with the SendMessage...() family of functions can be replied to. (Get|Peek)Message() will NEVER return a sent message, but will internally dispatch it to the target window's message procedure (only messages sent from another thread will be dispatched, messages sent to a window by the same thread that owns the window will bypass the message queue completely).
WM_PAINT is a queued message, so your event loop sees it. But WM_NCHITTEST is not queued, so your message loop will NEVER see it directly, it can only be seen in a message procedure.
What you have shown is NOT the right way to handle a Windows UI message loop. Since you are creating a UI window, you MUST provide it with a message procedure (if not by RegisterClass/Ex(), then by SetWindowLong/Ptr(GWL_WNDPROC) or SetWindowSubclass()). But DO NOT use DefWindowProc() for that procedure if you need to process messages manually. Provide your own message procedure that calls DefWindowProc() (or CallWindowProc() in the case of GWL_WNDPROC, or DefSubclassProc() in the case of SetWindowSubclass()) for any unhandled messages, eg:
LRESULT WINAPI MyWndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
switch (uMsg) {
case WM_PAINT: {
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
SelectObject(hdc, GetStockObject(BLACK_BRUSH));
Rectangle(hdc, 0, 0, 320, 200);
EndPaint(hwnd, &ps);
return 0;
}
case WM_NCHITTEST: {
LRESULT hit = DefWindowProc(hwnd, uMsg, wParam, lParam);
if (hit == HTCLIENT) {
hit = HTCAPTION;
}
return hit;
}
}
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
int main(int argc, char **argv)
{
/* anonymous scope: register window class */
{
WNDCLASSEXW wcx;
wcx.cbSize = sizeof(wcx);
wcx.style = CS_HREDRAW | CS_VREDRAW;
wcx.lpfnWndProc = MyWndProc; // <--
wcx.cbClsExtra = sizeof(void *);
wcx.cbWndExtra = sizeof(void *);
wcx.hInstance = (HINSTANCE)GetModuleHandle(NULL);
wcx.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wcx.hCursor = LoadCursor(NULL, IDC_ARROW);
wcx.hbrBackground = (HBRUSH)COLOR_WINDOWFRAME;
wcx.lpszMenuName = NULL;
wcx.lpszClassName = L"MyWindow";
wcx.hIconSm = wcx.hIcon;
RegisterClassExW(&wcx);
}
global.desktop = GetDesktopWindow();
global.window = CreateWindowExW (
0,
L"MyWindow",
NULL,
WS_POPUPWINDOW | WS_VISIBLE,
0,
0,
320,
200,
global.desktop,
NULL,
(HINSTANCE)GetModuleHandle(NULL),
NULL
);
/* anonymous scope, event loop */
{
MSG msg;
while (GetMessage(&msg, NULL, 0, 0) > 0) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
return 0;
}