[Interest] Handling WM_QUIT messages when no window is shown

[Julius Bullinger]

On 06.05.2020 14:57, Henry Skoglund wrote:
> Hi, when running apps on Windows without a window, you'll be missing out 
> on a lot of inter-process stuff like WM_QUIT (that's why it's common for 
> windowless apps to create a hidden, non-interactive window just to be 
> able to receive all those WM_XXX). So no aboutToQuit() will be emitted 
> for you when you skip creating that widget window.

How would one create a hidden, non-interactive window with Qt?

> But there's an API for windowless apps where you can receive a control 
> signal, e.g. Ctrl-C was pressed or someone (like the Task Manager) is 
> shutting down the app, more here: 
> https://docs.microsoft.com/en-us/windows/console/setconsolectrlhandler 
> and the callback: 
> https://docs.microsoft.com/en-us/windows/console/handlerroutine
> 
> There's no support for that API in Qt but it's pretty easy to use. 
> Here's an example, I created a vanilla Qt console app, and changed 
> main.cpp to this:
> -----------------------------------------
> #include <QCoreApplication>
> #include <windows.h>
> 
> BOOL WINAPI CtrlHandler(DWORD fdwCtrlType)
> {
>      switch (fdwCtrlType)
>      {
>      case CTRL_CLOSE_EVENT:
>           ::OutputDebugStringA("Ctrl-Close event"); // catch taskkill or 
> Task Manager End task
>          return TRUE;
> 
>      default:
>          return FALSE;
>      }
> }
> 
> int main(int argc, char *argv[])
> {
>      QCoreApplication app(argc, argv);
> 
>      SetConsoleCtrlHandler(CtrlHandler, TRUE);
> 
>      return app.exec();
> }
> -----------------------------------------
> Note: I'm using OutputDebugString and DbgView.exe to view the output, 
> qDebug() output has a tendency to disappear when you test with "End 
> task" from TaskManager.
> 
> Note 2: this API is very flakey if you try it from a GUI-built .exe, for 
> Qt apps I mean built with "QT += core gui etc" in the .pro file, and use 
> of QApplication in main.cpp. Instead you need to build your app like a 
> Qt console app, "QT -= gui" and CONFIG += console, and use 
> QCoreApplication in main.cpp, like in my example above.

That's the solution we're using in plain console QCoreApplications. 
However, in this case, we have a QApplication which is showing or not 
showing a GUI depending on some commandline args. So that will probably 
not work robustly in this situation.

Thanks!