Are you looking for QFileOpenEvent ?

I mark @SGaist answer as the right one, because he made me investigate again and deeper into QFileOpenEvent.

From this nice tutorial - at the very end - I have got the details, which are
needed to get QFileOpenEvent work on MacOS: https://youtu.be/o7Z0riL2kgE

After overwriting QApplication you can do this in main.cpp

#include <mainwindow.h>
#include "myapplication.h"

int main(int argc, char *argv[])
{
    MyApplication a(argc, argv);
    auto arguments = a.arguments();

    MainWindow w;
    w.show();

#ifdef Q_OS_MACOS
    QObject::connect(&a, &MyApplication::openFile,
                     &w, &MainWindow::messageSend);
#else
    w.messageSend(arguments.at(1));
#endif

    return a.exec();
}

    if(!listen())
        return 0;
    MainWindow w;
    w.show();
    return app.exec();

In the "listen()" part I try to connect a QLocalServer with certain name. If connected, I'll send some information to it (in you case you can send the arguments) and return false. If such server is not found, I'll create one, make it listening and return true.
QtSingleApplication seems to have similar design.

in my solution I can create multiple instances of the main window. The reason is by software concept. All processing parameters are bound to the window . It is like "Workspaces" with a fixed set of settings.

yes I did - still struggling to get this work
Let me send a minimal example later.

But this is mac only - right?
Does it need separate implementation for windows than?

An I am wondering if i really need to implement

• QtSingleApplication
• FileOpenEvent
• qApp-Arguments

To get a simple "Double Click" or "Drag to Icon" experience. Looks like this.

Are you looking for QFileOpenEvent ?

Let me try to describe in my own words:
main does initiate a "mainwindow" as an instance.

When I double click a file or do some of the other actions above I want to open this file in exact this already open window.
Hence somehow I need to tell the running instance about it. Right?

As what already got this approach might differ on MacOS and Windows.

I did what @jsulm recommended and added this to my MainWindow routine.

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QStringList arguments = qApp->arguments();
    if(arguments.size() > 1) {
        qDebug() << arguments.at(1);
        processArguments(arguments);
    }
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::processArguments(QStringList arguments)
{
    ui->plainTextEdit->clear();
    ui->plainTextEdit->appendPlainText(arguments.join("\n"));
}

This work only on initial launch of the application,
but not if the application is already running.

@jsulm Dear JS. the use cases are:

Double click a file on the desktop and get it open
Drag a file on the application Icon and get it opened
Use contact menu to send a file to this application to get it opened

But why you need a running instance for that?
I know, there are some examples but from my experience, if you have your program running and you open a file connected to your program, in most cases a new window / instance of the program will start. (because it's a lot easier, I guess)

• Double click a file on the desktop and get it open
• Drag a file on the application Icon and get it opened
• Use contact menu to send a file to this application to get it opened

All those on macOS as OnWindows.

Thx for the hint to "qApp" - I gave it a try - and yes it delivers what I wanted.
now I need to see how to achieve the above requirements.

But how to send those arguments to MainWindow if this is already active?

What is the use case?
And if you really need to do so then you can do that at any time because you can get them from QApplication instance (https://doc.qt.io/qt-6/qapplication.html#qApp) just like you do in the code snippet.