I need help with my Deno Qt/QML bindings - QGuiApplication::exec is blocking

[nxctr]

Hello,

I've been working on my QML bindings for Deno, the new JavaScript runtime. You can view them here: https://deno.land/x/qml@v0.1.5
But there are problems.

First off, some things you need to know.

• Deno has a new (unstable) FFI API for calling native code.
• Only C functions can be used. No C++ classes.
• Buffers (aka strings) can be passed as input parameters, but they cannot be return values.
• Because of this, the only way to make Qt work at this time is to declare a thread_local std::unique_ptr<App> global class and initialize QGuiApplication in an init() function.
• FFI blocks the entire Deno event loop until the function returns.
• It uses QGuiApplication and QQmlApplicationEngine.

The problem is that int exec() { return qGuiApp->exec(); } is blocking. I need to replace it with something like:

while (true) {
    processQtEvents();
    if (isDone()) break;
}

No matter how hard I tried, I can't figure out how to do it!

Things I've tried:

QEventLoop loop;
while (loop.isRunning()) {
    loop.processEvents(QEventLoop::AllEvents | QEventLoop::WaitForMoreEvents | QEventLoop::EventLoopExec);
}

while (true) {
    qGuiApp->processEvents(QEventLoop::AllEvents, 6000);
    if (qGuiApp->closingDown()) break;
}

qGuiApp->eventDispatcher()->processEvents(QEventLoop::AllEvents);

None of them work. The most common problem is that when the window is closed, the application still keeps running (aka the only way to end it is with CTRL+C). Signals such as QQmlApplicationEngine::quit and QQmlApplicationEngine::exit do not fire either.

I even tried making everything live in a global C++ thread class (not QThread) and passing data using mutexes and conditional variables, so that I could just have qGuiApp->exec() running in a separate thread and call isDone() from Deno. Still doesn't work. ThreadedApplication::operator()() and ThreadedApplication::setData() are in different threads, which Qt hates.

In simplest terms: can QGuiApplication::exec() be replaced with a while() loop?

I apologize if I'm not being clear about something. I'm at wits end to try and figure this out.

[SGaist]

Hi and welcome to devnet,

Did you try to integrate Qt's event loop with the one from Deno ?

AFAIK, QAbstractEventDispatcher is the starting for that.

[nxctr]

@SGaist Thanks for the reply. Unfortunately, Deno's event loop isn't exposed, at all. It's just calling functions from a C ABI using dlopen().