How to avoid Qt app.exec() blocking main thread

Question

I\'m new to Qt, but need to solve a difficult problem.

I\'ve created a VERY simple GUI that I need to add to an existing C++ application. The problem is, I\'m writin

Answer 1

Most of the time, "main thread" == "GUI thread", so people use those terms interchangeably -- even the official documentation does that. I agree that it's confusing though, because they don't have to be the same.^ The actual rule is this:

GUI classes must only be accessed from the thread which instantiates QApplication/QGuiApplication

With a plugin like yours, here is what you need to do:

1. Create a new std::thread (NOT a QThread)
2. Run an init function in that thread. Let it instantiate your QApplication/QGuiApplication and start the event loop
3. Ensure that all your GUI objects are accessed from that thread only.

Voila, you now have a GUI thread that is not your main thread.

---

^Note: It is a different story on Mac OS X. Due to restrictions in the Cocoa framework, the main thread MUST be the GUI thread. The steps I outlined above will work on Windows/Linux but not on Mac. For Mac, you need to inject your code into the main thread -- see Kuba Ober's comments below.

Answer 2

A good solution is found in: git@github.com:midjji/convenient_multithreaded_qt_gui.git

then it's just e.g.

run_in_gui_thread(new RunEventImpl([](){
        QMainWindow* window=new QMainWindow();
        window->show();
    }));

callable from any thread, at any time, while taking care of setting things up for you in the bg.

Note, this also takes care of creating a QApplication and executing it in a thread. But also works if you have already done so somewhere already.