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

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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.

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