UE5 Threads: FRunnable

I made a simple example project to demonstrate FRunnable thread usage:

https://github.com/XistGG/UE5-FRunnable-ThreadTest

The README in that repository contains a good description of it. It is definitely worth reading that, and taking a look at the few C++ methods described therein.

In general the primary takeaways for using FRunnable are:

• When you create the thread with FRunnableThread::Create (as I do in the example in the FRunnableNonsense::Start method), UE5 will automatically execute code similar to the following in the context of the new thread:

    if (Runnable->Init())
    {
        Runnable->Run();
        Runnable->Exit();
    }
  

• In your FRunnable destructor when you Thread->Kill(), it will automatically call Runnable->Stop()

• In your main game thread (or whatever thread is managing your FRunnable threads):

  • You can explicitly call Runnable->Stop() to schedule the thread to exit, but then you must wait for it to actually decide to stop, which will not be immediate.
  • When you are killing mass numbers of threads simultaneously, it can block your main thread for noticable time.
    • Whenever possible, wait for threads to fully exit before trying to rejoin them.

See Also

• Multithreading in Unreal Engine 5
  • Great high level overview
• UE5 Multithreading With FRunnable And Thread Workflow
• Creating UObjects From Async Threads