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Change the variables in a running function from the outside

  • There's an interface, and when you click the start button, the start function will execute, and you can see that as long as paused is always false,the start function will go on forever.

    Because I add a CoreApplication:: processEvents (), so now the interface is still responsive. When I click the pause button, the pause function will be called, and then start will exit. I tried this way, and there was nothing wrong with it, but I'm not sure whether there are other dangers that I hadn't seen.

    I think if this way is feasible, it's unnecessary to use multi-thread.

    int n =1;
    void start()
        while (1)
            if (paused)
            qDebug() << n++;
            QElapsedTimer timer;
            while (timer.elapsed() <= 500)
                QCoreApplication::processEvents(); // kepp the interface responsive
    void pause()
        paused = true;

  • Lifetime Qt Champion

    @Limer Well, you can do it this way, but using processEvents() is considered bad design. Depending on what you're doing inside the loop it can be that you are not calling processEvents() often enough to keep your UI responsive. Also, keep in mind that with this approach other slots can be called while start() is running, so you can still have parallel access to variables (not at the same time though). It is still better to move long lasting operations in a thread.

  • @jsulm Sorry, I forgot to add a while. The code above has changed.

  • @Limer
    This is an "odd" way to arrange things. I'm not even sure it will do things the way you want.

    If the n++ represents "doing your other work", note that it won't be executing once the while (timer.elapsed() ... is executing. It executes once, then it does nothing (other than keep the UI alive) for half a second. If the user does press "Pause" during that time, it continues doing "nothing" till the rest of the half second has elapsed, only then does it exit start(). (You might want to change the loop to while (timer.elapsed() <= 500 && !paused)

  • @Limer as mentioned in previous posts, you're approach isn't seem a good way to go. One advantage of using Qt framework is dealing with signals & slots to avoid dealing with event loops directly.

    Given that said, you may find this example about a file downloader interesting regarding the way the cancel action (pause in your case) is handled to stop (pause) an ongoing download.

  • @Pablo-J.-Rogina Thanks for your response.

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