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Help about QChart and repainting it

  • Hello everyone, I'm new in this community, as well as in QT programming and GUI programming. I'm using QT Creator to develop a project for the university and, since I don't have much knowledge about how programming a GUI except a bit of C#, I'm sure some of my questions could sound stupid.
    First of all, one of the page of my GUI should plot a QChart though a QChartView. In it there is a QScatterSeries with 0 points at first. Then, around every minute, in the final version of the project a thread should create a list of points to be added and a list of points to be removed, let's call it T1, which is a non-GUI thread (potentially written with standard C++ library, non QT) focused on collecting data (e.g. though socket connections on a LAN). At the moment, let's focus just on the list of points to add.
    My idea is the following: when the T1 has collected all the data (every minute, every 1.5 minutes) signals it with a std::condition_variable. I did that the GUI thread, when creating the layout, uses a thread (let's call T2) of the QThreadPool::globalInstance() to wait for the notification on this condition variable.
    Then, I tried to pass the pointer of the QScatterSeries to the istance of the subclass of QRunnable and in the method run I wrote the following code:

      *series << QPointF(x, y)<< QPointF(x, y+1)<< QPointF(x, y+2)<< QPointF(x, y+3)<< QPointF(x, y+4);

    And I noticed that these points are immadiately showed on the screen! But this T2 is a thread of the pool, not the GUI thread. How is it possible? Or it's normal but I should avoid? I thought that in the method run I should have emited a signal to slot which the GUi thread should have executed, to repaint the QChartView.
    Another question: this way am I paint a point at time? What could I do do add many points and showing them all together?

    I hope to have been the clearest as possible, and thank you for the attention!

  • Been a few months since I did some stuff with QCharts....

    If I remember right you add series to the chart - in which case when you modify the series, you will see that modification right away, From what I am seeing you are dereferencing a pointer that is likely to that series. Regardless of thread, if you have a pointer it will write to the original address and that is probably why you are getting this unexpected behavior.

  • Yes, you're perfectly right. So is it legit what I did, or is it against the principle of GUi Programming?

    Besides I tried to do the followinf double for cycle:

    for(x=0; x < 15; x+=0.2)
                for(y=0; y < 10; y +=0.2, count++)
                    series->insert(count, QPointF(x, y));

    And it works in both the GUI Thread and T2, but I imagined the GUi immediately freezes and can't continua to work, so I should find a way to process all the series together I suppose..

  • @KuriGohan

    1. QSeries << operator ( is just the same as QSeries::append(). I don't know whether QSeries::insert() is any slower than append() (try it and see?), otherwise I would expect them to take the same amount of time.

    2. The difference is your second code is adding a lot more points than your first!

    3. I think there was a thread recently where we were discussing the speed/GUI "freeze" when adding lots of points. You're adding say 2,500 points. That takes time! You can make the GUI be more responsive by calling intermittently, e.g. each time round your x loop (not too frequently, though, else it will actually slow things down; I think we found every 100 points or so was good). Also, you are adding your points one at a time; something like adds a list, I can't recall if we found that was any faster.

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