Multithreading on window constructors

[d_stranz]

Settings::Settings(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::Settings)

Settings::Settings(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::Settings)

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Why are you calling the QMainWindow constructor from the Settings constructor? Does this widget really inherit from QMainWindow? And if so, why are you adding something based on QMainWindow into a stacked widget which is itself inside a QMainWindow?

[nagendrababu]

Hi @d_stranz,
Yes all these are inherited from QMainWindow. The evolution of this application is, Initially started with creating individual screens of full screen dimension(800*480) But when changing between screens there is visible transition which looks like application running very slow. So then We have changed all windows to dimension of stacked widget(500*480) and added them to stacked widget of MainMenuWindow. From the qt forum posts I read, I understand this can be done so we went ahead with that approach instead of rewriting entire application.

Do you think this approach is causing this much delay? If that's the case from which template these screens should be inherited. QDialog or QWidget?

Thanks,
Nagendra.

[d_stranz]

That must be a very confusing GUI for users. You have the outer QMainWindow (that contains the stacked widget) and it has a menu. toolbar, and status bar, then inside that you have -more- QMainWindow classes, and each of them has a menu bar, toolbar, and status bar. Is that really what it looks like?

A typical application has a window hierarchy like this:

QApplication
    QMainWindow
        QWidget (central widget) - in your case this is QStackedWidget
            QWidget(s) - one for each page in the stack, including layouts

QApplication
    QMainWindow
        QWidget (central widget) - in your case this is QStackedWidget
            QWidget(s) - one for each page in the stack, including layouts

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Your application must look like this:

QApplication
    QMainWindow
         QStackedWidget (as central widget)
              QMainWindow 1
                    and what as the central widget?
              QMainWindow 2
                     and a different central widget?
              ...

QApplication
    QMainWindow
         QStackedWidget (as central widget)
              QMainWindow 1
                    and what as the central widget?
              QMainWindow 2
                     and a different central widget?
              ...

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Sounds very strange to me. I would replace all of the QMainWindow instance you use in the stacked widget with plain QWidget.

[nagendrababu]

Hi d_stranz,
I agree It might not be a good way, But as I told previously The evolution of app made us to take this kind of decision. And coming to UI, it don't has menubar, toolbar or status bar, So it looks cleaner and easy.

QMainWindow
     QStackedWidget
          QMainWindow:1
               centralwidget
                    all widgets under this screen
          QMainWindow:2
               centralwidget
                    all widgets under this screen

QMainWindow
     QStackedWidget
          QMainWindow:1
               centralwidget
                    all widgets under this screen
          QMainWindow:2
               centralwidget
                    all widgets under this screen

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I agree with the change, that All these windows should be Qwidgets, But it's like rewriting all my application again.
So do you think the constructors delay is caused by my approach only?

Thanks,
Nagendra

[d_stranz]

So do you think the constructors delay is caused by my approach only?

I can't see any other reason for it, but then you haven't showed much code. If your sub-main windows don't have menus or toolbars, then it makes no sense at all to do it the way you are doing it. It's a lot of overhead for absolutely nothing. You could just simply add your central widgets to the stacked widget and achieve the same thing.

[nagendrababu]

Hi @d_stranz,
I tried creating one of the window as Qwidget type instead of QMainWindow type and calculated time to execute the constructor in each case.
In case of QMainWinow:
Elapsed time Screen1: 1600ms
In case of QWidget:
Elapsed time Screen1: 1589ms

So from these metrics, I don't see much difference in time taken by constructor.

To be additional Question,
Is there any way we can implement threads to divide constructors between them to run them in parallel.

I tried something like this:

// Thread1 to init screens except Settings
    Thread1 = new QThread;
    connect(Thread1, SIGNAL(started()), this, SLOT(thread1_run()),Qt::DirectConnection);
    connect(this, SIGNAL(thread1Finished()), this, SLOT(thread1_finish()));
    // Thread2 to init Settings screen(Heavy Screen)
    Thread2 = new QThread;
    connect(Thread2, SIGNAL(started()), this, SLOT(thread2_run()),Qt::DirectConnection);
    connect(this, SIGNAL(thread2Finished()), this, SLOT(thread2_finish()));
    // Stack widgets
    connect(this,SIGNAL(threadsDone()),this,SLOT(stackWidgets()));
    Thread1->start(QThread::HighestPriority);
    Thread2->start(QThread::HighestPriority);

// Thread1 to init screens except Settings
    Thread1 = new QThread;
    connect(Thread1, SIGNAL(started()), this, SLOT(thread1_run()),Qt::DirectConnection);
    connect(this, SIGNAL(thread1Finished()), this, SLOT(thread1_finish()));
    // Thread2 to init Settings screen(Heavy Screen)
    Thread2 = new QThread;
    connect(Thread2, SIGNAL(started()), this, SLOT(thread2_run()),Qt::DirectConnection);
    connect(this, SIGNAL(thread2Finished()), this, SLOT(thread2_finish()));
    // Stack widgets
    connect(this,SIGNAL(threadsDone()),this,SLOT(stackWidgets()));
    Thread1->start(QThread::HighestPriority);
    Thread2->start(QThread::HighestPriority);

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But from Above code, The threads are executing one after another. They aren't running parallel. Can you help how to run these parallel?

Thanks,
Nagendra

[d_stranz]

Can you help how to run these parallel?

Not possible in Qt. GUI objects (things inheriting from QWidget or QWindow) can be created only in the thread that owns QApplication, and you can have only a single QApplication instance.

You need to do an experiment, I think. Create a new QMainWindow-based application, put a QStackedWidget as the central widget for the main window, and then just add a bunch of QWidget-based pages to them. Don't use your existing classes. Use QTreeView, QTableView, QGraphicsView, whatever. Time how long that takes. I think you'll find that your bottleneck is not in building your GUI, it's whatever you are doing in your application's sub-window constructors that's the cause.