How to create a QComboBox in another thread?

MasterBlade

Hello I need to use a second thread for performance optimization. But soon I found that Qt only allows to create widgets in the main thread. I then emitted a signal to the main thread asking it to create a QComboBox but it still failed. App output the follow errors. What should I do?

QObject: Cannot create children for a parent that is in a different thread.
(Parent is EffectTable(0xdac7480f0), parent's thread is QThread(0x1eaf27f09d0), current thread is QThreadPoolThread(0x1eaf4939fd0)

My code looks like this

    //Constructor
    connect(this, &EffectTable::NeedWidget,  this, &EffectTable::PrepWidget, Qt::QueuedConnection);
    QtConcurrent::run(&EffectTable::ReadINI, this);
    //Constructor

void EffectTable::ReadINI()
{
    QSettings set("...", QSettings::IniFormat, this);

    set.beginGroup("Basic");
    QStringList list = set.value("ID").toStringList();
    qint32      size = list.size();
    set.endGroup();

    emit NeedWidget(count);
}

void EffectTable::PrepWidget(qint32 count)
{
    QComboBox*   cb0 = new QComboBox[count];        //Error here
    QPushButton* pb0 = new QPushButton[count];

}

Christian Ehrlicher

You can not create widgets in a different thread than the main gui thread (and I don't see a reason for this here).

MasterBlade

@Christian-Ehrlicher said in How to create a QComboBox in another thread?:

You can not create widgets in a different thread than the main gui thread (and I don't see a reason for this here).

So my problem is, How to switch to the main thread?

jsulm

@MasterBlade said in How to create a QComboBox in another thread?:

So my problem is, How to switch to the main thread?

using signals/slots

MasterBlade

@jsulm said in How to create a QComboBox in another thread?:

@MasterBlade said in How to create a QComboBox in another thread?:

So my problem is, How to switch to the main thread?

using signals/slots

You see my code, I was trying to emit signals. Maybe it was not working as expected...

jsulm

@MasterBlade I'm not sure what the problem is, but I guess the reason is that you use same object in main thread and in an additional thread. I suggest you do not emit a signal, but use QFutureWatcher which has https://doc.qt.io/qt-5/qfuturewatcher.html#finished signal. So, your ReadINI() will return count which you then can get from the QFuture as soon as https://doc.qt.io/qt-5/qfuturewatcher.html#finished is emitted.