QMetaObject :: invokeMethod（）用于调用QListWidget :: addItem（）时返回false

Question

I am trying to add an item to a QListWidget owned by the main thread from a background thread in a threadsafe manner by using QMetaObject::invokeMethod().

QMetaObject::invokeMethod() works and returns true when I use it to call clear() on the QListWidget but returns false when I use it to call addItem(), as seen in this code:

void BlockingInvokeStringArg(QObject* widget, const char* functionName, const string& arg = "")
{
    QString argAsQString(arg.c_str());
    QGenericArgument genericArg = arg.empty() ? QGenericArgument() : Q_ARG(QString, argAsQString);
    if (!QMetaObject::invokeMethod(widget, functionName, Qt::BlockingQueuedConnection, genericArg))
    {
        throw runtime_error("QMetaObject::invokeMethod() returned false");
    }
}

void BacktestGui::on_buttonRunBacktest_clicked()
{
    auto runBacktest = [&]()
    {
        // Does not throw:
        BlockingInvokeStringArg(m_ui.listSymbols, "clear");
        // Throws:
        BlockingInvokeStringArg(m_ui.listSymbols, "addItem", "ItemName"); 
        // Does not throw but may be thread-unsafe due to a background thread interacting with a GUI component owned by the main thread:
        m_ui.listSymbols->addItem("ItemName");
    };
    QtConcurrent::run(runBacktest);
}

Why does QMetaObject::invokeMethod() return true when calling clear() on a QListWidget but return false when calling addItem()?

If QMetaObject::invokeMethod() cannot be used to call addItem(), do you know a threadsafe alternative for adding an item to a QListWidget from a background thread?

Answer 1

invokeMethod() is able to invoke a signal or a slot. QListWidget::clear() is a slot, so that works. QListWidget::addItem() is not a slot, therefore you can't invoke it.

The solution is simple. Write a slot that takes the QListWidget as an argument and which calls addItem() on it, and invoke that slot.