Need help on QtConcurrent usage.

ercyd

Hi to all,
I'm doing some heavy computing, like accessing a database, that blocks my UI. So, I'm implementing the QtConcurrent::run function to start some class methods. These methods retrieve data from database table to fill out a classic QComboBox. After the
@ui->setupUi(this);@

in my form ctor I'm running these commands:

@
QFuture<void> f1 = QtConcurrent::run(this, &frmPractices_CRUD::populateComboSubjects);
QFuture<void> f2 = QtConcurrent::run(this, &frmPractices_CRUD::populateComboSellPoints);
f1.waitForFinished();
f2.waitForFinished();
@

and the code is compiled but I get a Visual C++ Runtime Library errors!
Then I tried to debug the functions and maybe my debugging skill isn't so good yet. I get the program works without errors only making the following changes:

@
QFuture<void> f1 = QtConcurrent::run(this, &frmPractices_CRUD::populateComboSubjects);
f1.waitForFinished();
QFuture<void> f2 = QtConcurrent::run(this, &frmPractices_CRUD::populateComboSellPoints);
f2.waitForFinished();
@

And I don't think that's the right way to use QtConcurrent, my populateComboABCD functions contain the same code:

@
ui->cboSubject->clear();

QSqlDatabase db = QSqlDatabase::database();
QString tSqlCommand =
    QString(
        "SELECT "
        "sog_id, "
        "concat_ws(' ', sog_cogn, sog_nome) AS names, "
        "concat_ws(' - ', sog_codf, sog_piva) AS fiscals "
        "FROM %1.subjects "
        "WHERE (sog_tipo = 1) OR (sog_tipo = 3) "
        "ORDER BY names"
    ).arg(MyQtUtility::instance()->schemaName());
QSqlQuery tSqlQuery(db);
if (tSqlQuery.exec&#40;tSqlCommand&#41;) {
    while (tSqlQuery.next()) {
        QString tmpCboText;

        tmpCboText += tSqlQuery.value(1).toString().simplified();
        if (!tSqlQuery.value(2).toString().simplified().isEmpty()) {
            tmpCboText += " / " + tSqlQuery.value(2).toString().simplified();
        }

        ui->cboSubject->addItem(tmpCboText.trimmed(), tSqlQuery.value(0).toInt());
    }
}
tSqlQuery.finish();

@

Thanks in advance,
Danilo

Chris Kawa

@
QFuture<void> f1 = QtConcurrent::run(this, &frmPractices_CRUD::populateComboSubjects);
f1.waitForFinished();
QFuture<void> f2 = QtConcurrent::run(this, &frmPractices_CRUD::populateComboSellPoints);
f2.waitForFinished();
@
You're right. It's not good this way. This is a sequential code. It is the same as
@
populateComboSubjects();
populateComboSellPoints();
@
The problem is not with QtConcurrent. It's with the populate* methods. They run in separate threads and you can access UI only in the main thread, so calls to ui->cboSubject->clear() and ui->cboSubject->addItem() are not allowed there.

You can run your sql queries in the worker threads but you need to signal the main thread to do the UI update.

ercyd

Thanks Chris, but commenting the ui->cboSubject calls I get the same errors. I noted that the debug mode is stopping on MyQtUtility::instance()->schemaName() (it's a singleton class) command whithout breakpoint!! I don't know how strange it is but seems a wrong behavior.

ercyd

I googled a bit and got the perception that a QSqlDatabase can not be used within two threads.

SGaist

Hi,

AFAIK, you need to at least create a different connection for each thread in order to access the database.

ercyd

Thanks Sgaist, I'll investigate that and if the game is worth the candle.