QT return value from a signal?

Question

I'm running all my SQLite database operations in a separate thread to ensure that the GUI wont freeze up.

I'm doing this by connecting up signals and slots for the methods.

However now I need to find a way for the SQLite thread to return selected values to my main thread.

My header file:

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

// ....
// ....

signals:
    QString getUserNickS(QString channel);  
}  

class dbThread : public QObject
{
     Q_OBJECT

public:
     dbThread();

public slots:
     bool openDB(QString agentID);
     QString getUserNick(QString channel);

private:
     QSqlDatabase db;
};

I then open the DB connection inside MainWindow in a separate thread like this:

// Start database thread
QThread * thread = new QThread();
dbtrad = new dbThread();
dbtrad - > moveToThread(thread);

//dbtrad->openDB(agentID);
connect(this, SIGNAL(requestOpenDB(QString)), dbtrad, SLOT(openDB(QString)));
connect(this, SIGNAL(getUserNickS(QString)), dbtrad, SLOT(getUserNick(QString)));

thread - > start();

emit requestOpenDB(userID);

And finally at one point in my application I want to query the SQLite db for some info and receive a return value: (still inside MainWindow )

QString retVal = getUserNickS(channelId);
qDebug() << "RET VALUE -----> " + retVal;

The getUserNick method looks like this:

// Returns visitor nickname for a channelId
QString dbThread::getUserNick(QString channel) {
    bool ret = false;

    if (db.isOpen()) {
        QSqlQuery query(db);

        ret = query.exec(QString("select * from visitorInfo WHERE channelID = '%1' order by date(time) ASC LIMIT 1;").arg(channel));

        if (ret) {
            bool gotResults = false;
            while (query.next()) {
              gotResults = true;
              qDebug() << query.value(14).toString();
              return query.value(14).toString();
            }
            if (!gotResults) {
                qDebug() << "Name " + channel.replace("V", "");
                return "Name " + channel.replace("V", "");
            }

        } else {
            qDebug() << "Name " + channel.replace("V", "");
            return "Name " + channel.replace("V", "");
        }
    }
}

Any ideas how I can get the returned value?

Answer 1

You should have a connection of type Qt::BlockingQueuedConnection when the emitter and receiver are in different threads.

So it should be like:

connect(this, SIGNAL(getUserNickS(QString)), dbtrad, SLOT(getUserNick(QString)),Qt::BlockingQueuedConnection);

When you emit your signal, it causes the current thread to block until the slot is returned.