myDesign用于线程通信是否可以接受？

Question

I am currently trying to create a good and working Qt design for communication between several threads. I have a preference window, which emits different signals up on clicking on apply. For example one for creating the SQL connection and one changing other stuff. I want to change the preferences in a background thread in different classes and after making the changes they shall emit a result signal. In my preferences window I now want to wait until all signals are received (either with a true or a false result) before I either close the window or print an error message.

I tried to draw my design in the attached picture. Is that the correct way for my purpose? I am currently struggling with the way of waiting for all results. I was thinking of creating some kind of array to save every result and check the array, whether all signals are received. But that sounds pretty ugly... Is there a better method to wait until all signals are received?

Also is it a good idea to make the classes in the background thread as a singelton ? I only need one instance of the classes and that would make the access to the classes pretty easy since I do not need to drag the pointers to every object, which needs to know the classes.

Also i would like to know, whether it is a good Idea to store a public member in the MySQL Class, which tells me, whether the Database is connected and access it directly out of other threads?

Thank you!

Answer 1

The QStateMachine will do exactly what you want: it can transition between states when it receives signals.

The background threads might not need to be based on classes, and no matter what they definitely shouldn't be singletons. Most likely you can give a functor to QtConcurrent::run , and emit the signal there.

The logic should be factored out into a separate QObject :

// https://github.com/KubaO/stackoverflown/tree/master/questions/thread-jobs-39109247
#include <QtWidgets>
#include <QtConcurrent>
#include <functional>

class Controller : public QObject {
    Q_OBJECT
    QStateMachine m_machine{this};
    QState s_init{&m_machine};
    QState s_busy{&m_machine};
    QState s_idle{&m_machine};
    int m_activeTasks = 0;
    void onTaskStarted() {
        ++ m_activeTasks;
        emit taskRunning();
    }
    void onTaskDone() {
        if (--m_activeTasks == 0) emit allTasksDone();
    }
    Q_SIGNAL void taskRunning();
    Q_SIGNAL void allTasksDone();
    Q_SIGNAL void task1Done(int result);
    Q_SIGNAL void task2Done(int result);
public:
    Q_SIGNAL void active();
    Q_SIGNAL void finished();
    Q_SLOT void doTask1() {
        onTaskStarted();
        QtConcurrent::run([this]{
            QThread::sleep(2); // pretend we do some work
            emit task1Done(42);
        });
    }
    Q_SLOT void doTask2() {
        onTaskStarted();
        QtConcurrent::run([this]{
            QThread::sleep(5); // pretend we do some work
            emit task2Done(44);
        });
    }
    Controller(QObject * parent = nullptr) :
        QObject{parent}
    {
        // This describes the state machine
        s_init.addTransition(this, &Controller::taskRunning, &s_busy);
        s_idle.addTransition(this, &Controller::taskRunning, &s_busy);
        s_busy.addTransition(this, &Controller::allTasksDone, &s_idle);
        m_machine.setInitialState(&s_init);
        m_machine.start();
        //
        connect(this, &Controller::task1Done, this, [this](int result){
            onTaskDone();
            qDebug() << "task 1 is done with result" << result;
        });
        connect(this, &Controller::task2Done, this, [this](int result){
            onTaskDone();
            qDebug() << "task 2 is done with result" << result;
        });
        connect(&s_busy, &QState::entered, this, &Controller::active);
        connect(&s_idle, &QState::entered, this, &Controller::finished);
    }
};

Q_GLOBAL_STATIC(QStringListModel, model)
int main(int argc, char ** argv) {
    using Q = QObject;
    QApplication app{argc, argv};
    Controller ctl;
    QWidget w;
    QFormLayout layout{&w};
    QPushButton start1{"Start Task 1"};
    QPushButton start2{"Start Task 2"};
    QListView log;
    layout.addRow(&start1);
    layout.addRow(&start2);
    layout.addRow(&log);
    Q::connect(&start1, &QPushButton::clicked, &ctl, &Controller::doTask1);
    Q::connect(&start2, &QPushButton::clicked, &ctl, &Controller::doTask2);
    Q::connect(&ctl, &Controller::active, []{ qDebug() << "Active"; });
    Q::connect(&ctl, &Controller::finished, []{ qDebug() << "Finished"; });

    log.setModel(model);
    qInstallMessageHandler(+[](QtMsgType, const QMessageLogContext &, const QString & msg){
        auto row = model->rowCount();
        model->insertRow(row);
        model->setData(model->index(row), msg);
    });
    w.show();
    return app.exec();
}
#include "main.moc"