How to make the main thread wait for one of the functions of the child thread to end

I use qthread. Because I really don't know how to give a runnable example, I can only briefly describe it. A child thread will be generated when the main thread is running. This child thread will call A (), B (), C () in succession A value will be returned in B (). The main thread needs this value to continue the following calculation. However, it will waste a lot of time to wait for the end of the whole sub thread. I'm not familiar with threads. I hope I can get an answer.

Solution 1:

Mmm there is a bunch of ways of doing it... I'll show you one... probably bad one, but it is one way of doing it...

Read comments & ask questions when lost.

class mainWindow : public QWidget {
Q_OBJECT
    QLabel *mMyLabel;
Q_SIGNALS:
    void sHandleProcessedData(const QString &data);
private Q_SLOTS:
    inline void handleProcessedData(const QString &data) {
        mMyLabel->setText(data);
        /// This should be your Main Thread.
        qDebug() << "We are in thread : " << QThread::currentThread() << QThread::currentThread()->objectName();
    };
public:
    mainWindow() {
        /// Take a note of your thread 
        qDebug() << "We are in thread : " << QThread::currentThread() << QThread::currentThread()->objectName();
        /*!
         * Simple example gui to show processed data
         */
        auto lay = new QGridLayout(this);
        mMyLabel = new QLabel("I Will be replaced by worker thread data!");
        lay->addWidget(mMyLabel);
        auto btn = new QPushButton("Do Processing");
        connect(btn, &QPushButton::released, this, &mainWindow::spawnProcess);
        lay->addWidget(btn);
        /*!
         * Lazy thread message hockup using signals 
         */
        connect(this, &mainWindow::sHandleProcessedData, this, &mainWindow::handleProcessedData, Qt::QueuedConnection); // We want to FORCE queued connection as to not execute this function in worker thread context. We have to be in MAIN thread.
    }

    inline void spawnProcess() {
        /*!
         * I'll Use QtConcurrent coz I'm lazy. With Lambda using this as capture. 
         */
        QtConcurrent::run(this, [this]() {
            /// Lots and lots of processing in another thread.
            /// Once processing is done, we will send the result via signal to main app.
            qDebug() << "We are in thread : " << QThread::currentThread() << QThread::currentThread()->objectName();
            Q_EMIT sHandleProcessedData("Some Magical data"); // This will change the Label text to this.
        });
    }
};