qt5base-lts/examples/qtconcurrent/progressdialog/main.cpp

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#include <QtWidgets>
#include <QtConcurrent>

using namespace QtConcurrent;

const int iterations = 20;

void spin(int &iteration)
{
    const int work = 1000 * 1000 * 40;
    volatile int v = 0;
    for (int j = 0; j < work; ++j)
        ++v;

    qDebug() << "iteration" << iteration << "in thread" << QThread::currentThreadId();
}

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    // Prepare the vector.
    QVector<int> vector;
    for (int i = 0; i < iterations; ++i)
        vector.append(i);

    // Create a progress dialog.
    QProgressDialog dialog;
    dialog.setLabelText(QString("Progressing using %1 thread(s)...").arg(QThread::idealThreadCount()));

    // Create a QFutureWatcher and connect signals and slots.
    QFutureWatcher<void> futureWatcher;
    QObject::connect(&futureWatcher, SIGNAL(finished()), &dialog, SLOT(reset()));
    QObject::connect(&dialog, SIGNAL(canceled()), &futureWatcher, SLOT(cancel()));
    QObject::connect(&futureWatcher, SIGNAL(progressRangeChanged(int,int)), &dialog, SLOT(setRange(int,int)));
    QObject::connect(&futureWatcher, SIGNAL(progressValueChanged(int)), &dialog, SLOT(setValue(int)));

    // Start the computation.
    futureWatcher.setFuture(QtConcurrent::map(vector, spin));

    // Display the dialog and start the event loop.
    dialog.exec();

    futureWatcher.waitForFinished();

    // Query the future to check if was canceled.
    qDebug() << "Canceled?" << futureWatcher.future().isCanceled();
}