Using QEventLoop to block connected to multiple signals

Question

Using a QEventLoop to block execution in a Qt program, how can you connect 10 individual signals to the one loop in such a way that it won't unblock until all 10 signals are received?

Answer 1

Please avoid using nested loops when possible. But if you are absolutely sure that you have no way around, you need to have a way to store which of the signals have fired and which of them haven't, and quit the event loop (i.e. maybe by emitting a signal connected to your event loop's QEventLoop::quit) only when all signals have fired.

Here is a minimum example that uses 10 QTimers with different intervals, and waits for all of them to fire before quitting a nested event loop:

#include <QtCore>
#include <algorithm>

int main(int argc, char* argv[]) {
    QCoreApplication a(argc, argv);
    const int n = 10;
    //10 timers to emit timeout signals on different intervals
    QTimer timers[n];
    //an array that stores whether each timer has fired or not
    bool timerFired[n]= {};
    QEventLoop loop;
    //setup and connect timer signals
    for(int i=0; i<n; i++) {
        timers[i].setSingleShot(true);
        QObject::connect(&timers[i], &QTimer::timeout, [i, &timerFired, &loop]{
            qDebug() << "timer " << i << " fired";
            timerFired[i]=true;
            //if all timers have fired
            if(std::all_of(std::begin(timerFired), std::end(timerFired),
                           [](bool b){ return b; }))
                loop.quit(); //quit event loop
        });
        timers[i].start(i*i*100);
    }
    qDebug() << "executing loop";
    loop.exec();
    qDebug() << "loop finished";

    QTimer::singleShot(0, &a, &QCoreApplication::quit);
    return a.exec();
}