Why QtConcurrent::run freezing my program?

isan

I read QString from the database which has a lot of data
I want to split QString line by line and then use it,
When I use the for to split it, my app freezes and doesn't work,
I found that the run function QtConcurrent :: run a function in a separate thread
I did as below, but my application still freeze when calling function readfile

void MainWindow::getData(){
 QString getRecord=dbFile->getRecord(IdGet);

    QFuture<void> t1 = QtConcurrent::run(demoSimple,&ZoomScrollTrack2::readfile, getRecord);

    t1.waitForFinished();
}

void Track::readfile(QString Record){


    for(int u=0;u<Record.size();u++){
        QStringList line=Record.split("#");
         qDebug()<<"record list"<<line;

}
}

I try to use this way but I have error (no matching function for call to run)

QFuture<QList<QString>> future = QtConcurrent::run(IdGet, &QString::split, '#');
   QList<QString> result = future.result();
    qDebug()<<"list"<<result;

Is QtConcurrent::run function is a good way for my action or I need to use QThread with subclass?

Christian Ehrlicher

Since you're directly waiting for the result your gui freezes. You should connect to the QFuture signals and/or use QFutureWatcher. See https://doc.qt.io/qt-5/qtconcurrent-index.html

J.Hilk

@isan additional to what @Christian-Ehrlicher said, you may want to consider using splitRef('#') it returns a QVector<QStringRef> instead of a QList<QString> it is significantly faster, but it depends on the lifetime of the original QString!

KroMignon

@isan You need to use QFutureWatcher in combination with QEventLoop to not lock you main thread.
Something like this:

// start process in another thread
QFuture<void> future = QtConcurrent::run(demoSimple,&ZoomScrollTrack2::readfile, getRecord));

QFutureWatcher<void> watcher;
QEventLoop loop;
// QueuedConnection is necessary in case the signal finished is emitted before the loop starts
// (if the task is already finished when setFuture is called)
connect(&watcher, SIGNAL(finished()), &loop, SLOT(quit()),  Qt::QueuedConnection); 
watcher.setFuture(future);
qDebug()<<"wait for result";

// passive wait until QFuture is finished
loop.exec();
qDebug()<<"done";

isan

Thanks for your help
It works, but it takes a few minutes, is there a better way to make it faster?

J.Hilk

@isan compiling and running it in release mode will make it a good bit faster.

besides that, truncating your QString beforehand and running multiple QtConcurrent each one with part of the original string?

Christian Ehrlicher

@kromignon said in Why QtConcurrent::run freezing my program?:

You need to use QFutureWatcher in combination with QEventLoop to not lock you main thread.

This contradicts the whole idea to move the heavy work into an own thread...

Christian Ehrlicher

@isan said in Why QtConcurrent::run freezing my program?:

for(int u=0;u<Record.size();u++){
QStringList line=Record.split("#");
qDebug()<<"record list"<<line;

I hope this is not your real function since it basically does Record.size() times the same.

isan

@christian-ehrlicher
yes you said right, it should work without for()