Why QNetworkAccessManager::finished is fired on the main thread?

Dmitriano

I created a QThread with a code like this:

void MyThread::run()
{
    QEventLoop loop;

    QNetworkAccessManager nm;

    QObject::connect(&nm, &QNetworkAccessManager::finished, this, [=](QNetworkReply *reply)
    {
         //the lambda is called on the main thread
    }

    QNetworkRequest request;
    request.setUrl(QUrl("https://api.binance.com/api/v3/exchangeInfo"));
    nm.get(request);
    
    loop.exec();
}

and expected that the lambda is called on the thread nm object belongs to, but it is called on the main thread. why?

JKSH

@Dmitriano said in Why QNetworkAccessManager::finished is fired on the main thread?:

and expected that the lambda is called on the thread nm object belongs to, but it is called on the main thread. why?

Because this rule applies to all QObjects: A slot will execute in the thread that the QObject lives in. (See https://doc.qt.io/qt-5/qobject.html#thread-affinity )

A QThread is a QObject that manages a secondary thread. However, your QThread object lives in the main thread. Therefore, its slots will execute in the main thread.

QObject::connect(&nm, &QNetworkAccessManager::finished, this, [=](QNetworkReply *reply)
{
    // This lambda runs in this->thread()
}

QObject::connect(&nm, &QNetworkAccessManager::finished, &nm, [=](QNetworkReply *reply)
{
    // This lambda runs in nm.thread()
}

// To see which thread each object lives in, run this code:
qDebug() << qApp->thread();
qDebug() << this->thread();
qDebug() << this;
qDebug() << nm.thread();

Note:

You don't need QEventLoop. Just call this->exec()
You don't even need a thread. QNetworkAccessManager is asynchronous -- you can run it in your main thread.

SGaist

Hi,

How are you using your MyThread class ?

Dmitriano

@SGaist I declare my thread class

    class MyThread : public QThread
    {
        Q_OBJECT;

    protected:

        void run() override;
    };

then I declare its instance as a member of MyModel class derived from QAbstractListModel that is created in QML.

in MyModel's constructor I call

m_myThread.start();

and when the app closes I call

m_myThread.quit();
m_myThread.wait();

SGaist

How did you determine that the lambda was called in the main thread ?

Dmitriano

@SGaist I was able to debug my app, see the picture
alt text

JKSH

@Dmitriano said in Why QNetworkAccessManager::finished is fired on the main thread?:

and expected that the lambda is called on the thread nm object belongs to, but it is called on the main thread. why?

Because this rule applies to all QObjects: A slot will execute in the thread that the QObject lives in. (See https://doc.qt.io/qt-5/qobject.html#thread-affinity )

A QThread is a QObject that manages a secondary thread. However, your QThread object lives in the main thread. Therefore, its slots will execute in the main thread.

QObject::connect(&nm, &QNetworkAccessManager::finished, this, [=](QNetworkReply *reply)
{
    // This lambda runs in this->thread()
}

QObject::connect(&nm, &QNetworkAccessManager::finished, &nm, [=](QNetworkReply *reply)
{
    // This lambda runs in nm.thread()
}

// To see which thread each object lives in, run this code:
qDebug() << qApp->thread();
qDebug() << this->thread();
qDebug() << this;
qDebug() << nm.thread();

Note:

You don't need QEventLoop. Just call this->exec()
You don't even need a thread. QNetworkAccessManager is asynchronous -- you can run it in your main thread.

Christian Ehrlicher

One more note:

Again a nice example on how to misuse lambdas