QNetworkAccessManager即使在另一个线程中也会冻结GUI

Question

When my program opens, a connection is made to my server via QNetworkAccessManager::connectToHostEncrypted() which is called in the constructor of MainWindow . This freezes the GUI thread and causes a noticeable delay until the connection is finished. (Sometimes over a full second longer)

This problem is worsened by the fact that my program fades in at startup, so while the GUI thread is blocked, the fade in doesn't start until after the connection is done. In a default Qt project, this is noticeable in other ways like widgets not being painted.

To keep the GUI thread going, I moved QNetworkAccessManager into a completely different thread with QThread thinking this would solve the issue, however the GUI thread still freezes. This makes no sense to me.

Here is a minimally compilable example project.

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QNetworkAccessManager>
#include <QThread>

class Connection : public QNetworkAccessManager
{
    Q_OBJECT
public:
    Connection(QObject *parent) : QNetworkAccessManager(parent){}

public slots:
    void openConnection(){
        connectToHostEncrypted("https://www.url.com");
    }

signals:
    void closeThread(bool);
};


namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;

    QThread *connectionThread;
    Connection *connection;
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    connection = new Connection(this);

    connectionThread = new QThread();
    connection->moveToThread(connectionThread);
    connect(connectionThread, SIGNAL(started()), connection, SLOT(openConnection()));
    connect(connection, SIGNAL(closeThread(bool)), connectionThread, SLOT(quit()));
    connect(connectionThread, SIGNAL(finished()), connectionThread, SLOT(deleteLater()));

    connectionThread->start();
}

MainWindow::~MainWindow()
{
    delete connection;
    delete ui;
}

This example project creates an instance of Connection which is a subclass of QNetworkAccessManager that I then move to another thread via moveToThread() . This is how I do all my worker threads.

When the thread's start() signal is emitted, openConnection() calls connectToHostEncrypted() which is where the GUI thread freezes.

I have tried just calling a regular HTTP request instead, however the problem persists since an initial connection still needs to be made.

How come the GUI thread still freezes even though the connection is done in another thread?

Answer 1

尝试使用：

connect(connectionThread, SIGNAL(started()), connection, SLOT(openConnection(),Qt::QueuedConnection);

Answer 2

The issue was I passed this as a parent to the Connection instance which meant moveToThread() couldn't be completed. I just had to check the output log to see that, but I must have missed it!

By removing that parent, the connection was now in it's own thread.

I further tested this by calling QThread::sleep(3) inside that openConnection() call and there was still no delay.