When I start a thread second time my app is freezing

Hello! I'm trying to get title of a YouTube video and display it on main window. This operation is blocking my app second time when I press the button from main window so I created a class that represents a QObject and I moved it to another thread. But the same happens, when I start the thread second time my app is not responding.

My code:
title.h

Code:

---

#ifndef TITLE_H
#define TITLE_H
 
#include <QUrl>
#include <QWidget>
#include <QTextDocument>
#include <QFile>
 
class Title : public QObject
{
    Q_OBJECT
public:
    Title(QWidget *parent = nullptr);
    ~Title();
    void searchTile(const QUrl &);
    void htmlGet(const QUrl &);
 
signals:
    void titleReady(QString);
    void True(QString);
 
private:
    QFile file = QFile("data.json");
    QTextDocument h;
    bool find(QJsonArray, QString, const QUrl &);
};
 
#endif // TITLE_H

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title.cpp

Code:

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#include <title.h>
#include <QScopedPointer>
#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QRegularExpression>
#include <QJsonDocument>
#include <QJsonObject>
#include <QJsonArray>
 
Title::Title(QWidget *parent) : QObject(parent){
 
}
 
Title::~Title(){
 
}
 
void Title::searchTile(const QUrl &url){
    if(file.exists() && file.size() != 0){
        file.open(QIODevice::ReadOnly | QIODevice::Text); //I have a json file where I save data like title and url of a video
        QString val = file.readAll();
        file.close();
        QJsonDocument d = QJsonDocument::fromJson(val.toUtf8());
        QJsonArray list = d.array();
        bool checkUrl = this->find(list, "url", url);
        if(checkUrl)
            emit True("URL is already in download list!"); //this is for showing an error message box
        else
            this->htmlGet(url);
    }
    else
        this->htmlGet(url);
}
 
void Title::htmlGet(const QUrl &url) {
   QScopedPointer<QNetworkAccessManager> manager(new QNetworkAccessManager);
   QNetworkReply *response = manager->get(QNetworkRequest(url));
   qDebug() << "titluuuu";
   connect(response, &QNetworkReply::finished, [response, this]{
      response->deleteLater();
      response->manager()->deleteLater();
      if (response->error() != QNetworkReply::NoError) return;
      auto const contentType =
            response->header(QNetworkRequest::ContentTypeHeader).toString();
      static QRegularExpression re("charset=([!-~]+)");
      auto const match = re.match(contentType);
      if (!match.hasMatch() || 0 != match.captured(1).compare("utf-8", Qt::CaseInsensitive)) {
         qWarning() << "Content charsets other than utf-8 are not implemented yet:" << contentType;
         return;
      }
      auto const html = QString::fromUtf8(response->readAll());
      QRegularExpression rg("eow-title.*>");
      auto const m = rg.match(html);
      auto s = m.captured(0);
      QRegularExpression n("title=.*");
      auto a = n.match(s);
      h.setHtml(a.captured(0).split("\"")[1]);
      emit titleReady(QString(QChar(0x200E)) + h.toPlainText());
   }) && manager.take();
}
 
bool Title::find(QJsonArray arr, QString key, const QUrl &url){
    //this function is for searching if a url is already in the json file  
    for(const auto obj : arr){
        if(obj.toObject().value(key) == url.toString().split("&")[0])
            return true;
    }
    return false;
 
}

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mainwindow.h

Code:

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#include <QWidget>
#include <QThread>
#include <QJsonArray>
 
class mainWindow : public QWidget
{
    Q_OBJECT
 
signals:
    void titleReady(QUrl);
 
private:
    int i = 0;
    QThread th;
    QJsonArray json;
    void displayTitle(QString);
    void startWork();
    void buttonPushed();
}

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mainwindow.cpp

Code:

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#include "mainwindow.h"
#include "title.h"
 
mainWindow::mainWindow(QWidget *parent) :
    QWidget(parent)
{
    title = new Title();
    title->moveToThread(&th);
    connect(title, &Title::True, this, &mainWindow::errorDialog);
    connect(&th, &QThread::finished, title, [this] {qDebug() << "finished"; });
    connect(this, &mainWindow::titleReady, title, &Title::searchTile);
    connect(&th, &QThread::started, this, &mainWindow::startWork);
    connect(title, &Title::titleReady, this, &mainWindow::displayTitle);
}
 
void mainWindow::buttonPushed(){
    th.start()
}
 
void mainWindow::startWork(){
    qDebug() << "Started";
    emit titleReady(QUrl(txt->text()));
}
 
void mainWindow::displayTitle(QString html){
    QJsonObject info;
 
    //I have a QComboBox modified with a QTreeView
    QModelIndex index_type = type_box->currentView()->rootIndex();
    QVariant data_type = type_box->currentView()->model()->data(index_type);
    info.insert("type", data_type.toString());
 
    QModelIndex index_ext = type_box->currentView()->currentIndex();
    QVariant data_ext = type_box->currentView()->model()->data(index_ext);
    info.insert("ext", data_ext.toString());
 
    info.insert("loc", loc_txt->text());
    info.insert("title", QJsonValue(html));
    info.insert("url", txt->text().split("&")[0]);
 
    json.push_back(info);
    QJsonDocument doc(json);
    file.open(QIODevice::WriteOnly | QIODevice::Text);
    QTextStream text(&file);
    text << doc.toJson();
    file.close();
 
    //On main window I have a table widget where I want to display some data in it
    QTableWidgetItem *item  = new QTableWidgetItem(QString(html));
    QTableWidgetItem *item1 = new QTableWidgetItem(QString("%1(%2)").arg(data_type.toString(), data_ext.toString()));
    QTableWidgetItem *item2 = new QTableWidgetItem(QString("-"));
    QTableWidgetItem *item3 = new QTableWidgetItem(QString("-"));
    QTableWidgetItem *item4 = new QTableWidgetItem(QString("Waiting..."));
 
    item1->setTextAlignment(Qt::AlignCenter);
    item2->setTextAlignment(Qt::AlignCenter);
    item3->setTextAlignment(Qt::AlignCenter);
    tem4->setTextAlignment(Qt::AlignCenter);
 
    table->setItem(i, 0, item);
    table->setItem(i, 1, item1);
    table->setItem(i, 2, item2);
    table->setItem(i, 3, item3);
    table->setItem(i, 4, item4);
 
    i++;  
    title->thread()->quit();
}

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How can I solve this? Only the second time I start the thread, the GUI is freezing and I'm wondering why this happens. I also tried displayTitle() like this:

Code:

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void mainWindow:: displayTitle (){
    qDebug << "test";
    title->thread()->quit();
}

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but it's the same.

I'm glad if you give me advice for optimization of my code if it's something wrong. Thank you!

QTextDocument is a GUI class which should imo not be used inside a thread. I don't see a reason why a thread is needed here at all. Starting a thread a second time is also imo not supported (and also not needed for your task).

Quote:

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Originally Posted by ChristianEhrlicher

QTextDocument is a GUI class which should imo not be used inside a thread. I don't see a reason why a thread is needed here at all. Starting a thread a second time is also imo not supported (and also not needed for your task).

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I agree with you but what can I do to make my gui not freezing because I also tried without another thread and when I clicked the button second time after starting the application always my gui freezed?