can not quit the child event loop!

[jgxy1123]

Hello, All!

I have problem with QEvent loop.

I have created a child event loop in QApplication main loop and I want to quit the child loop by connectiong a signal of UI thread to QEventLoop::quit(), which does not work!

The work flow is as following!

1. create a window which contains a QTableWidget and run

   • QApplication.exec()

2. connect the itemDoubleclicked signal of tablewidet to a slot.

   • connect(tablewidiget, &QTableWidiget::itemDoubleClicked,this,[this](){
     .....do something;
     });

3. in the slot, I will process something

   • in the middle of the slot, I will create a child loop to stop the thread

• connect(tablewidiget, &QTableWidiget::itemDoubleClicked,this,[this](){
  .....do something;
  QEventLoop loop;
  connect(this, signal(mySignal()), &loop,&EventLoop::quit);
  do something else;
  loop.exec();
  do something else;
  });

The child loop does not quit even though I can successfully emit mySignal()

Please help me!!!

Thank you very much!!!

[JonB]

@jgxy1123 said in can not quit the child event loop!:

connect(this, signal(mySignal()), &loop,&EventLoop::quit);

This either does not or should not work (should not compile). Use proper connect() syntax.

[jgxy1123]

@JonB Thank you for your help!

sorry for the wrong description of my code.
actually, my actual code for connecting singal and slot is as below:
connect(this, &WidgetClass::mySinal, &loop, &QEventLoop::quit);

the other codes for connecting singal and slot all work well , but not only this one.

do you have any other idea?

[JonB]

@jgxy1123
Your original code was not copied and pasted. I am doubting this is copied and pasted either? For my part at least, I will help only if you copy and paste code now, and show some context of where this appears and what your loop actually looks like.

[jgxy1123]

@JonB Hello, please refer to the below.
As you can see, myFunction2() does not run even though I emit signalQuitLoop() to quit the child loop in myFunction1().

----myDemo.h-------

#pragma once

#include <QtWidgets/QWidget>
#include "ui_MyDemo.h"
#include <QTableWidget>
#include <QEventLoop>
#include <QDebug>
#include <QThread>
#include <QTableWidgetItem>

QT_BEGIN_NAMESPACE
namespace Ui { class MyDemoClass; };
QT_END_NAMESPACE

class MyDemo : public QWidget
{
Q_OBJECT

public:
MyDemo(QWidget *parent = nullptr);
~MyDemo();
void setupUI();
void setupEvent();
void myFunction1();
void myFunction2();

signals:
void signalQuitLoop();

public slots:

private:
Ui::MyDemoClass *ui;
QTableWidget * tbWidget;
};

--------myDemo.cpp-------------------
#include "MyDemo.h"

MyDemo::MyDemo(QWidget *parent)
: QWidget(parent)
, ui(new Ui::MyDemoClass())
{
tbWidget = new QTableWidget(this);
setupUI();
setupEvent();
//ui->setupUi(this);
}

MyDemo::~MyDemo()
{
delete ui;
}

void MyDemo::setupUI()
{
//tbWidget
tbWidget->setColumnCount(2);
tbWidget->setRowCount(2);
tbWidget->setEditTriggers(QAbstractItemView::NoEditTriggers);

for (auto row : { 0,1 }) {
    for (auto col : { 0,1 }) {

        QTableWidgetItem* item = new QTableWidgetItem(QString("row:%1,col:%2").arg(row).arg(col));
        tbWidget->setItem(row, col, item);
    }
}

//Main UI
setFixedSize(800, 600);

}

void MyDemo::setupEvent()
{

connect(tbWidget, &QTableWidget::itemDoubleClicked,this,[this](QTableWidgetItem *item) {

    qDebug() << "the slot for tbWidget triggered";

    QEventLoop loop;

    connect(this, &MyDemo::signalQuitLoop, &loop, &QEventLoop::quit);

    myFunction1();

    loop.exec();

    myFunction2();


    });

}

void MyDemo::myFunction1()
{

for (auto i : {1, 2, 3, 4, 5}) {

    qDebug() << "I am myFunction1()";
    QThread::sleep(2);
}

emit signalQuitLoop();

}

void MyDemo::myFunction2()
{
for (auto i : {1, 2, 3, 4, 5}) {

    qDebug() << "I am myFunction2()";
}

}

[jsulm]

@jgxy1123 said in can not quit the child event loop!:

myFunction1();

loop.exec();

You're starting the event loop AFTER function myFunction1() terminates! So, I guess QEventLoop::quit is called BEFORE you start the event loop and has no effect...

[jgxy1123]

@jsulm Thank you so much, you helped me a lot!
As you said, the slot to quit loop is called before loop.exec(), so there is never a chance to quit loop.