create qt thread event loop

Question

I am using Qt in order to write a GUI application.

A main thread is responsible for the GUI and creates an QThread in order to do some work with an object.

class Worker
{
    void start() {
        QTimer* timer = new Timer();
        connect(timer,SIGNAL(timeout()),this,SLOT(do()));
    }

    void do() {
        //do some stuff
        emit finished();
    }
}



class GUI
{
    //do some GUI work then call startWorker();

    void startWorker() {
        QThread* thread = new Thread();
        Worker* worker = new Worker();

        worker->moveToThread(thread);

        connect(thread, SIGNAL(started()), worker, SLOT(start()));
        connect(worker, SIGNAL(finished()), workerthread, SLOT(quit()));
        connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
    }
}

Now I have several problems:

The timer in my worker class does not work. Maybe it is because the new thread has no event loop, but I have no idea how to create such one. I tried

connect(workerthread, SIGNAL(started()), workerthread, SLOT(exec()));

but it does not work either.

When I try to wait on the new thread, the signal is never sent

class GUI
{
    void exit() {
        thread->wait();
    }
}

I think it also is because there is no event loop and because of that no signal is emitted.

Does anybody have an idea how to solve these problems?

Did you forget to call [start](http://qt-project.org/doc/qt-4.8/qthread.html#start) on your thread? It's not in your code example. — cgmb, Apr 02 '13 at 23:25

LemonCool · Answer 1 · 2013-04-02T22:25:28.077

why not use qthreadpool, than you make your task class inherits from qrunnable and qobject, this way you can use signals and slots to pass data from one thread to another, is much simpler to implement, and increase performance from not recreating a thread or having one sleeping all the time

class myTask : public QObject, public QRunnable{
Q_OBJECT

protected:
void run(); //where you actually implement what is supposed to do

signals:
void done(int data);//change int to whatever data type you need

}

//moc click example, or use a timer to call this function every x amount of time
void button_click(){
   myTask *task = new myTask();
   task->setAutoDelete(true);
   connect(task,SIGNAL(done(int)),this,SLOT(after_done(int)),Qt::QueuedConnection);
   QThreadPool::globalInstance()->start(task);
}

by default you application gets 1 thread automatically, which you can use to handle the graphic, than use the qthreadpool to process the data/object on demand, you can even set the max amount of threads your application can use to process new request, the others will stay in a queue until one thread is freed

QThreadPool::globalInstance()->setMaxThreadCount(5);

thank you, this is a really simple solution an I will use it in future. But now I decided to use another solution for my current problem, because I want so do some frequently work in another thread. Now my solution is to implement an own thread and use exec() to have an event loop in it — Sven Jung, Apr 04 '13 at 06:37
sounds good, I also want you to notice that it does use a second thread(in my case up to 5 threads at same time), and once the task is done the thread doesn't get destroyed, it gets reused increasing performance after all threads are expensive.Anyways glad could help — LemonCool, Apr 04 '13 at 16:31

score 2 · Answer 2 · edited May 17 '16 at 08:50

this is a sample code for you :

QThread* thread = new QThread();
Worker* worker = new Worker(3000);
worker->moveToThread(thread);
QObject::connect(thread, SIGNAL(started()), worker, SLOT(start()));
thread->start();`

class Worker : public QObject
{
    Q_OBJECT
public:
    explicit Worker(qint32,QObject *parent = 0);
    qint32 myTime;

signals:
    void   workFinished();

public slots:
    void doWork();
    void start();
    private:
    QTimer *timer;
};


#include "worker.h"
#include <QTimer>
#include <QDebug>
Worker::Worker(qint32 t,QObject *parent) :
    QObject(parent)
{
    myTime=t;
}

void Worker::start()
{
    timer = new QTimer();
    timer->start(myTime);
    qDebug()<<QString("start work in time:%1").arg(myTime);
    connect(timer,SIGNAL(timeout()),this,SLOT(doWork()));
}

void Worker::doWork()
{
    qDebug()<<"dowork";
    timer->stop();
    emit workFinished();
}

Debug results :

start work in time:3000

I hope this helps you.

yeah it works, but now I decided to use another solution. I am using an own implemented thread now — Sven Jung, Apr 04 '13 at 06:35
@SvenJung Is this the accept answer?I need reputation,so if it is,please set it. — Jack, Apr 06 '13 at 13:29

create qt thread event loop

2 Answers2