QThread never runs/finishes before it can be used?

Question

I have created a custom QObject class called EncodeThread, which looks as follows:

class EncodeThread : public QObject {
    Q_OBJECT

public:
    void set(SWSL::Video* v, QStringList f, QDir vDir);
    void run();

public slots:
    void encode();

signals:
    void encodeProgress(int i);

private:
    SWSL::Video* video;
    QStringList files;
    QDir videoDir;
};

As may be obvious, this class is used for encoding a video using an external library. Encode() contains the actual encoding routine, run() is a function I added while troubleshooting, though it's obviously non-functional:

void EncodeThread::run() {
    if (currentThread() != this) {
        // caller is in different thread.
        QMetaObject::invokeMethod(this, "encode", Qt::QueuedConnection);
    }
    else {
        encode();
    }
}

The problem is when I use a QThread and the moveToThread() function on the EncodeThread instance, namely that nothing seems to happen. No data is written, and the instance never emits the signal which should save the encoded file to disk.

encThread.set(video, files, videoDir);
connect(&encThread, SIGNAL(encodeProgress(int)), cookVideoProgress, SLOT(setValue(int)));
    connect(&encThread, SIGNAL(finished()), this, SLOT(videoCookEnd()));
    connect(this, SIGNAL(videoEncode()), &encThread, SLOT(encode()));
encThread.moveToThread(&thread);
    thread.start();

The above is how the whole setup is started. EncThread and thread variables are declared in the MainWindow class. I have made the set() function of EncodeThread call encode() after attempts to call encode() from the main thread using signals and QMetaObject failed.

I'm not new to threading, having used native Windows and Linux threads, as well as those of various cross-platform implementations, but QThreads really seem to baffle me. Any suggestions are more than welcome :)

I originally had the videoEncode() signal emitted right after starting the thread, but without any change in behaviour compared to the current problem. — MayaPosch

Michael Burr Michael Burr · Accepted Answer · 2011-05-06T07:17:49

Probably way to late to be any help to you, but here's a little demo program that puts an EncoderThread class to work. It probably doesn't quite mesh with your design (which your question only had fragments of), but it demonstrates running an object instance on its own thread and wiring 2 objects on different threads via signals/slots to let them communicate:

#include <stdio.h>
#include <QObject>
#include <QThread>
#include <QtCore/QCoreApplication>

// QSleeper is just a toy utility class that makes the
//  protected QThread::sleep() family of functions
//  publicly accessible.  It's only use is for demo
//  programs like this
class Sleeper : QThread
{
public:
    static void sleep(unsigned long secs) { QThread::sleep(secs); }
    static void msleep(unsigned long msecs) { QThread::msleep(msecs); }
    static void usleep(unsigned long usecs) { QThread::usleep(usecs); }

};


// an Encoder class that maintains itself on is own thread
class EncodeThread : public QObject {
    Q_OBJECT

public:
    EncodeThread();

public slots:
    void encode();

signals:
    void encodeProgress(int i);
    void finished();

private:
    QThread myThread;
};

EncodeThread::EncodeThread() {
    moveToThread(&myThread);
    myThread.start();
}


void EncodeThread::encode()
{
    printf("EncodeThread::encode() on thread %u\n", (unsigned int) QThread::currentThreadId());

    for (int i = 0; i < 6; ++i) {
        // encode for 1 second or so
        printf("EncodeThread::encode() working on thread %u\n", (unsigned int) QThread::currentThreadId());
        Sleeper::sleep(1);
        emit encodeProgress(i);
    }

    emit finished();
    printf("EncodeThread::encode() - done\n");
}




// a controller to manage and monitor an EncoderThread instance
class VideoEncoderController : public QObject
{
    Q_OBJECT
public:
    void start();

public slots:
    void setValue(int);
    void encodingDone();

signals:
    void encodingBegin();
};

void VideoEncoderController::start()
{
    printf("VideoEncoderController::start() on thread %u\n", (unsigned int) QThread::currentThreadId());
    emit encodingBegin();
}

void VideoEncoderController::setValue(int x)
{
    printf("VideoEncoderController::setValue(int %d) on thread %u\n", x, (unsigned int) QThread::currentThreadId());
}

void VideoEncoderController::encodingDone()
{
    printf("VideoEncoderController::encodingDone() on thread %u\n", (unsigned int) QThread::currentThreadId());
}




// a demo program that wires up a VideoEncoderController object to
//  an EncoderThread
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    EncodeThread encThread;
    VideoEncoderController controller;

    QObject::connect(&encThread, SIGNAL(encodeProgress(int)), &controller, SLOT(setValue(int)));
    QObject::connect(&encThread, SIGNAL(finished()), &controller, SLOT(encodingDone()));
    QObject::connect(&controller, SIGNAL(encodingBegin()), &encThread, SLOT(encode()));

    printf("hello world on thread %u\n", (unsigned int) QThread::currentThreadId ());

    controller.start();

    return a.exec();
}



#include "main.moc"

QThread never runs/finishes before it can be used?

3 Answers