QAudioDecoder Fails to start from a thread

DukeReddy

Hello all,

I have a weird issue with QAudioDecoder usage. When I call decoder->start() from a widget class (GUI) it works fine. bufferReady signal is emitted and it works fine.

I wanted to decode multiple WAV file at once. So, I created a QThread inherited class to decode concurrently. The problem is that decoder->start() does not result in the start of decoding process.

void AudioDecoder::run()
{
    m_pDecoder = new QAudioDecoder();
    m_pDecoder->setSource(m_fileUrl);

    // m_pDecoder->moveToThread(this);

    qDebug () << connect(m_pDecoder, &QAudioDecoder::bufferReady, this, &AudioDecoder::BufferReady);
    qDebug () << connect(m_pDecoder, &QAudioDecoder::finished, this, &AudioDecoder::DecodingFinished);
    qDebug () << connect(m_pDecoder, QOverload<QAudioDecoder::Error>::of(&QAudioDecoder::error), this, &AudioDecoder::HandleDecodingError);

    while(1)
    {
        //! Block for a result to be updated.
        m_pSem->acquire();

        if(this->isInterruptionRequested())
        {
            return;
        }

        m_pDecoder->start();

        qInfo() << "Source: " << m_pDecoder->source();
        qInfo() << "Error: " << m_pDecoder->errorString();
    }
}

Output

Source:  QUrl("file:///home/duke/Test/AudioSamples/Dummy/sample-s16-16k-256kbps.wav")
Error:  ""

But the slots that are connected are not called.

void AudioDecoder::BufferReady()
{
    LogOutput(LOG_DEBUG, "Buffer Ready");
    QAudioBuffer buffer = m_pDecoder->read();

    const quint16 *data = buffer.data<quint16>();
    m_decodedAudio.append((const char *)data, buffer.byteCount());

    qDebug() << "Decoded audio buffer ready, duration:" << buffer.duration() / 1000 << "ms";
    qDebug() << "Decoded audio buffer ready, position:" << m_pDecoder->position();
    qDebug() << "Decoded audio buffer ready, decodedBytes:" << m_decodedAudio.size();
}

Threaded class

class AudioDecoder : public QThread
{
    Q_OBJECT
public:
    explicit AudioDecoder(QObject *parent = nullptr);
    ~AudioDecoder();
...
...

This is how the threads are instantiated in the GUI.

void AudioStreamWidget::DecodeAudioFiles()
{
    StopAllThreads();

    foreach (QFileInfo fileInfo, m_fileInfoList)
    {
        AudioDecoder* pDecoder = new AudioDecoder(this);
        connect(pDecoder, &AudioDecoder::finished, this, &AudioStreamWidget::ThreadFinished);

        pDecoder->StartDecoding(fileInfo.absoluteFilePath());
        pDecoder->start(); //! Thread start
    }
}

Qt Version 6.5.1.

I believe the issue is with how I am using the threaded class.
Can anyone help me figure out the issue?

Best regards,
Duke

Pl45m4

@DukeReddy said in QAudioDecoder Fails to start from a thread:

foreach (QFileInfo fileInfo, m_fileInfoList)
{
    AudioDecoder* pDecoder = new AudioDecoder(this);
    connect(pDecoder, &AudioDecoder::finished, this, &AudioStreamWidget::ThreadFinished);

    pDecoder->StartDecoding(fileInfo.absoluteFilePath());
    pDecoder->start(); //! Thread start
}

Here you create a new AudioDecoder instance in the GUI thread

void AudioDecoder::run()
{
    m_pDecoder = new QAudioDecoder();
    m_pDecoder->setSource(m_fileUrl);
   // ..
}

and here you create a new instance in AudioDecoder

Check out the Qt Thread examples:

https://doc.qt.io/qt-6/examples-threadandconcurrent.html

and the general threading options with Qt since you are mixing two approaches.

https://doc.qt.io/qt-6/threads-technologies.html

You've subsclassed QThread and use an infinite loop while expecting to receive signals.
I would go for the "Worker-Thread" approach and make AudioDecoder a QObject (instead of QThread). Then create a basic QThread and move your object to the thread with moveToThread.

As also used here:

https://wiki.qt.io/QThreads_general_usage