[수정2]Qt mp3파일 앨범사진 추출에 관련해서 질문이 있습니다.

    QWidget *m_metaDataFields[QMediaMetaData::NumMetaData] = {};
    QLabel *m_metaDataLabels[QMediaMetaData::NumMetaData] = {};
 
 
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Player::Player(QWidget *parent) : QWidget(parent)
{
    //! [create-objs]
    m_player = new QMediaPlayer(this);
    m_audioOutput = new QAudioOutput(this);
    m_player->setAudioOutput(m_audioOutput);
    //! [create-objs]
    connect(m_player, &QMediaPlayer::durationChanged, this, &Player::durationChanged);
    connect(m_player, &QMediaPlayer::positionChanged, this, &Player::positionChanged);
    connect(m_player, QOverload<>::of(&QMediaPlayer::metaDataChanged), this,
            &Player::metaDataChanged);
    connect(m_player, &QMediaPlayer::mediaStatusChanged, this, &Player::statusChanged);
    connect(m_player, &QMediaPlayer::bufferProgressChanged, this, &Player::bufferingProgress);
    connect(m_player, &QMediaPlayer::hasVideoChanged, this, &Player::videoAvailableChanged);
    connect(m_player, &QMediaPlayer::errorChanged, this, &Player::displayErrorMessage);
    connect(m_player, &QMediaPlayer::tracksChanged, this, &Player::tracksChanged);
 
    //! [2]
    m_videoWidget = new VideoWidget(this);
    m_videoWidget->resize(1280, 720);
    m_player->setVideoOutput(m_videoWidget);
 
    m_playlistModel = new PlaylistModel(this);
    m_playlist = m_playlistModel->playlist();
    //! [2]
    connect(m_playlist, &QMediaPlaylist::currentIndexChanged, this,
            &Player::playlistPositionChanged);
 
    // player layout
    QBoxLayout *layout = new QVBoxLayout(this);
 
    // display
    QBoxLayout *displayLayout = new QHBoxLayout;
    displayLayout->addWidget(m_videoWidget, 2);
#if !defined(Q_OS_ANDROID) && !defined(Q_OS_IOS)
    m_playlistView = new QListView();
    m_playlistView->setModel(m_playlistModel);
    m_playlistView->setCurrentIndex(m_playlistModel->index(m_playlist->currentIndex(), 0));
    connect(m_playlistView, &QAbstractItemView::activated, this, &Player::jump);
    displayLayout->addWidget(m_playlistView);
#endif
    layout->addLayout(displayLayout);
 
    // duration slider and label
    QHBoxLayout *hLayout = new QHBoxLayout;
 
    m_slider = new QSlider(Qt::Horizontal, this);
    m_slider->setRange(0, m_player->duration());
    connect(m_slider, &QSlider::sliderMoved, this, &Player::seek);
    hLayout->addWidget(m_slider);
 
    m_labelDuration = new QLabel();
    m_labelDuration->setSizePolicy(QSizePolicy::Preferred, QSizePolicy::Fixed);
    hLayout->addWidget(m_labelDuration);
    layout->addLayout(hLayout);
 
    // controls
    QBoxLayout *controlLayout = new QHBoxLayout;
    controlLayout->setContentsMargins(0, 0, 0, 0);
 
    QPushButton *openButton = new QPushButton(tr("Open"), this);
    connect(openButton, &QPushButton::clicked, this, &Player::open);
    controlLayout->addWidget(openButton);
    controlLayout->addStretch(1);
 
    PlayerControls *controls = new PlayerControls();
    controls->setState(m_player->playbackState());
    controls->setVolume(m_audioOutput->volume());
    controls->setMuted(controls->isMuted());
 
    connect(controls, &PlayerControls::play, m_player, &QMediaPlayer::play);
    connect(controls, &PlayerControls::pause, m_player, &QMediaPlayer::pause);
    connect(controls, &PlayerControls::stop, m_player, &QMediaPlayer::stop);
    connect(controls, &PlayerControls::next, m_playlist, &QMediaPlaylist::next);
    connect(controls, &PlayerControls::previous, this, &Player::previousClicked);
    connect(controls, &PlayerControls::changeVolume, m_audioOutput, &QAudioOutput::setVolume);
    connect(controls, &PlayerControls::changeMuting, m_audioOutput, &QAudioOutput::setMuted);
    connect(controls, &PlayerControls::changeRate, m_player, &QMediaPlayer::setPlaybackRate);
    connect(controls, &PlayerControls::stop, m_videoWidget, QOverload<>::of(&QVideoWidget::update));
 
    connect(m_player, &QMediaPlayer::playbackStateChanged, controls, &PlayerControls::setState);
    connect(m_audioOutput, &QAudioOutput::volumeChanged, controls, &PlayerControls::setVolume);
    connect(m_audioOutput, &QAudioOutput::mutedChanged, controls, &PlayerControls::setMuted);
 
    controlLayout->addWidget(controls);
    controlLayout->addStretch(1);
 
    m_fullScreenButton = new QPushButton(tr("FullScreen"), this);
    m_fullScreenButton->setCheckable(true);
    controlLayout->addWidget(m_fullScreenButton);
 
#if !defined(Q_OS_ANDROID) && !defined(Q_OS_IOS)
    m_audioOutputCombo = new QComboBox(this);
    m_audioOutputCombo->addItem(QString::fromUtf8("Default"), QVariant::fromValue(QAudioDevice()));
    for (auto &deviceInfo : QMediaDevices::audioOutputs())
        m_audioOutputCombo->addItem(deviceInfo.description(), QVariant::fromValue(deviceInfo));
    connect(m_audioOutputCombo, QOverload<int>::of(&QComboBox::activated), this,
            &Player::audioOutputChanged);
    controlLayout->addWidget(m_audioOutputCombo);
#endif
 
    layout->addLayout(controlLayout);
 
    // tracks
    QGridLayout *tracksLayout = new QGridLayout;
 
    m_audioTracks = new QComboBox(this);
    connect(m_audioTracks, &QComboBox::activated, this, &Player::selectAudioStream);
    tracksLayout->addWidget(new QLabel(tr("Audio Tracks:")), 0, 0);
    tracksLayout->addWidget(m_audioTracks, 0, 1);
 
    m_videoTracks = new QComboBox(this);
    connect(m_videoTracks, &QComboBox::activated, this, &Player::selectVideoStream);
    tracksLayout->addWidget(new QLabel(tr("Video Tracks:")), 1, 0);
    tracksLayout->addWidget(m_videoTracks, 1, 1);
 
    m_subtitleTracks = new QComboBox(this);
    connect(m_subtitleTracks, &QComboBox::activated, this, &Player::selectSubtitleStream);
    tracksLayout->addWidget(new QLabel(tr("Subtitle Tracks:")), 2, 0);
    tracksLayout->addWidget(m_subtitleTracks, 2, 1);
 
    layout->addLayout(tracksLayout);
 
#if !defined(Q_OS_ANDROID) && !defined(Q_OS_IOS)
    // metadata
 
    QLabel *metaDataLabel = new QLabel(tr("Metadata for file:"));
    layout->addWidget(metaDataLabel);
 
    QGridLayout *metaDataLayout = new QGridLayout;
    int key = QMediaMetaData::Title;
    for (int i = 0; i < (QMediaMetaData::NumMetaData + 2) / 3; i++) {
        for (int j = 0; j < 6; j += 2) {
            m_metaDataLabels[key] = new QLabel(
                    QMediaMetaData::metaDataKeyToString(static_cast<QMediaMetaData::Key>(key)));
            if (key == QMediaMetaData::ThumbnailImage || key == QMediaMetaData::CoverArtImage)
                m_metaDataFields[key] = new QLabel;
            else
                m_metaDataFields[key] = new QLineEdit;
            m_metaDataLabels[key]->setDisabled(true);
            m_metaDataFields[key]->setDisabled(true);
            metaDataLayout->addWidget(m_metaDataLabels[key], i, j);
            metaDataLayout->addWidget(m_metaDataFields[key], i, j + 1);
            key++;
            if (key == QMediaMetaData::NumMetaData)
                break;
        }
    }
 
    layout->addLayout(metaDataLayout);
#endif
 
#if defined(Q_OS_QNX)
    // On QNX, the main window doesn't have a title bar (or any other decorations).
    // Create a status bar for the status information instead.
    m_statusLabel = new QLabel;
    m_statusBar = new QStatusBar;
    m_statusBar->addPermanentWidget(m_statusLabel);
    m_statusBar->setSizeGripEnabled(false); // Without mouse grabbing, it doesn't work very well.
    layout->addWidget(m_statusBar);
#endif
 
    setLayout(layout);
 
    if (!isPlayerAvailable()) {
        QMessageBox::warning(this, tr("Service not available"),
                             tr("The QMediaPlayer object does not have a valid service.\n"
                                "Please check the media service plugins are installed."));
 
        controls->setEnabled(false);
        if (m_playlistView)
            m_playlistView->setEnabled(false);
        openButton->setEnabled(false);
        m_fullScreenButton->setEnabled(false);
    }
 
    metaDataChanged();
}
 
----------------------------------------------------------------------
 
 
void Player::metaDataChanged()
{
    auto metaData = m_player->metaData();
    setTrackInfo(QString("%1 - %2")
                         .arg(metaData.value(QMediaMetaData::AlbumArtist).toString())
                         .arg(metaData.value(QMediaMetaData::Title).toString()));
 
#if !defined(Q_OS_ANDROID) && !defined(Q_OS_IOS)
    for (int i = 0; i < QMediaMetaData::NumMetaData; i++) {
        if (QLineEdit *field = qobject_cast<QLineEdit *>(m_metaDataFields[i]))
            field->clear();
        else if (QLabel *label = qobject_cast<QLabel *>(m_metaDataFields[i]))
            label->clear();
        m_metaDataFields[i]->setDisabled(true);
        m_metaDataLabels[i]->setDisabled(true);
    }
 
    for (auto &key : metaData.keys()) {
        int i = int(key);
        if (key == QMediaMetaData::CoverArtImage) {
            QVariant v = metaData.value(key);
            if (QLabel *cover = qobject_cast<QLabel *>(m_metaDataFields[key])) {
                QImage coverImage = v.value<QImage>();
                cover->setPixmap(QPixmap::fromImage(coverImage));
            }
        } else if (key == QMediaMetaData::ThumbnailImage) {
            QVariant v = metaData.value(key);
            if (QLabel *thumbnail = qobject_cast<QLabel *>(m_metaDataFields[key])) {
                QImage thumbnailImage = v.value<QImage>();
                thumbnail->setPixmap(QPixmap::fromImage(thumbnailImage));
            }
        } else if (QLineEdit *field = qobject_cast<QLineEdit *>(m_metaDataFields[key])) {
            QString stringValue = metaData.stringValue(key);
            field->setText(stringValue);
        }
        m_metaDataFields[i]->setDisabled(false);
        m_metaDataLabels[i]->setDisabled(false);
    }
#endif
}

void MainWindow::metaDataChanged() {
    metaData = mediaPlayer->metaData();
    titleLabel->setText(metaData.stringValue(QMediaMetaData::Title));
    albumLabel->setText(metaData.stringValue(QMediaMetaData::AlbumTitle));
    artistLabel->setText(metaData.stringValue(QMediaMetaData::AlbumArtist));
 
 
    QVariant var = metaData.value(QMediaMetaData::CoverArtImage);
    QImage image = var.value<QImage>();
 
    albumArtLabel->setPixmap(QPixmap::fromImage(image));
}