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Basic Media Player

Goal

This tutorial shows how to create a basic media player with Qt and QtGStreamer. It assumes that you are already familiar with the basics of Qt and GStreamer. If not, please refer to the other tutorials in this documentation.

In particular, you will learn:

How to create a basic pipeline
How to create a video output
Updating the GUI based on playback time

A media player with Qt

These files are located in the qt-gstreamer SDK's examples/ directory.

Due to the length of these samples, they are initially hidden. Click on each file to expand.

CMakeLists.txt

CMakeLists.txt

project(qtgst-example-player)
find_package(QtGStreamer REQUIRED)
## automoc is now a built-in tool since CMake 2.8.6.
if (${CMAKE_VERSION} VERSION_LESS "2.8.6")
    find_package(Automoc4 REQUIRED)
else()
    set(CMAKE_AUTOMOC TRUE)
    macro(automoc4_add_executable)
        add_executable(${ARGV})
    endmacro()
endif()
include_directories(${QTGSTREAMER_INCLUDES} ${CMAKE_CURRENT_BINARY_DIR} ${QT_QTWIDGETS_INCLUDE_DIRS})
add_definitions(${QTGSTREAMER_DEFINITIONS})
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${QTGSTREAMER_FLAGS}")
set(player_SOURCES main.cpp player.cpp mediaapp.cpp)
automoc4_add_executable(player ${player_SOURCES})
target_link_libraries(player ${QTGSTREAMER_UI_LIBRARIES} ${QT_QTOPENGL_LIBRARIES} ${QT_QTWIDGETS_LIBRARIES})

main.cpp

main.cpp

#include "mediaapp.h"
#include <QtWidgets/QApplication>
#include <QGst/Init>
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QGst::init(&argc, &argv);
    MediaApp media;
    media.show();
    if (argc == 2) {
        media.openFile(argv[1]);
    }
    return app.exec();
}

mediaapp.h

mediaapp.h

#ifndef MEDIAAPP_H
#define MEDIAAPP_H
#include <QtCore/QTimer>
#include <QtWidgets/QWidget>
#include <QtWidgets/QStyle>
class Player;
class QBoxLayout;
class QLabel;
class QSlider;
class QToolButton;
class QTimer;
class MediaApp : public QWidget
{
    Q_OBJECT
public:
    MediaApp(QWidget *parent = 0);
    ~MediaApp();
    void openFile(const QString & fileName);
private Q_SLOTS:
    void open();
    void toggleFullScreen();
    void onStateChanged();
    void onPositionChanged();
    void setPosition(int position);
    void showControls(bool show = true);
    void hideControls() { showControls(false); }
protected:
    void mouseMoveEvent(QMouseEvent *event);
private:
    QToolButton *initButton(QStyle::StandardPixmap icon, const QString & tip,
                            QObject *dstobj, const char *slot_method, QLayout *layout);
    void createUI(QBoxLayout *appLayout);
    QString m_baseDir;
    Player *m_player;
    QToolButton *m_openButton;
    QToolButton *m_fullScreenButton;
    QToolButton *m_playButton;
    QToolButton *m_pauseButton;
    QToolButton *m_stopButton;
    QSlider *m_positionSlider;
    QSlider *m_volumeSlider;
    QLabel *m_positionLabel;
    QLabel *m_volumeLabel;
    QTimer m_fullScreenTimer;
};
#endif

mediaapp.cpp

mediaapp.cpp

#include "mediaapp.h"
#include "player.h"
#if (QT_VERSION >= QT_VERSION_CHECK(5, 0, 0))
#include <QtWidgets/QBoxLayout>
#include <QtWidgets/QFileDialog>
#include <QtWidgets/QToolButton>
#include <QtWidgets/QLabel>
#include <QtWidgets/QSlider>
#else
#include <QtGui/QBoxLayout>
#include <QtGui/QFileDialog>
#include <QtGui/QToolButton>
#include <QtGui/QLabel>
#include <QtGui/QSlider>
#include <QtGui/QMouseEvent>
#endif
MediaApp::MediaApp(QWidget *parent)
    : QWidget(parent)
{
    //create the player
    m_player = new Player(this);
    connect(m_player, SIGNAL(positionChanged()), this, SLOT(onPositionChanged()));
    connect(m_player, SIGNAL(stateChanged()), this, SLOT(onStateChanged()));
    //m_baseDir is used to remember the last directory that was used.
    //defaults to the current working directory
    m_baseDir = QLatin1String(".");
    //this timer (re-)hides the controls after a few seconds when we are in fullscreen mode
    m_fullScreenTimer.setSingleShot(true);
    connect(&m_fullScreenTimer, SIGNAL(timeout()), this, SLOT(hideControls()));
    //create the UI
    QVBoxLayout *appLayout = new QVBoxLayout;
    appLayout->setContentsMargins(0, 0, 0, 0);
    createUI(appLayout);
    setLayout(appLayout);
    onStateChanged(); //set the controls to their default state
    setWindowTitle(tr("QtGStreamer example player"));
    resize(400, 400);
}
MediaApp::~MediaApp()
{
    delete m_player;
}
void MediaApp::openFile(const QString & fileName)
{
    m_baseDir = QFileInfo(fileName).path();
    m_player->stop();
    m_player->setUri(fileName);
    m_player->play();
}
void MediaApp::open()
{
    QString fileName = QFileDialog::getOpenFileName(this, tr("Open a Movie"), m_baseDir);
    if (!fileName.isEmpty()) {
        openFile(fileName);
    }
}
void MediaApp::toggleFullScreen()
{
    if (isFullScreen()) {
        setMouseTracking(false);
        m_player->setMouseTracking(false);
        m_fullScreenTimer.stop();
        showControls();
        showNormal();
    } else {
        setMouseTracking(true);
        m_player->setMouseTracking(true);
        hideControls();
        showFullScreen();
    }
}
void MediaApp::onStateChanged()
{
    QGst::State newState = m_player->state();
    m_playButton->setEnabled(newState != QGst::StatePlaying);
    m_pauseButton->setEnabled(newState == QGst::StatePlaying);
    m_stopButton->setEnabled(newState != QGst::StateNull);
    m_positionSlider->setEnabled(newState != QGst::StateNull);
    m_volumeSlider->setEnabled(newState != QGst::StateNull);
    m_volumeLabel->setEnabled(newState != QGst::StateNull);
    m_volumeSlider->setValue(m_player->volume());
    //if we are in Null state, call onPositionChanged() to restore
    //the position of the slider and the text on the label
    if (newState == QGst::StateNull) {
        onPositionChanged();
    }
}
/* Called when the positionChanged() is received from the player */
void MediaApp::onPositionChanged()
{
    QTime length(0,0);
    QTime curpos(0,0);
    if (m_player->state() != QGst::StateReady &&
        m_player->state() != QGst::StateNull)
    {
        length = m_player->length();
        curpos = m_player->position();
    }
    m_positionLabel->setText(curpos.toString("hh:mm:ss.zzz")
                                        + "/" +
                             length.toString("hh:mm:ss.zzz"));
    if (length != QTime(0,0)) {
        m_positionSlider->setValue(curpos.msecsTo(QTime(0,0)) * 1000 / length.msecsTo(QTime(0,0)));
    } else {
        m_positionSlider->setValue(0);
    }
    if (curpos != QTime(0,0)) {
        m_positionLabel->setEnabled(true);
        m_positionSlider->setEnabled(true);
    }
}
/* Called when the user changes the slider's position */
void MediaApp::setPosition(int value)
{
    uint length = -m_player->length().msecsTo(QTime(0,0));
    if (length != 0 && value > 0) {
        QTime pos(0,0);
        pos = pos.addMSecs(length * (value / 1000.0));
        m_player->setPosition(pos);
    }
}
void MediaApp::showControls(bool show)
{
    m_openButton->setVisible(show);
    m_playButton->setVisible(show);
    m_pauseButton->setVisible(show);
    m_stopButton->setVisible(show);
    m_fullScreenButton->setVisible(show);
    m_positionSlider->setVisible(show);
    m_volumeSlider->setVisible(show);
    m_volumeLabel->setVisible(show);
    m_positionLabel->setVisible(show);
}
void MediaApp::mouseMoveEvent(QMouseEvent *event)
{
    Q_UNUSED(event);
    if (isFullScreen()) {
        showControls();
        m_fullScreenTimer.start(3000); //re-hide controls after 3s
    }
}
QToolButton *MediaApp::initButton(QStyle::StandardPixmap icon, const QString & tip,
                                  QObject *dstobj, const char *slot_method, QLayout *layout)
{
    QToolButton *button = new QToolButton;
    button->setIcon(style()->standardIcon(icon));
    button->setIconSize(QSize(36, 36));
    button->setToolTip(tip);
    connect(button, SIGNAL(clicked()), dstobj, slot_method);
    layout->addWidget(button);
    return button;
}
void MediaApp::createUI(QBoxLayout *appLayout)
{
    appLayout->addWidget(m_player);
    m_positionLabel = new QLabel();
    m_positionSlider = new QSlider(Qt::Horizontal);
    m_positionSlider->setTickPosition(QSlider::TicksBelow);
    m_positionSlider->setTickInterval(10);
    m_positionSlider->setMaximum(1000);
    connect(m_positionSlider, SIGNAL(sliderMoved(int)), this, SLOT(setPosition(int)));
    m_volumeSlider = new QSlider(Qt::Horizontal);
    m_volumeSlider->setTickPosition(QSlider::TicksLeft);
    m_volumeSlider->setTickInterval(2);
    m_volumeSlider->setMaximum(10);
    m_volumeSlider->setMaximumSize(64,32);
    connect(m_volumeSlider, SIGNAL(sliderMoved(int)), m_player, SLOT(setVolume(int)));
    QGridLayout *posLayout = new QGridLayout;
    posLayout->addWidget(m_positionLabel, 1, 0);
    posLayout->addWidget(m_positionSlider, 1, 1, 1, 2);
    appLayout->addLayout(posLayout);
    QHBoxLayout *btnLayout = new QHBoxLayout;
    btnLayout->addStretch();
    m_openButton = initButton(QStyle::SP_DialogOpenButton, tr("Open File"),
                              this, SLOT(open()), btnLayout);
    m_playButton = initButton(QStyle::SP_MediaPlay, tr("Play"),
                              m_player, SLOT(play()), btnLayout);
    m_pauseButton = initButton(QStyle::SP_MediaPause, tr("Pause"),
                               m_player, SLOT(pause()), btnLayout);
    m_stopButton = initButton(QStyle::SP_MediaStop, tr("Stop"),
                              m_player, SLOT(stop()), btnLayout);
    m_fullScreenButton = initButton(QStyle::SP_TitleBarMaxButton, tr("Fullscreen"),
                                    this, SLOT(toggleFullScreen()), btnLayout);
    btnLayout->addStretch();
    m_volumeLabel = new QLabel();
    m_volumeLabel->setPixmap(
        style()->standardIcon(QStyle::SP_MediaVolume).pixmap(QSize(32, 32),
                QIcon::Normal, QIcon::On));
    btnLayout->addWidget(m_volumeLabel);
    btnLayout->addWidget(m_volumeSlider);
    appLayout->addLayout(btnLayout);
}
#include "moc_mediaapp.cpp"

player.h

player.h

#ifndef PLAYER_H
#define PLAYER_H
#include <QtCore/QTimer>
#include <QtCore/QTime>
#include <QGst/Pipeline>
#include <QGst/Ui/VideoWidget>
 
class Player : public QGst::Ui::VideoWidget
{
    Q_OBJECT
public:
    Player(QWidget *parent = 0);
    ~Player();
 
    void setUri(const QString &uri);
 
    QTime position() const;
    void setPosition(const QTime &pos);
    int volume() const;
    QTime length() const;
    QGst::State state() const;
 
public Q_SLOTS:
    void play();
    void pause();
    void stop();
    void setVolume(int volume);
 
Q_SIGNALS:
    void positionChanged();
    void stateChanged();
 
private:
    void onBusMessage(const QGst::MessagePtr &message);
    void handlePipelineStateChange(const QGst::StateChangedMessagePtr &scm);
 
    QGst::PipelinePtr m_pipeline;
    QTimer m_positionTimer;
};
 
#endif //PLAYER_H

player.cpp

player.cpp

#include "player.h"
#include <QtCore/QDir>
#include <QtCore/QUrl>
#include <QGlib/Connect>
#include <QGlib/Error>
#include <QGst/Pipeline>
#include <QGst/ElementFactory>
#include <QGst/Bus>
#include <QGst/Message>
#include <QGst/Query>
#include <QGst/ClockTime>
#include <QGst/Event>
#include <QGst/StreamVolume>
Player::Player(QWidget *parent)
    : QGst::Ui::VideoWidget(parent)
{
    //this timer is used to tell the ui to change its position slider & label
    //every 100 ms, but only when the pipeline is playing
    connect(&m_positionTimer, SIGNAL(timeout()), this, SIGNAL(positionChanged()));
}
Player::~Player()
{
    if (m_pipeline) {
        m_pipeline->setState(QGst::StateNull);
        stopPipelineWatch();
    }
}
void Player::setUri(const QString & uri)
{
    QString realUri = uri;
    //if uri is not a real uri, assume it is a file path
    if (realUri.indexOf("://") < 0) {
        realUri = QUrl::fromLocalFile(realUri).toEncoded();
    }
    if (!m_pipeline) {
        m_pipeline = QGst::ElementFactory::make("playbin").dynamicCast<QGst::Pipeline>();
        if (m_pipeline) {
            //let the video widget watch the pipeline for new video sinks
            watchPipeline(m_pipeline);
            //watch the bus for messages
            QGst::BusPtr bus = m_pipeline->bus();
            bus->addSignalWatch();
            QGlib::connect(bus, "message", this, &Player::onBusMessage);
        } else {
            qCritical() << "Failed to create the pipeline";
        }
    }
    if (m_pipeline) {
        m_pipeline->setProperty("uri", realUri);
    }
}
QTime Player::position() const
{
    if (m_pipeline) {
        //here we query the pipeline about its position
        //and we request that the result is returned in time format
        QGst::PositionQueryPtr query = QGst::PositionQuery::create(QGst::FormatTime);
        m_pipeline->query(query);
        return QGst::ClockTime(query->position()).toTime();
    } else {
        return QTime(0,0);
    }
}
void Player::setPosition(const QTime & pos)
{
    QGst::SeekEventPtr evt = QGst::SeekEvent::create(
        1.0, QGst::FormatTime, QGst::SeekFlagFlush,
        QGst::SeekTypeSet, QGst::ClockTime::fromTime(pos),
        QGst::SeekTypeNone, QGst::ClockTime::None
    );
    m_pipeline->sendEvent(evt);
}
int Player::volume() const
{
    if (m_pipeline) {
        QGst::StreamVolumePtr svp =
            m_pipeline.dynamicCast<QGst::StreamVolume>();
        if (svp) {
            return svp->volume(QGst::StreamVolumeFormatCubic) * 10;
        }
    }
    return 0;
}

void Player::setVolume(int volume)
{
    if (m_pipeline) {
        QGst::StreamVolumePtr svp =
            m_pipeline.dynamicCast<QGst::StreamVolume>();
        if(svp) {
            svp->setVolume((double)volume / 10, QGst::StreamVolumeFormatCubic);
        }
    }
}
QTime Player::length() const
{
    if (m_pipeline) {
        //here we query the pipeline about the content's duration
        //and we request that the result is returned in time format
        QGst::DurationQueryPtr query = QGst::DurationQuery::create(QGst::FormatTime);
        m_pipeline->query(query);
        return QGst::ClockTime(query->duration()).toTime();
    } else {
        return QTime(0,0);
    }
}
QGst::State Player::state() const
{
    return m_pipeline ? m_pipeline->currentState() : QGst::StateNull;
}
void Player::play()
{
    if (m_pipeline) {
        m_pipeline->setState(QGst::StatePlaying);
    }
}
void Player::pause()
{
    if (m_pipeline) {
        m_pipeline->setState(QGst::StatePaused);
    }
}
void Player::stop()
{
    if (m_pipeline) {
        m_pipeline->setState(QGst::StateNull);
        //once the pipeline stops, the bus is flushed so we will
        //not receive any StateChangedMessage about this.
        //so, to inform the ui, we have to emit this signal manually.
        Q_EMIT stateChanged();
    }
}
void Player::onBusMessage(const QGst::MessagePtr & message)
{
    switch (message->type()) {
    case QGst::MessageEos: //End of stream. We reached the end of the file.
        stop();
        break;
    case QGst::MessageError: //Some error occurred.
        qCritical() << message.staticCast<QGst::ErrorMessage>()->error();
        stop();
        break;
    case QGst::MessageStateChanged: //The element in message->source() has changed state
        if (message->source() == m_pipeline) {
            handlePipelineStateChange(message.staticCast<QGst::StateChangedMessage>());
        }
        break;
    default:
        break;
    }
}
void Player::handlePipelineStateChange(const QGst::StateChangedMessagePtr & scm)
{
    switch (scm->newState()) {
    case QGst::StatePlaying:
        //start the timer when the pipeline starts playing
        m_positionTimer.start(100);
        break;
    case QGst::StatePaused:
        //stop the timer when the pipeline pauses
        if(scm->oldState() == QGst::StatePlaying) {
            m_positionTimer.stop();
        }
        break;
    default:
        break;
    }
    Q_EMIT stateChanged();
}
#include "moc_player.cpp"

Walkthrough

Setting up GStreamer

We begin by looking at main():

main.cpp

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QGst::init(&argc, &argv);
    MediaApp media;
    media.show();
    if (argc == 2) {
        media.openFile(argv[1]);
    }
    return app.exec();
}

We first initialize QtGStreamer by calling QGst::init(), passing argc and argv. Internally, this ensures that the GLib type system and GStreamer plugin registry is configured and initialized, along with handling helpful environment variables such as GST_DEBUG and common command line options. Please see the Running GStreamer Applications section of the core reference manual for details.

Construction of the MediaApp (derived from QApplication) involves constructing the Player object and connecting its signals to the UI:

MediaApp::MediaApp()

    //create the player
    m_player = new Player(this);
    connect(m_player, SIGNAL(positionChanged()), this, SLOT(onPositionChanged()));
    connect(m_player, SIGNAL(stateChanged()), this, SLOT(onStateChanged()));

Next, we instruct the MediaApp to open the file given on the command line, if any:

MediaApp::openFile()

void MediaApp::openFile(const QString & fileName)
{
    m_baseDir = QFileInfo(fileName).path();
    m_player->stop();
    m_player->setUri(fileName);
    m_player->play();
}

This in turn instructs the Player to construct our GStreamer pipeline:

Player::setUri()

void Player::setUri(const QString & uri)
{
    QString realUri = uri;
    //if uri is not a real uri, assume it is a file path
    if (realUri.indexOf("://") < 0) {
        realUri = QUrl::fromLocalFile(realUri).toEncoded();
    }
    if (!m_pipeline) {
        m_pipeline = QGst::ElementFactory::make("playbin").dynamicCast<QGst::Pipeline>();
        if (m_pipeline) {
            //let the video widget watch the pipeline for new video sinks
            watchPipeline(m_pipeline);
            //watch the bus for messages
            QGst::BusPtr bus = m_pipeline->bus();
            bus->addSignalWatch();
            QGlib::connect(bus, "message", this, &Player::onBusMessage);
        } else {
            qCritical() << "Failed to create the pipeline";
        }
    }
    if (m_pipeline) {
        m_pipeline->setProperty("uri", realUri);
    }
}

Here, we first ensure that the pipeline will receive a proper URI. If Player::setUri() is called with /home/user/some/file.mp3, the path is modified to file:///home/user/some/file.mp3. playbin only accepts complete URIs.

The pipeline is created via QGst::ElementFactory::make(). The Player object inherits from the QGst::Ui::VideoWidget class, which includes a function to watch for the prepare-xwindow-id message, which associates the underlying video sink with a Qt widget used for rendering. For clarity, here is a portion of the implementation:

prepare-xwindow-id handling

    QGlib::connect(pipeline->bus(), "sync-message",
                  this, &PipelineWatch::onBusSyncMessage);
...
void PipelineWatch::onBusSyncMessage(const MessagePtr & msg)
{   
...
        if (msg->internalStructure()->name() == QLatin1String("prepare-xwindow-id")) {
            XOverlayPtr overlay = msg->source().dynamicCast<XOverlay>();
            m_renderer->setVideoSink(overlay);
        }

Once the pipeline is created, we connect to the bus' message signal (via QGlib::connect()) to dispatch state change signals:

void Player::onBusMessage(const QGst::MessagePtr & message)
{
    switch (message->type()) {
    case QGst::MessageEos: //End of stream. We reached the end of the file.
        stop();
        break;
    case QGst::MessageError: //Some error occurred.
        qCritical() << message.staticCast<QGst::ErrorMessage>()->error();
        stop();
        break;
    case QGst::MessageStateChanged: //The element in message->source() has changed state
        if (message->source() == m_pipeline) {
            handlePipelineStateChange(message.staticCast<QGst::StateChangedMessage>());
        }
        break;
    default:
        break;
    }
}
void Player::handlePipelineStateChange(const QGst::StateChangedMessagePtr & scm)
{
    switch (scm->newState()) {
    case QGst::StatePlaying:
        //start the timer when the pipeline starts playing
        m_positionTimer.start(100);
        break;
    case QGst::StatePaused:
        //stop the timer when the pipeline pauses
        if(scm->oldState() == QGst::StatePlaying) {
            m_positionTimer.stop();
        }
        break;
    default:
        break;
    }
    Q_EMIT stateChanged();
}

Finally, we tell playbin what to play by setting the uri property:

m_pipeline->setProperty("uri", realUri);

Starting Playback

After Player::setUri() is called, MediaApp::openFile() calls play() on the Player object:

Player::play()

void Player::play()
{
    if (m_pipeline) {
        m_pipeline->setState(QGst::StatePlaying);
    }
}

The other state control methods are equally simple:

Player state functions

void Player::pause()
{
    if (m_pipeline) {
        m_pipeline->setState(QGst::StatePaused);
    }
}
void Player::stop()
{
    if (m_pipeline) {
        m_pipeline->setState(QGst::StateNull);
        //once the pipeline stops, the bus is flushed so we will
        //not receive any StateChangedMessage about this.
        //so, to inform the ui, we have to emit this signal manually.
        Q_EMIT stateChanged();
    }
}

Once the pipeline has entered the playing state, a state change message is emitted on the GStreamer bus which gets picked up by the Player:

Player::onBusMessage()

void Player::onBusMessage(const QGst::MessagePtr & message)
{
    switch (message->type()) {
    case QGst::MessageEos: //End of stream. We reached the end of the file.
        stop();
        break;
    case QGst::MessageError: //Some error occurred.
        qCritical() << message.staticCast<QGst::ErrorMessage>()->error();
        stop();
        break;
    case QGst::MessageStateChanged: //The element in message->source() has changed state
        if (message->source() == m_pipeline) {
            handlePipelineStateChange(message.staticCast<QGst::StateChangedMessage>());
        }
        break;
    default:
        break;
    }
}

The stateChanged signal we connected to earlier is emitted and handled:

MediaApp::onStateChanged()

void MediaApp::onStateChanged()
{
    QGst::State newState = m_player->state();
    m_playButton->setEnabled(newState != QGst::StatePlaying);
    m_pauseButton->setEnabled(newState == QGst::StatePlaying);
    m_stopButton->setEnabled(newState != QGst::StateNull);
    m_positionSlider->setEnabled(newState != QGst::StateNull);
    m_volumeSlider->setEnabled(newState != QGst::StateNull);
    m_volumeLabel->setEnabled(newState != QGst::StateNull);
    m_volumeSlider->setValue(m_player->volume());
    //if we are in Null state, call onPositionChanged() to restore
    //the position of the slider and the text on the label
    if (newState == QGst::StateNull) {
        onPositionChanged();
    }
}

This updates the UI to reflect the current state of the player's pipeline.

Driven by a QTimer, the Player emits the positionChanged signal at regular intervals for the UI to handle:

MediaApp::onPositionChanged()

void MediaApp::onPositionChanged()
{
    QTime length(0,0);
    QTime curpos(0,0);
    if (m_player->state() != QGst::StateReady &&
        m_player->state() != QGst::StateNull)
    {
        length = m_player->length();
        curpos = m_player->position();
    }
    m_positionLabel->setText(curpos.toString("hh:mm:ss.zzz")
                                        + "/" +
                             length.toString("hh:mm:ss.zzz"));
    if (length != QTime(0,0)) {
        m_positionSlider->setValue(curpos.msecsTo(QTime(0,0)) * 1000 / length.msecsTo(QTime(0,0)));
    } else {
        m_positionSlider->setValue(0);
    }
    if (curpos != QTime(0,0)) {
        m_positionLabel->setEnabled(true);
        m_positionSlider->setEnabled(true);
    }
}

The MediaApp queries the pipeline via the Player's position() method, which submits a position query. This is analogous to gst_element_query_position():

Player::position()

QTime Player::position() const
{
    if (m_pipeline) {
        //here we query the pipeline about its position
        //and we request that the result is returned in time format
        QGst::PositionQueryPtr query = QGst::PositionQuery::create(QGst::FormatTime);
        m_pipeline->query(query);
        return QGst::ClockTime(query->position()).toTime();
    } else {
        return QTime(0,0);
    }
}

Due to the way Qt handles signals that cross threads, there is no need to worry about calling UI functions from outside the UI thread in this example.

Conclusion

This tutorial has shown:

How to create a basic pipeline
How to create a video output
Updating the GUI based on playback time

It has been a pleasure having you here, and see you soon!

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