# Basic Media Player
## Goal
This tutorial shows how to create a basic media player with
[Qt](http://qt-project.org/) and
[QtGStreamer](http://gstreamer.freedesktop.org/data/doc/gstreamer/head/qt-gstreamer/html/index.html).
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**
``` c
#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**
``` c
#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**
``` c
#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**
``` c
#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**
``` c
#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**
``` c
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](http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/gst-running.html)
section of the core reference manual for details.
Construction of the `MediaApp` (derived from
[`QApplication`](http://qt-project.org/doc/qt-5.0/qtwidgets/qapplication.html))
involves constructing the `Player` object and connecting its signals to
the UI:
**MediaApp::MediaApp()**
``` c
//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()**
``` c
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()**
``` c
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**
``` c
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:
``` c
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:
``` c
m_pipeline->setProperty("uri", realUri);
```
### Starting Playback
After `Player::setUri()` is called, `MediaApp::openFile()` calls
`play()` on the `Player` object:
**Player::play()**
``` c
void Player::play()
{
if (m_pipeline) {
m_pipeline->setState(QGst::StatePlaying);
}
}
```
The other state control methods are equally simple:
**Player state functions**
``` c
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()**
``` c
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()**
``` c
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`](http://qt-project.org/doc/qt-5.0/qtcore/qtimer.html), the
`Player` emits the `positionChanged` signal at regular intervals for the
UI to handle:
**MediaApp::onPositionChanged()**
``` c
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()**
``` c
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\!