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872 lines
25 KiB
Markdown
872 lines
25 KiB
Markdown
# Basic Media Player
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# Goal
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This tutorial shows how to create a basic media player with
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[Qt](http://qt-project.org/) and
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[QtGStreamer](http://gstreamer.freedesktop.org/data/doc/gstreamer/head/qt-gstreamer/html/index.html).
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It assumes that you are already familiar with the basics of Qt and
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GStreamer. If not, please refer to the other tutorials in this
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documentation.
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In particular, you will learn:
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- How to create a basic pipeline
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- How to create a video output
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- Updating the GUI based on playback time
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# A media player with Qt
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These files are located in the qt-gstreamer SDK's `examples/` directory.
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Due to the length of these samples, they are initially hidden. Click on
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each file to expand.
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CMakeLists.txt
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**CMakeLists.txt**
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```
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project(qtgst-example-player)
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find_package(QtGStreamer REQUIRED)
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# automoc is now a built-in tool since CMake 2.8.6.
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if (${CMAKE_VERSION} VERSION_LESS "2.8.6")
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find_package(Automoc4 REQUIRED)
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else()
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set(CMAKE_AUTOMOC TRUE)
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macro(automoc4_add_executable)
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add_executable(${ARGV})
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endmacro()
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endif()
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include_directories(${QTGSTREAMER_INCLUDES} ${CMAKE_CURRENT_BINARY_DIR} ${QT_QTWIDGETS_INCLUDE_DIRS})
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add_definitions(${QTGSTREAMER_DEFINITIONS})
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${QTGSTREAMER_FLAGS}")
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set(player_SOURCES main.cpp player.cpp mediaapp.cpp)
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automoc4_add_executable(player ${player_SOURCES})
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target_link_libraries(player ${QTGSTREAMER_UI_LIBRARIES} ${QT_QTOPENGL_LIBRARIES} ${QT_QTWIDGETS_LIBRARIES})
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```
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main.cpp
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**main.cpp**
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``` lang=c
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#include "mediaapp.h"
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#include <QtWidgets/QApplication>
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#include <QGst/Init>
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int main(int argc, char *argv[])
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{
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QApplication app(argc, argv);
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QGst::init(&argc, &argv);
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MediaApp media;
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media.show();
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if (argc == 2) {
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media.openFile(argv[1]);
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}
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return app.exec();
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}
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```
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mediaapp.h
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**mediaapp.h**
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``` lang=c
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#ifndef MEDIAAPP_H
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#define MEDIAAPP_H
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#include <QtCore/QTimer>
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#include <QtWidgets/QWidget>
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#include <QtWidgets/QStyle>
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class Player;
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class QBoxLayout;
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class QLabel;
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class QSlider;
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class QToolButton;
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class QTimer;
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class MediaApp : public QWidget
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{
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Q_OBJECT
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public:
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MediaApp(QWidget *parent = 0);
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~MediaApp();
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void openFile(const QString & fileName);
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private Q_SLOTS:
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void open();
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void toggleFullScreen();
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void onStateChanged();
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void onPositionChanged();
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void setPosition(int position);
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void showControls(bool show = true);
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void hideControls() { showControls(false); }
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protected:
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void mouseMoveEvent(QMouseEvent *event);
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private:
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QToolButton *initButton(QStyle::StandardPixmap icon, const QString & tip,
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QObject *dstobj, const char *slot_method, QLayout *layout);
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void createUI(QBoxLayout *appLayout);
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QString m_baseDir;
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Player *m_player;
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QToolButton *m_openButton;
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QToolButton *m_fullScreenButton;
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QToolButton *m_playButton;
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QToolButton *m_pauseButton;
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QToolButton *m_stopButton;
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QSlider *m_positionSlider;
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QSlider *m_volumeSlider;
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QLabel *m_positionLabel;
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QLabel *m_volumeLabel;
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QTimer m_fullScreenTimer;
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};
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#endif
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```
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mediaapp.cpp
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**mediaapp.cpp**
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``` lang=c
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#include "mediaapp.h"
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#include "player.h"
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#if (QT_VERSION >= QT_VERSION_CHECK(5, 0, 0))
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#include <QtWidgets/QBoxLayout>
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#include <QtWidgets/QFileDialog>
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#include <QtWidgets/QToolButton>
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#include <QtWidgets/QLabel>
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#include <QtWidgets/QSlider>
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#else
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#include <QtGui/QBoxLayout>
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#include <QtGui/QFileDialog>
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#include <QtGui/QToolButton>
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#include <QtGui/QLabel>
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#include <QtGui/QSlider>
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#include <QtGui/QMouseEvent>
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#endif
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MediaApp::MediaApp(QWidget *parent)
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: QWidget(parent)
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{
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//create the player
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m_player = new Player(this);
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connect(m_player, SIGNAL(positionChanged()), this, SLOT(onPositionChanged()));
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connect(m_player, SIGNAL(stateChanged()), this, SLOT(onStateChanged()));
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//m_baseDir is used to remember the last directory that was used.
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//defaults to the current working directory
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m_baseDir = QLatin1String(".");
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//this timer (re-)hides the controls after a few seconds when we are in fullscreen mode
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m_fullScreenTimer.setSingleShot(true);
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connect(&m_fullScreenTimer, SIGNAL(timeout()), this, SLOT(hideControls()));
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//create the UI
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QVBoxLayout *appLayout = new QVBoxLayout;
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appLayout->setContentsMargins(0, 0, 0, 0);
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createUI(appLayout);
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setLayout(appLayout);
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onStateChanged(); //set the controls to their default state
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setWindowTitle(tr("QtGStreamer example player"));
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resize(400, 400);
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}
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MediaApp::~MediaApp()
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{
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delete m_player;
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}
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void MediaApp::openFile(const QString & fileName)
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{
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m_baseDir = QFileInfo(fileName).path();
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m_player->stop();
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m_player->setUri(fileName);
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m_player->play();
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}
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void MediaApp::open()
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{
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QString fileName = QFileDialog::getOpenFileName(this, tr("Open a Movie"), m_baseDir);
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if (!fileName.isEmpty()) {
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openFile(fileName);
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}
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}
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void MediaApp::toggleFullScreen()
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{
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if (isFullScreen()) {
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setMouseTracking(false);
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m_player->setMouseTracking(false);
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m_fullScreenTimer.stop();
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showControls();
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showNormal();
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} else {
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setMouseTracking(true);
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m_player->setMouseTracking(true);
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hideControls();
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showFullScreen();
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}
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}
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void MediaApp::onStateChanged()
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{
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QGst::State newState = m_player->state();
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m_playButton->setEnabled(newState != QGst::StatePlaying);
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m_pauseButton->setEnabled(newState == QGst::StatePlaying);
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m_stopButton->setEnabled(newState != QGst::StateNull);
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m_positionSlider->setEnabled(newState != QGst::StateNull);
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m_volumeSlider->setEnabled(newState != QGst::StateNull);
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m_volumeLabel->setEnabled(newState != QGst::StateNull);
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m_volumeSlider->setValue(m_player->volume());
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//if we are in Null state, call onPositionChanged() to restore
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//the position of the slider and the text on the label
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if (newState == QGst::StateNull) {
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onPositionChanged();
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}
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}
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/* Called when the positionChanged() is received from the player */
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void MediaApp::onPositionChanged()
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{
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QTime length(0,0);
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QTime curpos(0,0);
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if (m_player->state() != QGst::StateReady &&
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m_player->state() != QGst::StateNull)
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{
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length = m_player->length();
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curpos = m_player->position();
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}
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m_positionLabel->setText(curpos.toString("hh:mm:ss.zzz")
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+ "/" +
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length.toString("hh:mm:ss.zzz"));
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if (length != QTime(0,0)) {
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m_positionSlider->setValue(curpos.msecsTo(QTime(0,0)) * 1000 / length.msecsTo(QTime(0,0)));
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} else {
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m_positionSlider->setValue(0);
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}
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if (curpos != QTime(0,0)) {
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m_positionLabel->setEnabled(true);
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m_positionSlider->setEnabled(true);
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}
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}
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/* Called when the user changes the slider's position */
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void MediaApp::setPosition(int value)
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{
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uint length = -m_player->length().msecsTo(QTime(0,0));
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if (length != 0 && value > 0) {
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QTime pos(0,0);
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pos = pos.addMSecs(length * (value / 1000.0));
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m_player->setPosition(pos);
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}
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}
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void MediaApp::showControls(bool show)
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{
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m_openButton->setVisible(show);
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m_playButton->setVisible(show);
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m_pauseButton->setVisible(show);
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m_stopButton->setVisible(show);
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m_fullScreenButton->setVisible(show);
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m_positionSlider->setVisible(show);
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m_volumeSlider->setVisible(show);
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m_volumeLabel->setVisible(show);
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m_positionLabel->setVisible(show);
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}
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void MediaApp::mouseMoveEvent(QMouseEvent *event)
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{
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Q_UNUSED(event);
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if (isFullScreen()) {
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showControls();
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m_fullScreenTimer.start(3000); //re-hide controls after 3s
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}
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}
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QToolButton *MediaApp::initButton(QStyle::StandardPixmap icon, const QString & tip,
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QObject *dstobj, const char *slot_method, QLayout *layout)
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{
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QToolButton *button = new QToolButton;
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button->setIcon(style()->standardIcon(icon));
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button->setIconSize(QSize(36, 36));
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button->setToolTip(tip);
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connect(button, SIGNAL(clicked()), dstobj, slot_method);
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layout->addWidget(button);
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return button;
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}
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void MediaApp::createUI(QBoxLayout *appLayout)
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{
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appLayout->addWidget(m_player);
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m_positionLabel = new QLabel();
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m_positionSlider = new QSlider(Qt::Horizontal);
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m_positionSlider->setTickPosition(QSlider::TicksBelow);
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m_positionSlider->setTickInterval(10);
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m_positionSlider->setMaximum(1000);
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connect(m_positionSlider, SIGNAL(sliderMoved(int)), this, SLOT(setPosition(int)));
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m_volumeSlider = new QSlider(Qt::Horizontal);
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m_volumeSlider->setTickPosition(QSlider::TicksLeft);
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m_volumeSlider->setTickInterval(2);
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m_volumeSlider->setMaximum(10);
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m_volumeSlider->setMaximumSize(64,32);
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connect(m_volumeSlider, SIGNAL(sliderMoved(int)), m_player, SLOT(setVolume(int)));
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QGridLayout *posLayout = new QGridLayout;
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posLayout->addWidget(m_positionLabel, 1, 0);
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posLayout->addWidget(m_positionSlider, 1, 1, 1, 2);
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appLayout->addLayout(posLayout);
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QHBoxLayout *btnLayout = new QHBoxLayout;
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btnLayout->addStretch();
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m_openButton = initButton(QStyle::SP_DialogOpenButton, tr("Open File"),
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this, SLOT(open()), btnLayout);
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m_playButton = initButton(QStyle::SP_MediaPlay, tr("Play"),
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m_player, SLOT(play()), btnLayout);
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m_pauseButton = initButton(QStyle::SP_MediaPause, tr("Pause"),
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m_player, SLOT(pause()), btnLayout);
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m_stopButton = initButton(QStyle::SP_MediaStop, tr("Stop"),
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m_player, SLOT(stop()), btnLayout);
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m_fullScreenButton = initButton(QStyle::SP_TitleBarMaxButton, tr("Fullscreen"),
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this, SLOT(toggleFullScreen()), btnLayout);
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btnLayout->addStretch();
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m_volumeLabel = new QLabel();
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m_volumeLabel->setPixmap(
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style()->standardIcon(QStyle::SP_MediaVolume).pixmap(QSize(32, 32),
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QIcon::Normal, QIcon::On));
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btnLayout->addWidget(m_volumeLabel);
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btnLayout->addWidget(m_volumeSlider);
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appLayout->addLayout(btnLayout);
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}
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#include "moc_mediaapp.cpp"
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```
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player.h
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**player.h**
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``` lang=c
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#ifndef PLAYER_H
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#define PLAYER_H
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#include <QtCore/QTimer>
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#include <QtCore/QTime>
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#include <QGst/Pipeline>
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#include <QGst/Ui/VideoWidget>
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class Player : public QGst::Ui::VideoWidget
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{
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Q_OBJECT
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public:
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Player(QWidget *parent = 0);
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~Player();
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void setUri(const QString &uri);
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QTime position() const;
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void setPosition(const QTime &pos);
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int volume() const;
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QTime length() const;
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QGst::State state() const;
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public Q_SLOTS:
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void play();
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void pause();
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void stop();
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void setVolume(int volume);
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Q_SIGNALS:
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void positionChanged();
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void stateChanged();
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private:
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void onBusMessage(const QGst::MessagePtr &message);
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void handlePipelineStateChange(const QGst::StateChangedMessagePtr &scm);
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QGst::PipelinePtr m_pipeline;
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QTimer m_positionTimer;
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};
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#endif //PLAYER_H
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```
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player.cpp
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**player.cpp**
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``` lang=c
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#include "player.h"
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#include <QtCore/QDir>
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#include <QtCore/QUrl>
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#include <QGlib/Connect>
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#include <QGlib/Error>
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#include <QGst/Pipeline>
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#include <QGst/ElementFactory>
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#include <QGst/Bus>
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#include <QGst/Message>
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#include <QGst/Query>
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#include <QGst/ClockTime>
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#include <QGst/Event>
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#include <QGst/StreamVolume>
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Player::Player(QWidget *parent)
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: QGst::Ui::VideoWidget(parent)
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{
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//this timer is used to tell the ui to change its position slider & label
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//every 100 ms, but only when the pipeline is playing
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connect(&m_positionTimer, SIGNAL(timeout()), this, SIGNAL(positionChanged()));
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}
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Player::~Player()
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{
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if (m_pipeline) {
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m_pipeline->setState(QGst::StateNull);
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stopPipelineWatch();
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}
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}
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void Player::setUri(const QString & uri)
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{
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QString realUri = uri;
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//if uri is not a real uri, assume it is a file path
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if (realUri.indexOf("://") < 0) {
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realUri = QUrl::fromLocalFile(realUri).toEncoded();
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}
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if (!m_pipeline) {
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m_pipeline = QGst::ElementFactory::make("playbin2").dynamicCast<QGst::Pipeline>();
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if (m_pipeline) {
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//let the video widget watch the pipeline for new video sinks
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watchPipeline(m_pipeline);
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//watch the bus for messages
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QGst::BusPtr bus = m_pipeline->bus();
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bus->addSignalWatch();
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QGlib::connect(bus, "message", this, &Player::onBusMessage);
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} else {
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qCritical() << "Failed to create the pipeline";
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}
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}
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if (m_pipeline) {
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m_pipeline->setProperty("uri", realUri);
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}
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}
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QTime Player::position() const
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{
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if (m_pipeline) {
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//here we query the pipeline about its position
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//and we request that the result is returned in time format
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QGst::PositionQueryPtr query = QGst::PositionQuery::create(QGst::FormatTime);
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m_pipeline->query(query);
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return QGst::ClockTime(query->position()).toTime();
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} else {
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return QTime(0,0);
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}
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}
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void Player::setPosition(const QTime & pos)
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{
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QGst::SeekEventPtr evt = QGst::SeekEvent::create(
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1.0, QGst::FormatTime, QGst::SeekFlagFlush,
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QGst::SeekTypeSet, QGst::ClockTime::fromTime(pos),
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QGst::SeekTypeNone, QGst::ClockTime::None
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);
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m_pipeline->sendEvent(evt);
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}
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int Player::volume() const
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{
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if (m_pipeline) {
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QGst::StreamVolumePtr svp =
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m_pipeline.dynamicCast<QGst::StreamVolume>();
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if (svp) {
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return svp->volume(QGst::StreamVolumeFormatCubic) * 10;
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}
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}
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return 0;
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}
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void Player::setVolume(int volume)
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{
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if (m_pipeline) {
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QGst::StreamVolumePtr svp =
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m_pipeline.dynamicCast<QGst::StreamVolume>();
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if(svp) {
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svp->setVolume((double)volume / 10, QGst::StreamVolumeFormatCubic);
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}
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}
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}
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QTime Player::length() const
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{
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if (m_pipeline) {
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//here we query the pipeline about the content's duration
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//and we request that the result is returned in time format
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QGst::DurationQueryPtr query = QGst::DurationQuery::create(QGst::FormatTime);
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m_pipeline->query(query);
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return QGst::ClockTime(query->duration()).toTime();
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} else {
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return QTime(0,0);
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}
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}
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QGst::State Player::state() const
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{
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return m_pipeline ? m_pipeline->currentState() : QGst::StateNull;
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}
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void Player::play()
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{
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if (m_pipeline) {
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m_pipeline->setState(QGst::StatePlaying);
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}
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}
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void Player::pause()
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{
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if (m_pipeline) {
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m_pipeline->setState(QGst::StatePaused);
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}
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}
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void Player::stop()
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{
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if (m_pipeline) {
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m_pipeline->setState(QGst::StateNull);
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//once the pipeline stops, the bus is flushed so we will
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//not receive any StateChangedMessage about this.
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//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**
|
||
|
||
``` lang=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()**
|
||
|
||
``` lang=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()**
|
||
|
||
``` lang=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()**
|
||
|
||
``` lang=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("playbin2").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`. `playbin2` 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**
|
||
|
||
``` lang=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:
|
||
|
||
``` lang=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 `playbin2` what to play by setting the `uri` property:
|
||
|
||
``` lang=c
|
||
m_pipeline->setProperty("uri", realUri);
|
||
```
|
||
|
||
## Starting Playback
|
||
|
||
After `Player::setUri()` is called, `MediaApp::openFile()` calls
|
||
`play()` on the `Player` object:
|
||
|
||
**Player::play()**
|
||
|
||
``` lang=c
|
||
void Player::play()
|
||
{
|
||
if (m_pipeline) {
|
||
m_pipeline->setState(QGst::StatePlaying);
|
||
}
|
||
}
|
||
```
|
||
|
||
The other state control methods are equally simple:
|
||
|
||
**Player state functions**
|
||
|
||
``` lang=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()**
|
||
|
||
``` lang=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()**
|
||
|
||
``` lang=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()**
|
||
|
||
``` lang=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()**
|
||
|
||
``` lang=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\!
|