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241 lines
7.2 KiB
Markdown
241 lines
7.2 KiB
Markdown
# Using appsink/appsrc in Qt
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## Goal
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For those times when you need to stream data into or out of GStreamer
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through your application, GStreamer includes two helpful elements:
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- `appsink` - Allows applications to easily extract data from a
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GStreamer pipeline
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- `appsrc` - Allows applications to easily stream data into a
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GStreamer pipeline
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This tutorial will demonstrate how to use both of them by constructing a
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pipeline to decode an audio file, stream it into an application's code,
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then stream it back into your audio output device. All this, using
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QtGStreamer.
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## Steps
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First, the files. These are also available in the
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`examples/appsink-src` directory of the QGstreamer SDK.
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**CMakeLists.txt**
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```
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project(qtgst-example-appsink-src)
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find_package(QtGStreamer REQUIRED)
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find_package(Qt4 REQUIRED)
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include_directories(${QTGSTREAMER_INCLUDES} ${QT_QTCORE_INCLUDE_DIRS})
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add_definitions(${QTGSTREAMER_DEFINITIONS})
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${QTGSTREAMER_FLAGS}")
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add_executable(appsink-src main.cpp)
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target_link_libraries(appsink-src ${QTGSTREAMER_UTILS_LIBRARIES} ${QT_QTCORE_LIBRARIES})
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```
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**main.cpp**
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``` c
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#include <iostream>
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#include <QtCore/QCoreApplication>
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#include <QGlib/Error>
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#include <QGlib/Connect>
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#include <QGst/Init>
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#include <QGst/Bus>
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#include <QGst/Pipeline>
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#include <QGst/Parse>
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#include <QGst/Message>
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#include <QGst/Utils/ApplicationSink>
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#include <QGst/Utils/ApplicationSource>
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class MySink : public QGst::Utils::ApplicationSink
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{
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public:
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MySink(QGst::Utils::ApplicationSource *src)
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: QGst::Utils::ApplicationSink(), m_src(src) {}
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protected:
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virtual void eos()
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{
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m_src->endOfStream();
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}
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virtual QGst::FlowReturn newBuffer()
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{
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m_src->pushBuffer(pullBuffer());
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return QGst::FlowOk;
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}
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private:
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QGst::Utils::ApplicationSource *m_src;
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};
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class Player : public QCoreApplication
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{
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public:
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Player(int argc, char **argv);
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~Player();
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private:
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void onBusMessage(const QGst::MessagePtr & message);
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private:
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QGst::Utils::ApplicationSource m_src;
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MySink m_sink;
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QGst::PipelinePtr pipeline1;
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QGst::PipelinePtr pipeline2;
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};
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Player::Player(int argc, char **argv)
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: QCoreApplication(argc, argv), m_sink(&m_src)
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{
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QGst::init(&argc, &argv);
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if (argc <= 1) {
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std::cerr << "Usage: " << argv[0] << " <audio_file>" << std::endl;
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std::exit(1);
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}
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const char *caps = "audio/x-raw-int,channels=1,rate=8000,"
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"signed=(boolean)true,width=16,depth=16,endianness=1234";
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/* source pipeline */
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QString pipe1Descr = QString("filesrc location=\"%1\" ! "
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"decodebin2 ! "
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"audioconvert ! "
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"audioresample ! "
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"appsink name=\"mysink\" caps=\"%2\"").arg(argv[1], caps);
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pipeline1 = QGst::Parse::launch(pipe1Descr).dynamicCast<QGst::Pipeline>();
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m_sink.setElement(pipeline1->getElementByName("mysink"));
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QGlib::connect(pipeline1->bus(), "message::error", this, &Player::onBusMessage);
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pipeline1->bus()->addSignalWatch();
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/* sink pipeline */
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QString pipe2Descr = QString("appsrc name=\"mysrc\" caps=\"%1\" ! autoaudiosink").arg(caps);
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pipeline2 = QGst::Parse::launch(pipe2Descr).dynamicCast<QGst::Pipeline>();
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m_src.setElement(pipeline2->getElementByName("mysrc"));
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QGlib::connect(pipeline2->bus(), "message", this, &Player::onBusMessage);
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pipeline2->bus()->addSignalWatch();
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/* start playing */
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pipeline1->setState(QGst::StatePlaying);
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pipeline2->setState(QGst::StatePlaying);
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}
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Player::~Player()
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{
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pipeline1->setState(QGst::StateNull);
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pipeline2->setState(QGst::StateNull);
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}
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void Player::onBusMessage(const QGst::MessagePtr & message)
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{
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switch (message->type()) {
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case QGst::MessageEos:
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quit();
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break;
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case QGst::MessageError:
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qCritical() << message.staticCast<QGst::ErrorMessage>()->error();
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break;
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default:
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break;
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}
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}
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int main(int argc, char **argv)
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{
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Player p(argc, argv);
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return p.exec();
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}
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```
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### Walkthrough
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As this is a very simple example, most of the action happens in the
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`Player`'s constructor. First, GStreamer is initialized through
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`QGst::init()`:
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**GStreamer Initialization**
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``` c
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QGst::init(&argc, &argv);
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```
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Now we can construct the first half of the pipeline:
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**Pipeline Setup**
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``` c
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const char *caps = "audio/x-raw-int,channels=1,rate=8000,"
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"signed=(boolean)true,width=16,depth=16,endianness=1234";
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/* source pipeline */
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QString pipe1Descr = QString("filesrc location=\"%1\" ! "
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"decodebin2 ! "
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"audioconvert ! "
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"audioresample ! "
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"appsink name=\"mysink\" caps=\"%2\"").arg(argv[1], caps);
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pipeline1 = QGst::Parse::launch(pipe1Descr).dynamicCast<QGst::Pipeline>();
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m_sink.setElement(pipeline1->getElementByName("mysink"));
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QGlib::connect(pipeline1->bus(), "message::error", this, &Player::onBusMessage);
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pipeline1->bus()->addSignalWatch();
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```
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`QGst::Parse::launch()` parses the text description of a pipeline and
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returns a `QGst::PipelinePtr`. In this case, the pipeline is composed
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of:
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- A `filesrc` element to read the file
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- `decodebin2` to automatically examine the stream and pick the right
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decoder(s)
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- `audioconvert` and `audioresample` to convert the output of the
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`decodebin2` into the caps specified for the `appsink`
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- An `appsink` element with specific caps
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Next, we tell our `MySink` class (which is a subclass
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of `QGst::Utils::ApplicationSink`) what `appsink` element to use.
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The second half of the pipeline is created similarly:
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**Second Pipeline**
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``` c
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/* sink pipeline */
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QString pipe2Descr = QString("appsrc name=\"mysrc\" caps=\"%1\" ! autoaudiosink").arg(caps);
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pipeline2 = QGst::Parse::launch(pipe2Descr).dynamicCast<QGst::Pipeline>();
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m_src.setElement(pipeline2->getElementByName("mysrc"));
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QGlib::connect(pipeline2->bus(), "message", this, &Player::onBusMessage);
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pipeline2->bus()->addSignalWatch();
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```
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Finally, the pipeline is started:
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**Starting the pipeline**
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``` c
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/* start playing */
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pipeline1->setState(QGst::StatePlaying);
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pipeline2->setState(QGst::StatePlaying);
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```
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Once the pipelines are started, the first one begins pushing buffers
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into the `appsink` element. Our `MySink` class implements the
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`newBuffer()` method, which is called by QGStreamer when a new buffer is
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ready for processing:
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**MySink::newBuffer()**
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``` c
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virtual QGst::FlowReturn newBuffer()
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{
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m_src->pushBuffer(pullBuffer());
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return QGst::FlowOk;
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}
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```
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Our implementation takes the new buffer and pushes it into the
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`appsrc` element, which got assigned in the `Player` constructor:
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**Player::Player()**
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``` c
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Player::Player(int argc, char **argv)
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: QCoreApplication(argc, argv), m_sink(&m_src)
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```
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From there, buffers flow into the `autoaudiosink` element, which
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automatically figures out a way to send it to your speakers.
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## Conclusion
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You should now have an understanding of how to push and pull arbitrary
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data into and out of a GStreamer pipeline.
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It has been a pleasure having you here, and see you soon\!
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