How to build-in a static Qt plugin into an application with CMake?

devel

http://stackoverflow.com/questions/41977236/how-to-build-in-a-static-qt-plugin-into-an-application-with-cmake

I'd like to embed a custom virtual audio device plugin into an application. The QAudioSystemPlugin looks suitable for this task.

But it doesn't load.

CMakeLists.txt:

cmake_minimum_required(VERSION 2.8.11)

project(myproject)

find_package(Qt5 5.5.0 REQUIRED COMPONENTS
    Core
    Multimedia
)

set(CMAKE_AUTOMOC ON)
add_definitions("-DQT_STATICPLUGIN")

add_executable(${PROJECT_NAME}
    main.cpp
    VirtualQAudioSystemPlugin.cpp
)

target_link_libraries(${PROJECT_NAME}
    Qt5::Core
    Qt5::Multimedia
)

set_property(TARGET ${PROJECT_NAME} PROPERTY CXX_STANDARD 11)

main.cpp:

#include <iostream>

#include <QCoreApplication>
#include <QAudioDeviceInfo>
#include <QtPlugin>

Q_IMPORT_PLUGIN(VirtualQAudioSystemPlugin)

int main(int argc, char *argv[])
{
	QCoreApplication app(argc, argv);

	for (const auto& dev : QAudioDeviceInfo::availableDevices(QAudio::AudioInput))
		std::cerr << dev.deviceName().toStdString() << "\n";

	return app.exec();
}

VirtualQAudioSystemPlugin.hpp:

#pragma once

#include <QtMultimedia/QAudioSystemPlugin>

class VirtualQAudioSystemPlugin: public QAudioSystemPlugin
{
	Q_OBJECT
	Q_PLUGIN_METADATA(IID "org.qt-project.qt.audiosystemfactory/5.0" FILE "virtual.json")

public:
	VirtualQAudioSystemPlugin(QObject *parent = 0);
	~VirtualQAudioSystemPlugin() {}

	virtual QList<QByteArray> availableDevices(QAudio::Mode mode) const override;
	virtual QAbstractAudioInput* createInput(const QByteArray& device) override;
	virtual QAbstractAudioOutput* createOutput(const QByteArray& device) override;
	virtual QAbstractAudioDeviceInfo* createDeviceInfo(const QByteArray& device, QAudio::Mode mode) override;
};

VirtualQAudioSystemPlugin.cpp:

#include "VirtualQAudioSystemPlugin.hpp"

#include <iostream>

VirtualQAudioSystemPlugin::VirtualQAudioSystemPlugin(QObject *parent) :
		QAudioSystemPlugin(parent)
{
	std::cerr << "VirtualQAudioSystemPlugin\n";
}

QList<QByteArray> VirtualQAudioSystemPlugin::availableDevices(QAudio::Mode mode) const
{
	std::cerr << "VirtualQAudioSystemPlugin\n";
}

QAbstractAudioInput* VirtualQAudioSystemPlugin::createInput(const QByteArray &device)
{
	std::cerr << "VirtualQAudioSystemPlugin\n";
}

QAbstractAudioOutput* VirtualQAudioSystemPlugin::createOutput(const QByteArray &device)
{
	std::cerr << "VirtualQAudioSystemPlugin\n";
}

QAbstractAudioDeviceInfo* VirtualQAudioSystemPlugin::createDeviceInfo(const QByteArray &device, QAudio::Mode mode)
{
	std::cerr << "VirtualQAudioSystemPlugin\n";
}

virtual.json:

{
    "Keys": ["default"]
}

Build and launch: the constructor of the VirtualQAudioSystemPlugin is not executed.

How to make it load?

SGaist

Hi,

From what I can see, it should rather be something like:

class VirtualQAudioSystemPlugin : public QAudioSystemPlugin, public QAudioSystemPluginExtension
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "org.qt-project.qt.audiosystemfactory/5.0" FILE "virtual.json")
    Q_INTERFACES(QAudioSystemPluginExtension)
etc.

You can also set the QT_DEBUG_PLUGINS environment variable to 1 to get more information about what is happening.

devel

That class isn't present in the current Qt version.

Can anyone reproduce the problem?

devel

The problem was because of the 'default' in virtual.json: it was overridden by other plugins. If there are no other plugins, the object is instantiated.

Had to change to something different like 'virtual'.

{
    "Keys": ["virtual"]
}