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"]
}