QML Plugin load from C++

Question

Is it possible to load QML plugin from C++, say, from QPluginLoader to work with it's functions? In my project, I have a qml plugin with a version information in it and I want to read it from C++.

Example:

main() {
    // ...
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:///ui/views/mainwindow.qml")));

    if (parser.isSet(verionsOption)) {
        QSharedPointer<QQmlExtensionPlugin> myPlugin = // load plugin
        std::cout << "Version of plugin: " << myPlugin->version() << std::endl;
    }

    return app.exec();
}

Answer 1

The anwser is.. yes . After few months I was able to get to this task back again.

So, in the qt source code we can see what QQmlExtensionPlugin . It is really a qt plugin (which we are able to open through QPluginLoader ) - QPlugin . In my opinion it should be listed in Qt's plugin type list .

To be able to open your qml plugin in the application your plugin class must implement some interface to which you will cast with qobject_cast in the application. In my project it looks like:

#ifndef MYPLUGIN_H
#define MYPLUGIN_H

#include <QQmlExtensionPlugin>

class ExternalInterface
{
public:
    virtual const QString pluginVersion() const = 0;
    virtual const QString qxmppVersion() const = 0;
    virtual ~ExternalInterface() {}
};

Q_DECLARE_INTERFACE(ExternalInterface, "com.my.ExternalInterface")

class MyPlugin : public QQmlExtensionPlugin, public ExternalInterface
{
    Q_OBJECT
    Q_INTERFACES(ExternalInterface)
    Q_PLUGIN_METADATA(IID "com.MyPlugin")

    public:
        void registerTypes(const char *uri);
        void initializeEngine(QQmlEngine *engine, const char *uri);

        const QString pluginVersion() const final override;
        const QString qxmppVersion() const final override;
};

#endif // MYPLUGIN_H

Next, in main.cpp of the application which opens the plugin:

ExternalInterface* pluginVersion = nullptr;
#ifdef Q_OS_MACOS
    const QString fileSuffix = ".dylib";
#else
    const QString fileSuffix = ".so";
#endif
    QPluginLoader qmlPlugin(QApplication::applicationDirPath() + "../PlugIns/quick/libmyplugin" + fileSuffix);
    qmlPlugin.load();
    if (qmlPlugin.isLoaded()) {
        pluginVersion = qobject_cast<ExternalInterface*>(qmlPlugin.instance());
    } else {
            qmlPlugin.setFileName(QLibraryInfo::location(QLibraryInfo::Qml2ImportsPath) + "/fx/my/libmyplugin" + fileSuffix);
        qmlPlugin.unload();
        qmlPlugin.load();
        if (qmlPlugin.isLoaded()) {
            pluginVersion = qobject_cast<ExternalInterface*>(qmlPlugin.instance());
        } else {
            qDebug() << "ERROR while opening plugin: " << qmlPlugin.errorString();
        }
    }

    if (pluginVersion) {
        qDebug() << "Plugin: \n" << pluginVersion->pluginVersion() << "\n"
                 << pluginVersion->qxmppVersion() << "\n"
                 << "Location: " << qmlPlugin.fileName();
    } else {
        qDebug() << "Can't obtain version information from the plugin";
    }

The worst thing here is that you must provide full path to the plugin (shared library) and you need to know where it installs on different systems.