Create QML plugin with CMake

Question

I'm trying to create simple QML C++ plugin using CMake. There are my sources:

CMakeLists.txt:

cmake_minimum_required(VERSION 2.8.12)

project(qmltest LANGUAGES CXX)

set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

# Path to installed Qt 5.15
set(Qt5Core_DIR ".../Qt/5.15.0/gcc_64/lib/cmake/Qt5Core")
set(Qt5Qml_DIR ".../Qt/5.15.0/gcc_64/lib/cmake/Qt5Qml")
set(Qt5Quick_DIR ".../Qt/5.15.0/gcc_64/lib/cmake/Qt5Quick")

find_package(Qt5 COMPONENTS Core Quick Qml REQUIRED)

LINK_DIRECTORIES(.../Qt/5.15.0/gcc_64/lib)
INCLUDE_DIRECTORIES(.../Qt/5.15.0/gcc_64/include)
INCLUDE_DIRECTORIES(.../Qt/5.15.0/gcc_64/include/QtQml)
set(QML_IMPORT_PATH ${CMAKE_CURRENT_BINARY_DIR} CACHE STRING "" FORCE)

add_library(dummy SHARED
  "dummy.h" "dummy.cpp" "myplugin.h" "myplugin.cpp"
)
target_link_libraries(dummy Qt5::Core Qt5::Quick Qt5::Qml)

add_executable(${PROJECT_NAME} "main.cpp" "qml.qrc")

target_link_libraries(${PROJECT_NAME} dummy Qt5::Core Qt5::Quick Qt5::Qml)

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QObject>
#include <QtPlugin>

int main(int argc, char *argv[])
{
  qputenv("QT_IM_MODULE", QByteArray("qtvirtualkeyboard"));

  QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

  QGuiApplication app(argc, argv);

  QQmlApplicationEngine engine;
  engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
  if (engine.rootObjects().isEmpty())
    return -1;

  return app.exec();
}

dummy.h:

#ifndef DUMMY_H
#define DUMMY_H

#include <QObject>

class Dummy : public QObject
{
  Q_OBJECT
  Q_PROPERTY(QString name READ name WRITE setName NOTIFY nameChanged)

public:
  explicit Dummy(QObject *parent = nullptr);
  QString name();
  void setName(const QString x);

signals:
  void nameChanged();

public slots:

private:
  QString _name;
};

#endif // DUMMY_H

myplugin.h:

#ifndef MYPLUGIN_H
#define MYPLUGIN_H

#include <QtQml/QQmlEngineExtensionPlugin>
#include <QObject>
#include <QQmlApplicationEngine>
#include <iostream>

#include <dummy.h>

class MyPlugin : public QQmlEngineExtensionPlugin
{
  Q_OBJECT
  Q_PLUGIN_METADATA(IID QQmlEngineExtensionInterface_iid)

public:
  void registerTypes(const char* uri) {
     std::cout << "GGGGGGGGGGGGGGGG\n";
     qmlRegisterType<Dummy>(uri, 1, 0, "Dummy");
     qmlRegisterModule(uri, 1, 0);
     qmlProtectModule(uri, 1);
  }
};

#endif // MYPLUGIN_H

main.qml:

import MyPlugin 1.0

Window {
    Dummy {}
}

I also created directory in my build directory named MyPlugin with link to built libdummy.so and qmldir file as below:

module MyPlugin
plugin dummy

Project builds, but when I run it, I get an error:

plugin cannot be loaded for module "MyPlugin": Cannot protect module MyPlugin 1 as it was never registered

My debug output from registerTypes function have not been called (no output on stdout), so I assume that types have not been registered by some reasons. If I try register type statcally by add this line to myplugin.cpp:

static int unused = qmlRegisterType<Dummy>("MyPlugin", 1, 0, "Dummy");

, I get an error:

plugin cannot be loaded for module "MyPlugin": Namespace 'MyPlugin' has already been used for type registration

What do I wrong in this plugin and how to make it works with CMake?

Answer 1

Qt contains 2 different base class for qml plugins: QQmlEngineExtensionPlugin (This class was introduced in Qt 5.14) and QQmlExtensionPlugin

myplugin.h:

#ifndef MYPLUGIN_H
#define MYPLUGIN_H

#include <QtQml/QQmlExtensionPlugin>
#include "dummy.h"

class MyPlugin : public QQmlExtensionPlugin
{
  Q_OBJECT
  Q_PLUGIN_METADATA(IID QQmlEngineExtensionInterface_iid)

public:
  void registerTypes(const char* uri) {
     qmlRegisterType<Dummy>(uri, 1, 0, "Dummy");
  }
};

#endif // MYPLUGIN_H

Answer 2

as of Qt 6.2, the plugin class is generated for you by qt6_add_qml_module:

https://doc.qt.io/qt-6/cmake-build-reusable-qml-module.html