QMLにおけるシングルトン

pragma Singleton

set_source_files_properties(MySingleton.qml
    PROPERTIES QT_QML_SINGLETON_TYPE TRUE)

set(plain_qml_files
    MyItem1.qml
    MyItem2.qml
    FancyButton.qml
)
set(qml_singletons
    MySingleton.qml
    MyOtherSingleton.qml
)
set_source_files_properties(${qml_singletons}
    PROPERTIES QT_QML_SINGLETON_TYPE TRUE)
qt_add_qml_module(myapp
    URI MyModule
    QML_FILES ${plain_qml_files} ${qml_singletons}
)

module MyModule
singleton MySingleton 1.0 MySingleton.qml
singleton MyOtherSingleton 1.0 MyOtherSingleton.qml

class MySingleton : public QObject
{
    Q_OBJECT
    QML_SINGLETON
    QML_ELEMENT
public:
    MySingleton(QObject *parent = nullptr) : QObject(parent) {
        // ...
    }
};

class MyNonDefaultConstructibleSingleton : public QObject
{
    Q_OBJECT
    QML_SINGLETON
    QML_NAMED_ELEMENT(MySingleton)
public:
    MyNonDefaultConstructibleSingleton(QJSValue id, QObject *parent = nullptr)
        : QObject(parent)
        , m_symbol(std::move(id))
    {}

    static MyNonDefaultConstructibleSingleton *create(QQmlEngine *qmlEngine, QJSEngine *)
    {
         return new MyNonDefaultConstructibleSingleton(qmlEngine->newSymbol(u"MySingleton"_s));
    }

private:
    QJSValue m_symbol;
};

struct SingletonForeign
{
    Q_GADGET
    QML_FOREIGN(MySingleton)
    QML_SINGLETON
    QML_NAMED_ELEMENT(MySingleton)
public:

    inline static MySingleton *s_singletonInstance = nullptr;

    static MySingleton *create(QQmlEngine *, QJSEngine *engine)
    {
        // The instance has to exist before it is used. We cannot replace it.
        Q_ASSERT(s_singletonInstance);

        // The engine has to have the same thread affinity as the singleton.
        Q_ASSERT(engine->thread() == s_singletonInstance->thread());

        // There can only be one engine accessing the singleton.
        if (s_engine)
            Q_ASSERT(engine == s_engine);
        else
            s_engine = engine;

        // Explicitly specify C++ ownership so that the engine doesn't delete
        // the instance.
        QJSEngine::setObjectOwnership(s_singletonInstance,
                                      QJSEngine::CppOwnership);
        return s_singletonInstance;
    }

private:
    inline static QJSEngine *s_engine = nullptr;
};

SingletonForeign::s_singletonInstance = getSingletonInstance();
QQmlApplicationEngine engine;
engine.loadFromModule("MyModule", "Main");

import QtQuick
import MyModule

Item {
    x: MySingleton.posX
    Component.onCompleted: MySingleton.ready = true;
}

import QtQuick
import MyModule

Item {
    id: root
    Binding {
        target: MySingleton
        property: "posX"
        value: root.x
    }
}

class GlobalState : QObject
{
    Q_OBJECT
    QML_ELEMENT
    QML_SINGLETON
    Q_PROPERTY(QAbstractItemModel* localBooks MEMBER localBooks)
    Q_PROPERTY(QAbstractItemModel* digitalStoreFront MEMBER digitalStoreFront)
    Q_PROPERTY(QAbstractItemModel* publicLibrary MEMBER publicLibrary)
public:
    QAbstractItemModel* localBooks;
    QAbstractItemModel* digitalStoreFront;
    QAbstractItemModel* publicLibrary
};

int main() {
    QQmlApplicationEngine engine;
    auto globalState = engine.singletonInstance<GlobalState *>("MyModule", "GlobalState");
    globalState->localBooks = getLocalBooks();
    globalState->digitalStoreFront = setupLoalStoreFront();
    globalState->publicLibrary = accessPublicLibrary();
    engine.loadFromModule("MyModule", "Main");
}

// remotebookmodel.h
class RemoteBookModel : public QAbstractItemModel
{
    Q_OBJECT
    QML_ELEMENT
    Q_PROPERTY(QUrl url READ url WRITE setUrl NOTIFY urlChanged)
    // ...
};

// bookbrowser.qml
Row {
    ListView {
        model: RemoteBookModel { url: "www.public-lib.example"}
    }
    ListView {
        model: RemoteBookModel { url: "www.store-front.example"}
    }
}

QQmlApplicationEngine engine;
if (parser.isSet(fullScreenOption)) {
    // assumes root item is ApplicationWindow
    engine.setInitialProperties(
        { "visibility", QVariant::fromValue(QWindow::FullScreen)}
    );
}
engine.loadFromModule("MyModule, "Main");