How to make a JSON-like QObject - qt

Question

In short, I want to make an JSON-like object, which is easily accessible to both QML / Qt C++ side.
In QML, I can make a settings object like this:
Item {
id: settings
property alias snapshot: snapshot
QtObject {
id: snapshot
property string saveDirectory: "~/hello/world/"
}
}
And make it a singleton, now we can access data via settings.snapshot.saveDirectory .
But accessing it in C++ is very inconvenience, so I'm trying to do this task in C++: create a class Settings, establish instance as settings, then pass it into QQmlApplicationEngine as a context. (the whole application only has only one instance of Settings)
state.cpp
// =============================================================
/// Settings is parent
class Settings : public QObject
{
Q_OBJECT
Q_PROPERTY(SnapshotSettings* snapshot READ snapshot) // <- It's pointer, very important!
public:
explicit Settings(QObject *parent=0) : m_snapshot_settings(new SnapshotSettings) {}
private:
SnapshotSettings* m_snapshot_settings;
}
// =============================================================
// SnapshotSettings is the child of Settings
class SnapshotSettings : public QObject
{
Q_OBJECT
Q_PROPERTY(QString saveDirectory READ saveDirectory)
public:
explicit Settings(QObject *parent=0) : m_save_directory("~/hello/world/") {}
private:
QUrl m_save_directory;
}
main.cpp
I register SnapshotSettings* type. Otherwise, it said:
QMetaProperty::read: Unable to handle unregistered datatype 'SnapshotSettings' for property 'Settings::snapshot'
main.qml:52: TypeError: Cannot read property saveDirectory' of undefined
qmlRegisterType<>() seems unnecessary because I pass its instance into QML as a context, instead of instantialize it in QML?
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQml>
#include <QQmlContext>
#include "state.cpp"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
Settings settings;
// ....Am I right?
qRegisterMetaType<SnapshotSettings*>("SnapshotSettings");
// This seems unnecessary because I use its instance as a context?
// qmlRegisterType<Settings>("com.aisaka.taiga", 0, 1, "Settings");
engine.rootContext()->setContextProperty("settings", &settings);
engine.addImportPath(QStringLiteral(":/qml"));
engine.load(QUrl("qrc:/qml/main.qml"));
return app.exec();
}
One told me I made an anti-pattern of C++/QML…, saying "I never see anyone do like this".
Now it seems ok, I can use settings.snapshot.saveDirectory in QML. But in QtCreator, auto-completion work incorrectly.(saveDirectory won't show after settings.snapshot.)
My question is: Maybe I really did an anti-pattern? If yes, is there any better way to do this?
Thanks.

Answer 1

Your snapshot property needs to be of type SnapshotSettings*, i.e. a pointer to SnapshotSettings because that class is a QObject and cannot be copied.
You should also mark that property as CONSTANT, since the pointer never changes.
The saveDirectory property will either need a NOTIFY signal or also CONSTANT depending on whether it can change during runtime.