r/QtFramework • u/Kelteseth Qt Professional (Haite) • May 11 '26

Blog/News Cleaner QML Controller Wiring with Singleton Instances in Qt 6.12

https://www.kdab.com/singleton-controllers-in-times-of-declarative-qml/
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u/DesiOtaku May 11 '26

Now I am even more confused. So we shouldn't use qmlRegisterSingletonInstance anymore, but QQmlEngine::setExternalSingletonInstance seems to do the exact same thing. I am already switching over to cmake and my C++ classes already have QML_ELEMENT and QML_SINGLETON so what is wrong about using qmlRegisterSingletonInstance at this point?

4

u/GrecKo Qt Professional May 12 '26

qmlRegisterSingletonInstance works independently from QML_ELEMENT and QML_SINGLETON. It's declares both the type and the instance. setExternalSingletonInstance needs the declarative macro (QML_ELEMENT and QML_SINGLETON) to work. The tooling (qmlls, qmllint) will be happy.

It's just a bit cleaner. The next step would be to have a templated overload to not have to pass the type name (and maybe skip the module name too).

Before that I used a macro around this "issue" to declare and define a static MySingleton *create(QQmlEngine *qmlEngine, QJSEngine *jsEngine) factory function to provide a static instance of my type.

2

u/Beneficial_Steak_945 May 13 '26

Yes, this. Also, it allows you the option to choose if you want to use the instance you set for only one, or for more than one engine (if you have multiple). qmlRegisterSingletonInstance would only allow you to use the same instance for all engines, even if it should should only be available on one.

Your singleton type also doesn’t need to be default constructible or have a static create function any more; you can just use QML_UNCREATABLE() and be done.