QmlSingletonRegisterType and QmlSingletonRegisterIstance
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wrote 20 days ago last edited by Shrishashank
what is the difference between qmlregisterSingletotype and qmlregisterSingletoinstance...?
I didn't see much key diffrence as I go through documentation .Please help me to know the key difference ... -
Hi,
From a quick look, the former is to registry a QML type while the latter is to register an actual C++ object instance.
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wrote 19 days ago last edited by
The important thing to understand is that the "singleton" that QML knows about does not necessarily have to be a singleton on the C++ side. You might already have an object that you want to make available to QML using the singleton access syntax but the object already exists and is not a singleton in the usually understood C++ sense. I find this useful because typically my C++ classes have dependencies that need to be passed in at construction, whereas
qmlRegisterSingletonTypecreates the object for QML when it is needed and doesn't provide an easy means to pass in constructor arguments.Having said that, I believe that using
qmlRegisterSingletonInstanceis discouraged these days and that a certain combination of macros should be used to achieve a similar thing, but I have not switched over to this yet. -
wrote 16 days ago last edited by
The existing answers are correct, but to amend them: With qmlRegisterSingletonInstance, the same instance is shared by all engines (which can be somewhat problematic when there's more than one), whereas with qmlRegisterSingletonType, each engine will end up with its own instance.
Please see also https://doc.qt.io/qt-6/qml-singleton.html , especially the sections on "Exposing an existing object as a singleton", "Imperative type registration" and "Guidelines for (not) using singletons".
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