[Interest] controlling QML singleton lifetime from C++

[Ulf Hermann]

Hi Stefan,


> I'd like to be able to replace that singleton instance over the course 
> of the application's workflow, but I couldn't find any indication in the 
> docs for whether that's allowed, or whether that might lead to undefined 
> behaviour.

There is QQmlEngine::clearSingletons(), but it's not for the faint of heart:

>   This function drops all singleton instances, deleting any QObjects owned by
>   the engine among them. This is useful to make sure that no QML-created objects
>   are left before calling clearComponentCache().
> 
>   QML properties holding QObject-based singleton instances become null if the
>   engine owns the singleton or retain their value if the engine doesn't own it.
>   The singletons are not automatically re-created by accessing existing
>   QML-created objects. Only when new components are instantiated, the singletons
>   are re-created.
> 
>   \sa clearComponentCache()

After this, the engine will have "forgotten" your C++-owned singleton, 
and you can delete it, making it null in all properties that still hold 
it. Then, if you instantiate a component that uses the singleton, it 
will be re-created from your create() function.

If you just delete the singleton without clearSingletons() you've 
created a null singleton. There is code that should deal with that case, 
but I don't think it's exercised very often.

best regards,
Ulf