[Interest] How to avoid qml engine repeatedly copying and garbage collecting qstringlists?

[NIkolai Marchenko]

Ahem... I tried searching with https://codebrowser.dev/ and doesn't
QtQuickGuiProvider do the exact same thing with QfontDatabase::families?
Unless I misunderstand something it should hit exactly the same problem
with copying the stringlist and then Qt.fontFamilies isn't the solution?


On Mon, Oct 23, 2023 at 6:38 PM Ulf Hermann <ulf.hermann at qt.io> wrote:

> > The question though is whether this would be a singleton per qmlengine.
> > If it will, then that's a nogo as the list of loaded fonts needs to
> > exist once regardless of how much engines are running
>
> Singletons are created once per engine, even C++ ones. You should not
> subvert this. And you cannot safely share JavaScript objects across
> engines anyway.
>
> What you _can_ do is have your SingletonBase redirect the property
> access to some other place that holds a unique list of font families.
> You can write the getter for a Q_PROPERTY any way you like after all. If
> you have a QList<QString> for the font families, redirecting through one
> more function call is not a big performance hit, due to implicit
> sharing. And since your actual access to the font families is via the
> QML-defined property, each engine will access the original font families
> only once. So:
>
> // singletonbase.h
> class SingletonBase : public QObject
> {
>       Q_OBJECT
>       QML_ELEMENT
>       // Not a singleton!
>       Q_PROPERTY(QStringList internalFontFamilies READ
> internalFontFamilies CONSTANT FINAL)
> public:
>      QStringList internalFontFamilies() {
>          // The enclosing object does not have to be a singleton
>          // in order to return singular data from some getter!
>          return QFontDatabase::families()
>      }
> };
>
> I feel we're touching on a common misconception that many people
> struggle with. Just because some data is conceptually static, you don't
> need to nail it down to a single static QObject. You can just return the
> static data from some method. Each QML engine you expose a single static
> QObject to will try to create its own QObject wrapper. Since we can only
> store one QObject wrapper in the pointer exposed by QObjectPrivate, this
> leads to a hilarious amount of complexity on the side of the QML
> engines. It is also a giant foot gun waiting to trigger when you create
> more than one thread. Yes, it is possible to manage all of this, but
> just don't do it. Let each engine each have its own singleton object and
> dispatch the data in the C++ code you have to write anyway.
>
> By the way, there is also Qt.fontFamilies(). If that is all you need,
> you don't need a special C++ backing type for your singleton at all.
>
> best regards,
> Ulf
>
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