[Interest] Problem with unintentional circular dependencies between QML singletons

[Ben Lau]

I usually have many singleton components in my app. I would break them down
into different packages to avoid circular dependence.

On 30 March 2016 at 17:06, Elvis Stansvik <elvstone at gmail.com> wrote:

> 2016-03-30 11:00 GMT+02:00 Elvis Stansvik <elvstone at gmail.com>:
> > Hi all,
> >
> > I have a problem that I think is due to QTBUG-39703 [1] and
> > QTBUG-34418 [2], but perhaps there's a smart workaround.
> >
> > Imagine the following setup:
> >
> > dir/
> >     main.qml           The main file
> >     MyItem.qml       Some item, uses Style
> >     Style.qml           Some colors et.c.
> >     State.qml           Some application-wide state (uses MyItem, see
> below!)
> >     qmldir
> >
> > with contents as follows:
> >
> >
> > //// main.qml
> > import QtQuick 2.4
> >
> > MyItem {
> > }
> >
> >
> > //// MyItem.qml
> > import QtQuick 2.4
> > import "." // required due to QTBUG-34418
> >
> > Rectangle {
> >     color: Style.backgroundColor
> > }
> >
> >
> > //// Style.qml  --  Some colors et.c.
> > pragma Singleton
> >
> > import QtQuick 2.4
> >
> > QtObject {
> >     readonly property color backgroundColor: "#abc"
> > }
> >
> >
> > //// State.qml  --  Some application-wide state.
> > pragma Singleton
> >
> > import QtQuick 2.4
> >
> > QtObject {
> >     property int someState: 1  // This is fine.
> >
> >     // But what if we need to keep a MyItem as state
> >     // like below? It would lead to a process exit with
> >     // status 255 because this singleton will implicitly
> >     // import MyItem.qml, which in turn must import "."
> >     // to access the Style.qml singleton (due to
> >     // QTBUG-34418), and this will bring in State.qml
> >     // again -> circular dependency :/
> >
> >     //property MyItem someItem: MyItem {}
> >
> >     // Uncomment the above -> Process exits with 255 due to QTBUG-39703
> > }
> >
> > This last file shows the problem, if the application state in the
> > State.qml singleton includes a MyItem, I get a circular dependency
> > leading to a process exit with status 255.
> >
> > Is there some way around this conundrum? It's the requirement to make
> > an explicit import "." to get the Style singleton (due to QTBUG-39703)
> > that really spoils it I think :/
> >
> > The qmldir file is:
> >
> > singleton Style 1.0 Style.qml
> > singleton State 1.0 State.qml
> > MyItem 1.0 MyItem.qml
> >
> > Surely someone has used multiple singletons like this?
>
> And I realized that, eventhough my example did not show it, it's
> likely that MyItem would have a dependency on State.qml (the
> application-wide state singleton), so there would indeed be a circular
> dependency as far as imports go, but not a real circular dependency of
> course. That is, I would not make use of the MyItem property of the
> singleton from MyItem itself. Can QML not cope with such a situation
> currently?
>
> Elvis
>
> >
> > Cheers,
> > Elvis
> >
> > [1] https://bugreports.qt.io/browse/QTBUG-39703
> > [2] https://bugreports.qt.io/browse/QTBUG-34418
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