[Interest] Problem with unintentional circular dependencies between QML singletons

[Ben Lau]

Hi Elvis,

It is not allowed in QML. However, you may pass the instance of State to
MyItem as var type during construction.

On 30 March 2016 at 17:33, Elvis Stansvik <elvstone at gmail.com> wrote:

> Hi Ben,
>
> 2016-03-30 11:19 GMT+02:00 Ben Lau <xbenlau at gmail.com>:
> >
> > I usually have many singleton components in my app. I would break them
> down
> > into different packages to avoid circular dependence.
>
> Yes, I realized that this is a possible workaround: If I would move
> the Style.qml in my example into a package "styles" and do import
> "styles" instead of import "." in MyItem.qml, it would work without
> problems.
>
> But what about my latest message, what if MyItem needs to access some
> state in the State singleton, but the State singleton also needs to
> have a MyItem property (as part of the state). Would that be in any
> way possible, or is it simply not allowed? The more I think about it
> the more I realize that's probably the case.
>
> Elvis
>
> >
> > 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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> >
> >
>
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