[Interest] Problem with unintentional circular dependencies between QML singletons

[Elvis Stansvik]

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