How to create a "creatable" qml singleton? - qt

Question

I have a use case where I need to instantiate a singleton in qml (for property bindings).
i.e:
File: main.qml
Item{
MainQuery{id: mainquery
graphql: `query MainQuery{...}`
}
Text{
text: mainquery.data
}
}
This other file should use the query defined in main.qml
File: otherfile.qml
Item{
import MyLib 1.0 as Gql
Text{
text: Gql.MainQuery.data
}
}
However if you use QML_SINGLETON you can't initialize it in QML
and in order to bind to the graphql property above you would have to use
Connection.
Is there a better solution?
Note:
The object should be a singleton in CPP / Python side as well.
currently it is implemented in python like this:
class QSingleton(type(QObject)): # type: ignore
def __init__(cls, name, bases, dict):
super().__init__(name, bases, dict)
cls.instance = None
def __call__(cls, *args, **kw):
if cls.instance is None:
cls.instance = super().__call__(*args, **kw)
return cls.instance
class BaseQuery(QObjcet, QSingleton):
...
#QmlElement
class MainQuery(BaseQuery):
...
And gives this nice error when the qml engine tries to initiate the type:
QEventLoop: Cannot be used without QApplication
Process finished with exit code 139 (interrupted by signal 11: SIGSEGV)
(I do have an eventloop running)

Answer 1

Whether the underlying Python object is or is not a singleton it doesn't matter. We can always declare a singleton wrapper in QML by defining a qmldir and using pragma Singleton in our source file. To demonstrate this approach, I wrap TextMetrics in my singleton object and expose TextMetrics properties as properties of my singleton so that I can use property binding with singleton syntax:
import QtQuick
import QtQuick.Controls
import QtQuick.Layouts
Page {
Frame {
anchors.centerIn: parent
ColumnLayout {
TextField {
text: "Hello"
onTextChanged: MyWrapper.tmText = text
}
Text {
text: MyWrapper.tmWidth
}
}
}
}
// qmldir
module ExampleWrapper
singleton MyWrapper 1.0 MyWrapper.qml
// MyWrapper.qml
pragma Singleton
import QtQuick
import QtQuick.Controls
Item {
property alias tmText: tm.text
property alias tmWidth: tm.width
TextMetrics {
id: tm
text: "Hello"
}
}
You can Try it Online!
References:
https://doc.qt.io/qt-6/qtqml-modules-qmldir.html