QML Loader, passing arguments to source

mzimmers

Hi all -

I have a page that will select a sub-page to display based on a property. I'm using a Loader for this.
This works:

required property Equipment equipment

Loader {
    id: loader
    
    Component.onCompleted: {
        var sourceFile
        switch (equipmentCopy.category) {
        case NgaUI.CATEGORY_VSP:
            sourceFile = "edits/VspEdit.qml"
            break;
        ...
        default:
            sourceFile = ""
            break;
        }
        setSource(sourceFile,
                  {
                      "equipment": equipmentEdit.equipment,
                  })
    }
}

What I'd prefer to do is something like this (for brevity and clarity):

required property Equipment equipment

Loader {
    id: loader
	source: {
        switch (equipmentCopy.category) {
            case NgaUI.CATEGORY_VSP:
				"edits/VspEdit.qml"
            ...
            default:
                return null
        }
        equipment: equipmentEdit.equipment
    }
}

But I get a runtime error: "Unable to assign Equipment to QUrl." So, it appears that I can't pass arguments this way.

So...is it even possible to specify arguments within my Loader definition? Or, do I need to use the onCompleted() method?

Thanks...

GrecKo

required properties can't work in Loader's components unless you call setSource

https://bugreports.qt.io/browse/QTBUG-125072
https://bugreports.qt.io/browse/QTBUG-93086

A workaround is mentioned at the end of the description here : https://bugreports.qt.io/browse/QTBUG-125071

Using StackView could also be an alternative if it fits your use case.

Also is this for a delegate? A DelegateChooser could be a good solution.

ankou29666

If i'm understanding properly your need :

https://www.youtube.com/watch?v=nteJeojg07k at 8min50s

J.Hilk

@mzimmers Connections and Bindings are your friend hier

//From Component to Outside
Connections{
    target: loader.item
    enabled: loader.status == Loader.Ready
    function mySignal(myArgument) {
        myOutsideProperty = myArgument
    }
}
//from file to Loader Component:
Binding {
    target:loader.item
    property:"myComponentPropert"
    value: myOutsideProperty
    when: loader.status == Loader.Ready
}

Also IIRC properties defined in the scope of the Loader are also passed to the Component loader. Not 100% on this though

mzimmers

@ankou29666 @J-Hilk thanks for the replies - the KDAB tutorial seems to agree with what @J-Hilk posted. Somehow, though, it's not working for me. Here's what I did:

Pane {
    id: equipmentEdit
    required property Equipment equipment

    Loader {
        id: loader
            source: {
                switch (equipmentCopy.category) {
                    case NgaUI.CATEGORY_VSP:
                        "edits/VspEdit.qml"
                    default:
                        return null
                }
            }
        }
    }
    Binding {
        target: loader.item
        property: "equipment"
        value: equipmentEdit.equipment
        when: loader.status === Loader.Ready
    }
}

in "edits/VspEdit.qml," the equipment property is required. At run-time, I get an error:

Required property equipment was not initialized.

Did I miss a step? Or, is this possibly a race condition where the QML engine attempts to initialize the property before the Loader is ready?

Thanks...

ankou29666

hmmm I see nothing in the doc on how to deal with it when the source property belongs to the Binding's parent.

have you tried value: equipment in the Binding ?

or

when: loader.source==="edits/VspEdit.qml" && loader.status===Loader.Ready"

if ever relevant depending on other cases for the switch ?

mzimmers

@ankou29666 said in QML Loader, passing arguments to source:

have you tried value: equipment in the Binding

Yes, and I even created properties inside the loader, and used those instead -- same result.

ankou29666

and what happens if you remove the requirement for that "equipment" property inside your "edits/VspEdit.qml" ?

or maybe just when: loader.source==="edits/VspEdit.qml", without checking the status ?

I'm actually thinking about a race condition as you suggested.
As far as I understand,

the component requires the property binding BEFORE it's instantiation, but
the binding is made only AFTER the loader has done the job, due to the "when" condition
and thus the binding is made AFTER the instantiation of the component, which is contradictory with the first statement

so my guess would be removing the loader.status case in the "when" clause.

mzimmers

@ankou29666 said in QML Loader, passing arguments to source:

so my guess would be removing the loader.status case in the "when" clause.

That was my guess too, but it doesn't work. Maybe a loader isn't what I should be using here. I'll look at alternatives.

J.Hilk

@mzimmers I noticed some strange behaviour with bindings since I startet working with them in Qt6 and had undefined's when they clearly shouldn't be. A work around that I found for me would be:

Pane {
    id: equipmentEdit
    required property Equipment equipment
    property bool completed: false
    Component.onCompleted: equipmentEdit.completed = true

    Loader {
        id: loader
            source: {
                switch (equipmentCopy.category) {
                    case NgaUI.CATEGORY_VSP:
                        "edits/VspEdit.qml"
                    default:
                        return null
                }
            }
        }
    }
    Binding {
        target: loader.item
        property: "equipment"
        value: equipmentEdit.equipment
        when: loader.status === Loader.Ready && equipmentEdit.completed 
    }
}

Don't know why, I never had to rely on this tricks before

mzimmers

@J-Hilk it was worth a try, but still didn't work. As I said above, maybe I don't need a Loader here - I just need something to insert one of my Components into a screen, selecting the component based on a property of one of the properties in my main Component (in this case, equipmentEdit.category).

J.Hilk

@mzimmers Ah I missunderstood

you have your equipment property in the component markt as required ! Right ?

thats an easy fix, set your loader active property to false

and set it to true in the parent on the onCompleted signal

GrecKo

required properties can't work in Loader's components unless you call setSource

https://bugreports.qt.io/browse/QTBUG-125072
https://bugreports.qt.io/browse/QTBUG-93086

A workaround is mentioned at the end of the description here : https://bugreports.qt.io/browse/QTBUG-125071

Using StackView could also be an alternative if it fits your use case.

Also is this for a delegate? A DelegateChooser could be a good solution.

mzimmers

@GrecKo said in QML Loader, passing arguments to source:

required properties can't work in Loader's components unless you call setSource

Well, that's that. Looks like I'll be using setSource() after all; a Loader is better for my use case than a StackView.

Thanks to all who looked at this.