passing lists of data to a Component

[mzimmers]

Hi all -

I'm in need of an idea for passing data into a Component that I've defined. Some but not all of the data originates from a model.

The model maintains a list of equipment items (Equipment is a QObject-based class). Properties of the Equipment class have been exposed to QML. One property is the equipment category, which can be one of ~40 values. The display will vary based on this category. For example, for a pump I may expose several speed settings, while for a heater I'll expose one or more temperature settings.

Currently I've hard-coded the implementation for one category. The code looks like this:

Flickable {
    EquipmentConfigList { 
        equipment: equipmentCopy // from the model
    }
}

// EquipmentConfigList.qml
ColumnLayout {

    Loader {
        sourceComponent: equipmentListLine
        property string title: "Equipment Name"
        property string value: equipment.name
        property color valueColor: Colors.primaryDarkBlue
    }
    Loader {
        sourceComponent: equipmentListLine
        property string title: "Model Name"
        property string value: equipment.modelId
        property color valueColor: Colors.primaryDarkBlue
    } // etc.
    
    Component {
        id: equipmentListLine
        ColumnLayout {
        ...

This approach is obviously highly imperfect. I could use a Repeater instead of the loader, but I'm stuck on how to "feed" it data. A ListModel won't allow me to use elements from my object (the "not from a script" error). Any suggestions on what I could use here?

Thanks for any ideas...

[GrecKo]

@mzimmers I didn't realize you didn't want to use this is a model/view but yes it is indeed supported to have a single object as a model. You were 1 link away ;) https://doc.qt.io/qt-6/qtquick-modelviewsdata-modelview.html#object-instances-as-models

using Loader like mentioned by TomZ is an alternative.

[TomZ]

Yap, that's a bit messy...

You obviously know about the Loader.setSource second argument. Which doesn't work for a Component.

Here is one solution:

Loader {
  onLoaded: item.someProperty = someValue
}

Your component should have that property and your code here can set it to anything.
Biggest downside is that the property is null for a small while which may cause you to have warnings unless you check for that in usages.

Now, the main upside here is that you can call into C++ code and pass in a 'parent' object for memory management (QObject constructor arg). If you pass in the 'item' (property of loader) then your created QObject's livespan will be tied to the loaded item.

I use this method often in things like a delegate. Your delegate is a thin wrapper around a Loader, the loader selects the Compenent on some properties in the 'modelData' and then assigns the modelData to some property of the component on load.

[mzimmers]

@TomZ thanks for the reply. One of my goals here is to make the EquipmentConfigList.qml file reusable. I'm looking for a way to aggregate the data I need to display (I would expect to do this inside my Flickable or my reference to the EquipmentConfigList) so that I can use a Repeater (or something similar) to display all the information that's passed to EquipmentConfigList.qml. In C++ I'd use an array of structs (probably) but I don't know how to do this in QML. Any thoughts?

EDIT:

This is sort of pseudocode outline of what I'd like to do:

EquipmentA.qml
EquipmentB.qml
EquipmentC.qml

// EquipmentConfig.qml

if (category is A) {
    list = makeListForCategoryA // choosing only the properties I want to display for A
    EquipmentConfigList(list)
} else if (category is B) {
    list = makeListForCategoryB
    EquipmentConfigList(list)
} // etc.

My EquipmentA/B/C.qml files ideally wouldn't have to be QML, though I'm not sure what I'd replace them with. But the gist is to have a reusable list that I can conditionally populate based on the category of the equipment object.

Thanks...

[TomZ]

@mzimmers I think you are on the right track, but you seem to be stuck on some minor issues.

First, remember the strong seperation of model/view. In this specific case you can easily have one model that feeds all 3 types of views. You KNOW that a view will only ask for certain properties if the data is a certain type, as such you can optimize your model code to match (add asserts, even).

So, in short, to feed your 3 types of views you should only have one backing model. It simply has different properties you query for a specific view.

Second,
you can use a Loader with a construction like:

ListView {
   model: someModel
   Loader {
     source: {
       if (model.someProperty)
         return "./Page1.qml";
      if (model.foo)
         return "./Page2.qml";
      return "./Page3.qml";
   }
}

Then your pages simply need to use the model property to fetch the data they want to use.

Page1.qml

Label {
   text: model.something
}

[mzimmers]

@TomZ I think this is moving in the right direction. A couple points:

I am indeed maintaining a single model. It contains a list of all the equipment items in the system.

There will be multiple equipment items of the same category in a system. For this reason, I need to somehow inform my Page1/2/3.qml files of the equipment object I'm using. By the time I'm in this QML area, I've already accessed the model and have a single instance of Equipment in my QML.

Also (minor) when I changed my Flickable to a ListView, it no longer flicks. Here's the complete code for the section:

// Flickable {
ListView {
    boundsBehavior: Flickable.StopAtBounds
    clip: true
    contentHeight: contentItem.childrenRect.height
    implicitWidth: parent.width
    implicitHeight: innerPane.availableHeight
    EquipmentConfigList { // this would become Page1/2/3.qml
        id: equipmentConfigList
        Layout.fillHeight: true
        Layout.preferredWidth: rightColumn.availableWidth
        equipment: equipmentCopy
    }
}

Any idea why it doesn't flick for me anymore?

Thanks for the help...

[GrecKo]

If you want to have different delegate components based on a type role, use DelegateChooser instead of Loader for delegates.

If you want to have a single generic delegate, you could define all the possible fields and conditionally show them if there is a corresponding property. Or fetch the properties from the meta-object. Do you want to do that though? What if you want to display a gauge/slider? How would you define the min/max value?

[mzimmers]

@GrecKo said in passing lists of data to a Component:

What if you want to display a gauge/slider? How would you define the min/max value?

You're a mind reader - I will indeed need to do that elsewhere (the customer wants separate pages for viewing and editing the Equipment fields). I was starting with the easy (readonly) stuff.

I'm still having trouble putting this all together, though. I need a Flickable (or ListView) that will conditionally display properties of the Equipment object based on the category of the equipment.

Here's the basis of the Equipment class:

enum EquipmentCategory {
    CATEGORY_UNKNOWN,
    CATEGORY_VSP,
    CATEGORY_HEATER,
    CATEGORY_HEATPUMP,
    // ...
}
class Equipment : public QObject
{
    Q_OBJECT
    QML_ELEMENT

    EquipmentCategory m_category = CATEGORY_UNKNOWN; // exposed to QML
    // ...
}

So, how do I use a DelegateChooser here:

Flickable {
    // what is my model?
    DelagateChooser {
        role: category
        DelegateChoice {
            role: CATEGORY_VSP // this doesn't seem right
            VspDelegate {} // contained an another file
        }
        DelegateChoice {
            role: CATEGORY_HEATER
            HeaterDelegate {} // contained an another file
        }
    }
}

Is this on the right track?

Thanks...

[GrecKo]

@mzimmers said in passing lists of data to a Component:

role: CATEGORY_VSP // this doesn't seem right

use roleValue here. And properly expose your enum type to QML.

The role property of DelegateChooser is a string, so specify the role name as a string here.

[mzimmers]

This post is deleted!

[mzimmers]

@GrecKo so, this is what I have now (and it seems to work):

ListView {
    anchors.fill: parent
    model: equipmentCopy
    clip: true
    boundsBehavior: Flickable.StopAtBounds
    delegate: DelegateChooser {
        role: "category"
        DelegateChoice {
            roleValue: NgaUI.CATEGORY_VSP
            delegate: EquipmentConfigList {
                equipment: equipmentCopy
            }
        }
    }
}

It's a little foreign to me, as I've never used an object as a model, but everything seems to work. Thanks for all the help on this.

[mzimmers]

@GrecKo Before I close this topic, I do have a couple of follow-up questions:

1. I don't seem to be using a list in the "classical" sense, in that I'm only displaying one item. I realize that a delegate (and therefore, a DelegateChooser) needs a list, but...is there a preferred way to implement what I'm doing? Without a list?

2. (sort of related) is it OK to use an object as the model for a ListView? It seems to work OK, but there's nothing in the docs that explicitly says I can do this, so I want to make sure this is licit.

Thanks...

[GrecKo]

@mzimmers I didn't realize you didn't want to use this is a model/view but yes it is indeed supported to have a single object as a model. You were 1 link away ;) https://doc.qt.io/qt-6/qtquick-modelviewsdata-modelview.html#object-instances-as-models

using Loader like mentioned by TomZ is an alternative.

[mzimmers]

@GrecKo yeah, this is a weird use case. I actually do want a list, just not a list in the Qt model/view/delegate sense. Ideally, I'd like to create a list (array/vector/whatever) of the properties I want to display, and pass it into a universal component for display, but creating and using that structure seems to be a bit out of reach for JS/QML/me. It's OK; this approach is working, if a bit inelegant.