accessing aggregates (QVector of a struct)

[mzimmers]

@LeLev (et al): kshugenov's example does indeed work. I tried modifying it as follows:

    Column {
       property var myArray: []
       Component.onCompleted: DataSource.getData().forEach(function(bottle) {
//           template.createObject(this, { text: bottle.name });
           myArray.push(bottle.name)
       }, this)
    Text {
        text: myArray[0]
    }

But I get a runtime error that myArray is not defined. Am I supposed to use something other than a property var for this?

Thanks...

[ODБOï]

@mzimmers said in accessing aggregates (QVector of a struct):

myArray is not defined

you need to assign the id of Column so you can access its properties from outside, it is qml basics

Column {
id :col
property var myArray: []
Text {
text: col.myArray[0]
}
}

@mzimmers said in accessing aggregates (QVector of a struct):

kshugenov's example does indeed work.

it does almost exactly the same thing as my very first example/answer.

[mzimmers]

@LeLev ah. So, now my code looks like this:

    Column {
        id: myColumn
        property var myArray: []
        Component.onCompleted: DataSource.getData().forEach(function(bottle) {
            //           template.createObject(this, { text: bottle.name });
            myArray.push(bottle.name)
        }, this)
        Text {
            text: myColumn.myArray[0]
        }
        Text {
            text:myColumn.myArray[1]
        }
        Text {
            text: myColumn.myArray[2]
        }
    }

and I'm getting this error: "Unable to assign [undefined] to QString"

EDIT: I've stepped through this in the debugger, and the array is most definitely being created.

[ODБOï]

@mzimmers said in accessing aggregates (QVector of a struct):

Column {
id: myColumn
property var myArray: []
Component.onCompleted: DataSource.getData().forEach(function(bottle) {
// template.createObject(this, { text: bottle.name });
myArray.push(bottle.name)
}, this)

you should not modify myArray directly in the function but create a local array then copy its content into myArray

Column {
        id: myColumn
        property var myArray: []
        Component.onCompleted: {
           var arr = []
           DataSource.getData().forEach(function(bottle) {           
            //myArray.push(bottle.name)
              arr.push(bottle.name)
        }, this);
      myColumn.myArray = arr;          
}

[mzimmers]

@LeLev that works! Thanks for your patience on this.

May I ask why my approach didn't work?

[ODБOï]

@mzimmers
here is an example to illustrate, uncomment one or the other Component.onCompleted, you will observe that when we call
col.myArr.push("N° " + i) directly, the view is empty at the beginning but myArr actually contains the items

Window {
    width: 640
    height: 480
    visible: true

    Column{
        id:col
        property var myArr : []

// calling myArr.push() directly in the function.
//   You will need to somehow "refresh" the view, (in this example see the button that resets the repeater model)
        
//        Component.onCompleted: {
//            var arr = []
//            for(var i=0;i<10;i++){
//                col.myArr.push("N° " + i)
//            }
//        }
//OR ------------------------------------------
// creating a local array        
        
//        Component.onCompleted: {
//            var arr = []
//            for(var i=0;i<10;i++){
//                arr.push("N° " + i)
//            }
//            col.myArr = arr;
//        }

        //--------------------------------------------------
        Repeater{
            id:rep
            model: col.myArr.length

            Text {
                id: txt
                text: col.myArr[index]
            }
        }
        //--------------------------------------------------
        Button{ 
            text: 'refresh'
            onClicked:{ rep.model = 0; rep.model = col.myArr.length;  console.log(col.myArr) } // just to reset the repeater
        }
    }
}

It's great that your solution worked but why not implement something where you can simply write

 Text {
            text: DataSource.getDataByIndex(0).name 
        }

[mzimmers]

@LeLev I may have spoken a bit too soon...when I try to apply this from the example to my application, I get the error again.

Column {
	id: myColumn
	property var myArray: []
	Component.onCompleted: {
		var arr = []
		BottleList.getBottleListQv().forEach(function(bottle) {
			arr.push(bottle.name)
		}, this);
		myColumn.myArray = arr;
	}

	Bottle {
		cellText: myColumn.myArray[0]//"W7"
	}

The array element notation seems like it's correct; can you see what I'm doing wrong?

Regarding writing a function for the name: I would do that, and I still may, but I want to grow this to return several values in the struct: name, position, capacity, fill level, etc. So far, this approach looks better for that.

[ODБOï]

@mzimmers said in accessing aggregates (QVector of a struct):

I get the error again.

what error do you get ? Did you read/test my example with all the comments about the arrays, i think the error you get is related to that

[kshegunov]

@mzimmers said in accessing aggregates (QVector of a struct):

@LeLev I may have spoken a bit too soon...when I try to apply this from the example to my application, I get the error again.

At the time you reference the array it's empty. That's the reason. Component.onCompleted is executed after the property to Bottle.cellText is bound, so the JS engine throws you an error, there's no such thing as myArray[0] at this point; the array is empty.

[kshegunov]

@mzimmers said in accessing aggregates (QVector of a struct):

I still don't understand how to retrieve the information to use it in my bottle objects.

You can instantiate them at runtime (as I did with the text item), you just need to replace the Text with your template for the Bottle. Alternatively you can use the Repeater @LeLev's been pushing, but then set the delegate property to a component which is going to create your bottle items (the repeater with a set delegate pretty much does what my example does). Note I don't know if the model property is going to work with a list of gadgets, you need to check.

@LeLev said in accessing aggregates (QVector of a struct):

it does almost exactly the same thing as my very first example/answer.

Almost but with fewer objects created under the hood. Not that it really matters as the QML engine is an elephant anyway.

[ODБOï]

@kshegunov said in accessing aggregates (QVector of a struct):

Alternatively you can use the Repeater @LeLev's been pushing

I just use a Repeater to answer to this question :

@mzimmers said in accessing aggregates (QVector of a struct):

@LeLev ah. So, now my code looks like this:
Column {
id: myColumn
property var myArray: []
Component.onCompleted: DataSource.getData().forEach(function(bottle) {
// template.createObject(this, { text: bottle.name });
myArray.push(bottle.name)
}, this)
Text {
text: myColumn.myArray[0]
}
Text {
text:myColumn.myArray[1]
}
Text {
text: myColumn.myArray[2]
}
}

just shorter to write with a repeater
i'm really not pushing mzimmers to use it. it was just an example to show what is happening with "property var myArray: []" initialisation

[mzimmers]

@kshegunov said in accessing aggregates (QVector of a struct):

@mzimmers said in accessing aggregates (QVector of a struct):

@LeLev I may have spoken a bit too soon...when I try to apply this from the example to my application, I get the error again.

At the time you reference the array it's empty. That's the reason. Component.onCompleted is executed after the property to Bottle.cellText is bound, so the JS engine throws you an error, there's no such thing as myArray[0] at this point; the array is empty.

That makes sense. So, according to the docs, I can use the onCompleted() handler with any object. Can I somehow use it with my Bottle (defined in QML) object?

[mzimmers]

@LeLev it's a good suggestion; I'm still looking at the code.

I probably don't really need the Column any more, as far as that goes. But I guess I need something to attach the Component.onCompleted() to.

[kshegunov]

@LeLev said in accessing aggregates (QVector of a struct):

i'm really not pushing mzimmers to use it. it was just an example to show what is happening with "property var myArray: []" initialisation

That came out badly. I didn't mean to imply it's wrong in any way, or that your answer is bad. A poor choice of words, sorry.

@mzimmers said in accessing aggregates (QVector of a struct):

That makes sense. So, according to the docs, I can use the onCompleted() handler with any object. Can I somehow use it with my Bottle (defined in QML) object?

Yes all the items can attach to it. Think of it as a "global" signal (sort of).

[ODБOï]

@kshegunov no worries , i just wanted to clarify

@mzimmers said in accessing aggregates (QVector of a struct):

Can I somehow use it with my Bottle (defined in QML) object?

you can refactor the code in the Component.onCompleted into a function and call it when you need

[mzimmers]

@LeLev somehow, I'm overcomplicating this. I already have a function:

    Q_INVOKABLE QString getBottleName(int index) { return m_bottleList[index].m_name; }

The obstacle here is, anything I do is getting called at program startup, before my data structure is populated (as ksheg indicated). I can't figure out how to "delay" my assignment of the cellText property of the bottle until after that vector/list is built.

[kshegunov]

@mzimmers said in accessing aggregates (QVector of a struct):

The obstacle here is, anything I do is getting called at program startup, before my data structure is populated (as ksheg indicated). I can't figure out how to "delay" my assignment of the cellText property of the bottle until after that vector/list is built.

You can't, not without going to dynamic creation of the items. The properties are bound (think similar to QObject::connect) when the items are instantiated. Either you use dynamic creation (e.g. what @LeLev and I suggested), or you need to attach to each of the item's you have statically added to your interface and fetch the value in the completed() handler.

<rant>
The docs sing a very smooth song about integrating QML and C++, but the truth is ... well you already know what the truth is. With a couple of exceptions none of the QQuickItems have public interfaces that can be used from C++ and you're stuck jumping trough hoops to tie up the API ...
</rant>

[mzimmers]

So, as currently implemented, even if my backing data is updated, it doesn't do anything for updating the QML screen...is that about right?

If I use the repeater idea, I guess I'll have to use it for my entire object -- in fact, my entire array of objects. Because using it to create an array won't updating anything that uses that array (for the reasons you cited), will it?

[kshegunov]

@mzimmers said in accessing aggregates (QVector of a struct):

So, as currently implemented, even if my backing data is updated, it doesn't do anything for updating the QML screen...is that about right?

Very much, yes.

If I use the repeater idea, I guess I'll have to use it for my entire object -- in fact, my entire array of objects. Because using it to create an array won't updating anything that uses that array (for the reasons you cited), will it?

Yes, and you still won't get notifications if the data changes.

If you want to get notifications about data changing in QML, switch to QObject derived classes. And provide a QObject * to the QML, then when you create the items dynamically and you pass on the properties, you pass on as bindings, not values. Then the QObject being changed from the C++ is going to be reflected in the QtQuick scene.

<rant 2>
If QML had a C++ API, you could've derived from the correct item and added the data to the item, not create and instantiate yet another type just for keeping the data ...
</rant 2>

[mzimmers]

@kshegunov thanks. I have some follow-on questions, but as they don't focus on the original intent of this thread, and as this thread has gotten fairly long already, I'm going to mark this as solved, and open a new thread.

Thanks to everyone for the assistance on this.