ContentChildren doesn't update after Adding a Component Dynamically in QML

[carles.sole.grau]

Hi everyone,
I have a problem in a project when I'm trying to adding dynamically a Component into a Toolbar (qtcontrols 2). My sample code is:

ToolBar {
    id: toolbar    
    property var myNewComponent;
    Component.onCompleted: {
        var component = Qt.createComponent("MyNewComponent.qml"); 
        if (component.status === Component.Ready) {
            myNewComponent = component.createObject(toolbar, { width: toolbar.width,  height: toolbar.implicitHeight});
        }

        var totalHeight = 0;
        console.log("Num of Children: "  + toolbar.contentChildren.length)
        for(var i in toolbar.contentChildren)
        {
            console.log("Children " + i + " is a: " + toolbar.contentChildren[i])
            console.log("Children " + i + " height is: " + toolbar.contentChildren[i].height)
        }
    }
    Rectangle {
        id: myRect
        color: "green"
        width: toolbar.width
        height: toolbar.implicitHeight
        anchors.bottom: parent.bottom
    }
}

It seems like after adding the component myNewComponent, the contentChildren does not update because it seems like there is just one item (myRect).

Console:

qml: Num of Children: 1
qml: Children 0 is a: QQuickRectangle(0x2496f40)
qml: Children 0 height is: 48

Anyone knows if it's a bug, or if there is a code error?

Thank you very much.

[A Former User]

Hi! You need to use the toolbar's contentItem as the parent for your items:

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    function add_item() {
        var component = Qt.createComponent("MyNewComponent.qml");
        var posX = toolbar.contentItem.children.length * 25
        component.createObject(toolbar.contentItem, {x: posX});
        console.log("Num of Children: "  + toolbar.contentItem.children.length)
    }

    header: ToolBar {
        id: toolbar
        Component.onCompleted: console.log("Num of Children: "  + toolbar.contentItem.children.length)
    }

    Button {
        text: "Add another item"
        anchors.centerIn: parent
        onClicked: add_item()
    }
}

It's also hidden in the documentation ;-)

Items declared as children of a Pane are automatically parented to the Pane's contentItem. Items created dynamically need to be explicitly parented to the contentItem.

[carles.sole.grau]

@Wieland
Thank you very much.
It helped me a lot.
Thanks!

[carles.sole.grau]

@Wieland
And for destruction? component.destroy() is not enough?
Thanks

[A Former User]

No need to destroy component, once it goes out of scope, the JS garabage collector will destroy it (sooner or later), just like every other JS variable. Also, destroying the component does not affect your items. The component is like the blueprint for a car: If you build cars using a blueprint and then burn the blueprint, the cars will still be there. To remove items from the scene your have to remove them from the toolbar's contentItem children list. The easiest way to do that is to just call destroy() on the item you want to get rid of. The contentItem's children list is a QObject and will notice when one of it's children get's destroyed. The list will then remove said item automatically. The following code does exactly this:

MyItem.qml

import QtQuick 2.0

Rectangle {
    property string text: ""

    width: 20
    height: 20
    anchors.verticalCenter: parent.verticalCenter
    color: "orange"

    Text {
        text: parent.text
        anchors.centerIn: parent
    }

    Component.onDestruction: console.log("Destructing item " + text)
}

main.qml

import QtQuick 2.7
import QtQuick.Controls 2.0

ApplicationWindow {
    id: mainWindow
    visible: true
    width: 640
    height: 480
    title: "Fun with items"
    color: "darkgrey"

    function push_item() {
        var item_number = toolbar.contentItem.children.length
        var posX = item_number * 25 + 5
        var component = Qt.createComponent("MyItem.qml");
        component.createObject(toolbar.contentItem, {x: posX, text: item_number});
    }

    function pop_item() {
        var len = toolbar.contentItem.children.length
        if (len === 0)
            return
        toolbar.contentItem.children[len-1].destroy()
    }

    header: ToolBar {
        id: toolbar
    }

    footer: Rectangle {
        height: 32
        width: mainWindow.width
        color: "black"
        Text {
           x: 5
           anchors.verticalCenter: parent.verticalCenter
           color: "white"
           text: "Number of items: " + toolbar.contentItem.children.length
        }
    }

    Row {
        anchors.centerIn: parent
        spacing: 5
        Button {
            text: "Add item"
            onClicked: push_item()
        }
        Button {
            text: "Remove last item"
            onClicked: pop_item()
        }
        Button {
            text: "Exit"
            onClicked: mainWindow.close()
        }
    }
}

And this is what it looks like:

[carles.sole.grau]

@Wieland
Thanks again!
The example is very clear to understand.
But from my needs, I still have a problem,
Regarding on your example, if in pop function I add:

function pop_item() {
        var len = toolbar.contentItem.children.length
        if (len === 0)
            return
        toolbar.contentItem.children[len-1].destroy() 
       console.log("NUM OF CHILDS: " + toolbar.contentItem.children.length)
    }

It's like in the moment when console.log it's done, there is still this item.
Is it because this item is still destorying? And what can we do? Is there a signal like onDestroyed?

I try with that (but I don't know if it's really the most correct way):

header: ToolBar {
        id: toolbar
        onContentChildrenChanged:{
            console.log("NUM OF CHILDS: " + toolbar.contentItem.children.length)
        }
    }

Thanks again.

[A Former User]

.destroy() doesn't destruct the object immediately but schedules it for deletion. It will be destroyed once the eventloop processes the next events. So in this case, destruction takes place when pop_item() returns. Your solution to use onContentChildrenChanged looks good to me.

[carles.sole.grau]

@Wieland
Thank you for your help