Qml implement my own changable contentItem property

[aatwo]

Hello. I have a custom QML component (MyComponent) which has a content item it animates in various ways. I want users to be able to change this default content item using a contentItem property similar to how many other QML components work. How can this be achieved?

For example:

ApplicationWindow {
    MyComponent {
        contentItem: Rectangle {
            color: "yellow"
        }
    }
}

In my custom component I originally had a standard Rectangle item which I converted to a property in the following way, but as soon as I turn a child item into a property I can no longer see it:

// This rectangle is invisible
property Rectangle contentItem: Rectangle {
    id: content_item
    x: (root.width / 2) - (content_item.width / 2)
    y: 0
    width: Math.min(root.width * 0.7, 400)
    height: Math.min(root.height * 0.7, 400)
    color: "blue"
}

// This rectangle is visible but can't be changed in another file
Rectangle {
    id: content_item
    x: (root.width / 2) - (content_item.width / 2)
    y: 0
    width: Math.min(root.width * 0.7, 400)
    height: Math.min(root.height * 0.7, 400)
    color: "blue"
}

I also tried using a property alias but got the error it was read only when I tried to reassign it in an external QML file.

Any help would be appreciated, thanks.

[dheerendra]

Can you post complete example ?

[aatwo]

Well since i dont know how to achieve what I want it is hard to provide code. But I can provide code demonstrating what I showed above where the property item is invisible for some reason when converted to a property:

main.qml

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.0

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

    MyComponent {
        anchors.fill: parent
    }
}

MyComponent.qml

import QtQuick 2.0

Rectangle {
    id: root
    color: "red"
    property Rectangle contentItem: Rectangle {
        id: content_item
        anchors.fill: parent
        color: "blue"
    }
}

With this code (Qt 5.15.2) the application is red, but should be blue.

[IntruderExcluder]

If I understand you correctly - default property is what you need. You can see this example for more info.