How to get id of child in qml - qt

Question

When I tried to get children ID's slightly modifying this http://qmlbook.github.io/en/ch01/index.html example
// animate.qml
import QtQuick 2.0
Item
{
id: root
width: background.width
height: background.height
property string information: "this is root"
property int rotationStep: 45
Image {
id: background
source: "images/background.png"
}
Image {
id: pole
property string information: "this is pole"
x: (root.width - width)/2+2
y: root.height - height
source: "images/pole.png"
}
Image {
id: pinwheel
anchors.centerIn: parent
source: "images/pinwheel.png"
property string information: "this is pinweel"
// visible: false
Behavior on rotation {
NumberAnimation { duration: 125 }
}
}
Image {
id: blur
opacity: 0
anchors.centerIn: parent
source: "images/blur.png"
property string information: "this is blur"
// visible: false
Behavior on rotation {
NumberAnimation { duration: 125 }
}
Behavior on opacity {
NumberAnimation { duration: 125 }
}
}
// M1>>
focus: true
Keys.onLeftPressed: {
blur.opacity = 0.8
pinwheel.rotation -= root.rotationStep
blur.rotation -= root.rotationStep
}
Keys.onRightPressed: {
blur.opacity = 0.5
pinwheel.rotation += root.rotationStep
blur.rotation += root.rotationStep
}
Keys.onReleased: {
blur.opacity = 0
}
Keys.onSpacePressed:
{
for (var i=0; i<root.children.length; ++i)
console.info(root.children[i].information)
}
Keys.onDeletePressed:
{
for (var i=0; i<root.children.length; ++i)
console.info(root.children[i].id)
}
// <<M1
}
Unfortunately pressing Delete key gives me an error:
qml: undefined
qml: undefined
qml: undefined
qml: undefined
as opposed to pressing spacebar:
qml: undefined
qml: this is pole
qml: this is pinweel
qml: this is blur
Why this script returns undefined id's ?
I need to traverse some objects and be able to tell what is what - so I need to know how to traverse root tree to get id's of its childs and their object types.
Unfortunately I was unable to print the most trival id's and had to add some simple property to get it done but this means a lot of work in real life project since every object needs information property :(
So to reiterate:
Why the id's in this example are undefined?
How to traverse object tree using qml and print its id's and types ?

Answer 1

Id is not an ordinary object property, so it is undefined when you try to assess it through js. And qml doesn't provide operators like typeof. So you need to add type or objectname property manually. I would consider subclassing Image and adding type. Ref: How to do a "is_a", "typeof" or instanceof in QML?
Item
{
id: root
Image {
id: background
type: "image"
}
Image {
id: pole
type: "image"
}
function iter(){
for(var i = 0; i < root.children.length; ++i)
if(root.children[i].type==="image"){
//balabala
}
}
}
}