Detection of event "property changed" - qt

Question

There is a property var1 declared in file1.qml. The property is visible in other qml files, but it is not possible to detect its event onChange.
file1.qml:
Item {
id: obj1Id
property int var1: 0
...
Component.onCompleted: {
var1=2
var1Changed() <<<<<<<<<<<< emit signal "changed"
}
onVar1Changed: {} <<<<<<<<<<<< signal is detected
...
}
file2.qml:
Item {
id: obj2Id
property int local_var: 0
...
Component.onCompleted: {
local_var = obj1Id.var1 <<<<<<<<<<<<<<<<< It is OK
}
onVar1Changed: {} <<<<<<<<<<<<<<<<< Error
obj1Id.onVar1Changed: {} <<<<<<<<<<<<<<<<< Error
...
}
Is it possible this approach or in file2.qml have to add property int local_var: obj1Id.var1 and then onLocal_varChanged: {}?

Answer 1

There are few moments in your code that could bring misunderstanding. Makes sense to figure out it first.
You wrote:
Component.onCompleted: {
var1=2
var1Changed() <<<<<<<<<<<< emit signal "changed"
}
but you do not need to manually emit that signal. QML will do it for you. So, actually nothing will change without that line.
You wrote
property int local_var: 0
...
Component.onCompleted: {
local_var = obj1Id.var1 <<<<<<<<<<<<<<<<< It is OK
}
this is totally valid code, but you need to understand that this is one-time assign and not a binding of a value obj1Id.var1 to local_var, e.g. if you change obj1Id.var1 -- do not expect local_var to be changed respectively.
To make a real binding of a value you may use Binding component or Qt.binding for creating property bindings from JavaScript or as you've proposed -- add property int local_var: obj1Id.var1.
In any case you need to have a valid reference to your obj1Id in your file2.qml.
If bindings approach doesn't fit you -- you can use Connections QML Type.
In your case this code should be placed inside file2.qml:
Connections {
target: obj1Id
onVar1Changed: console.log("obj1Id.var1 changed")
}