qml: DelegateModel + Package makes delegate have null parent transiently - qt

Question

When using using Package, State and ParentChange to re-parent a delegate to one of several views, the delegate seems to momentarily have a null parent while bindings are re-evaluated. This causes errors if the delegate has bindings to parent.width or similar.
This is disturbing because Qt's published Package example displays the same problem if the initial state is changed to the other alternative
(To reproduce: copy ..../Qt/Examples/Qt-5.14.1/quick/views/package/{view.qml,Delegate.qml} and change the initial upTo value from 0 to 7 in view.qml and then run "qmlscene view.qml".).
Below is a simplification of Qt's example, with a constant state. The active State uses a ParentChange which defines width; this somehow causes the delegate to momentarily have a null parent, and the binding for width:parent.width gets an error:
test.qml line 26: TypeError: Cannot read property 'width' of null
What causes this, or rather, how to avoid it? And, how could the Package mechanism ever work given that Qt's example has the same problem as noted above?
import QtQuick 2.0
import QtQml.Models 2.1
Rectangle {
width: 300; height: 400
ListModel {
id: myModel
ListElement { display: "One" }
ListElement { display: "Two" }
ListElement { display: "Three" }
}
DelegateModel {
id: visualModel
model: myModel
delegate:
Package {
Rectangle { id: rectA;
width: 40; height: 25; Package.name: 'pkgA'
}
Rectangle { id: rectB;
width: 40; height: 25; Package.name: 'pkgB'
}
Rectangle {
id: wrapper
width: parent.width; // ERROR HERE: parent is sometimes null!
height: 25
color: 'lightsteelblue'
state: 'state1'
states: [
State {
name: 'state1'
ParentChange {
target: wrapper; parent: rectB
width: rectB.width; height: rectB.height // **CAUSES ERROR**
}
}
]
Component.onCompleted: {
console.log("wrapper onCompleted: parent is",parent);
}
onParentChanged: {
console.log("wrapper parent changed to",parent);
}
}
}
}
ListView {
width: 300; height: 200
model: visualModel.parts.pkgA
}
ListView {
width: 300; height: 200
model: visualModel.parts.pkgB
}
}

Answer 1

Ok, finally figured this out. You are starting with this:
width: parent.width
which is a Qt binding expression. It will update whenever parent or parent.width changes. And when the wrapper Rectangle first is created, parent starts rightly as null - it has no visual parent yet.
But then you do a ParentChange like this:
ParentChange {
target: wrapper; parent: rectB
width: rectB.width; height: rectB.height // **CAUSES ERROR**
}
The width assignment in the ParentChange is removing your previous binding and placing a new one of rectB.width width on it. This is why console logging on the original binding stops firing at this point. That binding has been removed.
So, bottom line, you are misinterpreting what is happening. Parent is null to start with and gets set via the state initial value before Component.onCompleted fires. Then when you do the ParentChange, you are wiping out the original binding so the debugging you went through was misleading.
So back to your original question, to stop the error, just remove the width and height bindings altogether from wrapper and let the ParentChange apply the ones there which won't throw errors.