stale-property-read.qdoc

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// Copyright (C) 2026 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\ingroup qmllint-warnings-and-errors
 
\title Stale property read
\brief [stale-property-read] Reading non-constant and non-notifiable property.
 
\qmllintwarningcategory stale-property-read
 
\section1 Reading non-constant and non-notifiable property
 
\section2 What happened?
A property that is both non-constant and non-notifyable is used in a binding.
 
\section2 Why is that bad?
\l{The Property System}{Constant properties} never change in value.
\l{The Property System}{Notifiable properties} notify bindings depending on
them when they change in value so the bindings can re-evaluate.
 
Because this property is neither constant nor notifiable, it could change
during the program's execution without notifying dependent bindings. This
would leave the bindings in a potentially outdated state, as their value
depends on the property.
 
\section2 Example
\code
class Circle : public QObject
{
    Q_OBJECT
    QML_ELEMENT
    Q_PROPERTY(double radius READ radius WRITE setRadius FINAL)
public:
    double radius() const { return m_radius; }
    void setRadius(double radius) { m_radius = radius; }
private:
    double m_radius = 1;
};
\endcode
\qml
import QtQuick
 
Item {
    Circle {
        id: circle
        property double area: Math.PI * radius * radius
    }
 
    Component.onCompleted: {
        console.log(circle.area) // 3.14159...
        circle.radius = 2
        console.log(circle.area) // 3.14159...
    }
}
\endqml
To fix this warning, either mark the property as
\l{The Property System}{constant} if it will not change in value or make it
\l{The Property System}{notifiable}.
\code
class Circle : public QObject
{
    Q_OBJECT
    QML_ELEMENT
    Q_PROPERTY(double radius READ radius WRITE setRadius NOTIFY radiusChanged FINAL)
public:
    double radius() const { return m_radius; }
    void setRadius(double radius) {
        if (radius != m_radius) {
            m_radius = radius;
            emit radiusChanged();
        }
    }
signals:
    void radiusChanged();
private:
    double m_radius = 1;
};
\endcode
\qml
import QtQuick
 
Item {
    Circle {
        id: circle
        property double area: Math.PI * radius * radius
    }
 
    Component.onCompleted: {
        console.log(circle.area) // 3.14159...
        circle.radius = 2
        console.log(circle.area) // 12.5663...
    }
}
\endqml
*/