Stale property read

This warning category is spelled [stale-property-read] by qmllint.

Reading non-constant and non-notifiable property

What happened?

A property that is both non-constant and non-notifyable is used in a binding.

Why is that bad?

Constant properties never change in value. 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 This would leave the bindings in a potentially outdated state, as their value depends on the property.

Example

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;
};

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...
    }
}

To fix this warning, either mark the property as constant if it will not change in value or make it notifiable.

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;
};

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...
    }
}

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