attached-property-reuse.qdoc

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// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\ingroup qmllint-warnings-and-errors
 
\title Attached property reuse
\brief [attached-property-reuse] Attached type was initialized multiple times.
 
\qmllintwarningcategory attached-property-reuse
 
\section1 Using attached type already initialized in a parent scope
 
\section2 What happened?
You initialized a propagating
\l{Attached Properties and Attached Signal Handlers}{attached type}
multiple times.
 
\note This mostly happens for attached types that inherit from
\l{QQuickAttachedPropertyPropagator}.
 
\section2 Why is this bad?
Propagating attached objects consume memory for each instantiation but only need to be initialized once.
 
\section2 Example
\qml
import QtQuick
import QtQuick.Templates as T
import QtQuick.Controls.Material // contains the Material attached type
 
T.ToolBar {
    id: control
 
    // first instantiation of Material's attached property
    property color c: Material.toolBarColor
 
    background: Rectangle {
         // second instantiation of Material's attached property, wrong!
        color: Material.toolBarColor
    }
}
 
\endqml
To fix this warning, query the attached type from the parent:
\qml
import QtQuick
import QtQuick.Templates as T
import QtQuick.Controls.Material // contains the Material attached type
 
T.ToolBar {
    id: control
 
    // first instantiation of Material's attached property
    property color c: Material.toolBarColor
 
    background: Rectangle {
        // use control's attached property, correct!
        color: control.Material.toolBarColor
    }
}
\endqml
*/