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diff --git a/src/qml/doc/src/qmllint/duplicate-property-binding.qdoc b/src/qml/doc/src/qmllint/duplicate-property-binding.qdoc new file mode 100644 index 0000000000..4826a552ea --- /dev/null +++ b/src/qml/doc/src/qmllint/duplicate-property-binding.qdoc @@ -0,0 +1,122 @@ +// Copyright (C) 2023 The Qt Company Ltd. +// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only + +/*! +\page qmllint-warnings-and-errors-duplicate-property-binding.html +\ingroup qmllint-warnings-and-errors + +\title Duplicate Bindings +\brief A property was bound multiple times. + +This warning category has multiple warnings: +\list + \li \l{Duplicate Interceptor On Property} + \li \l{Cannot Combine Value Source And Binding} + \li \l{Duplicate Value Source On Property} +\endlist + +\section1 Duplicate Interceptor On Property + +\section2 What happened? +One property has multiple \l{Property Modifier Types}{interceptors}. + +\section2 Why is this bad? +Setting multiple interceptors on the same property is unsupported by the QML engine. + +\section2 Example + +Lets use \l{Behavior} as interceptor twice on the same property: +\qml +import QtQuick + +Rectangle { + Behavior on width { + NumberAnimation { duration: 1000 } + } + Behavior on width { // not ok: Duplicate interceptor on property "width" [duplicate-property-binding] + NumberAnimation { duration: 2000 } + } +} +\endqml +You can fix this warning by removing all but one \l{Behavior}: +\qml +import QtQuick + +Rectangle { + Behavior on width { + NumberAnimation { duration: 2000 } + } +} +\endqml + +\b {See also} \l {Property Modifier Types}. + +\section1 Duplicate Value Source On Property + +\section2 What happened? +One property has multiple \l{Property Value Sources}{value sources}. + +\section2 Why is this bad? +The value sources will show unexpected behavior when combined. See \l{Example}{example} below. + +\section2 Example + +Lets use \l{NumberAnimation} as value source twice on the same property: +\qml +import QtQuick + +Rectangle { + NumberAnimation on x { to: 50; duration: 1000 } + NumberAnimation on x { to: 10; duration: 100 } // not ok: Duplicate value source on property "x" [duplicate-property-binding] + + onXChanged: console.log(x) +} +\endqml + +If you check the output of that program, you will see that the two NumberAnimation will interleave +each other, which is probably not the effect that was intended. +You can fix this warning by removing all but one \l{NumberAnimation}: +\qml +import QtQuick + +Rectangle { + NumberAnimation on x { to: 50; duration: 1000 } +} +\endqml + + +\section1 Cannot Combine Value Source And Binding + +\section2 What happened? +One property has a \l{Property Value Sources}{value source} and a binding on the same property. + +\section2 Why is this bad? +The binding will updated the property value before the value source starts updating this property. +This may lead to unexpected behavior, and is also harder to read. + +\section2 Example + +Lets use \l{NumberAnimation} as value source on the same property: +\qml +import QtQuick + +Rectangle { + NumberAnimation on x { to: 50; duration: 1000 } // not ok: Cannot combine value source and binding on property "x" [duplicate-property-binding] + x: 55 + + onXChanged: console.log(x) +} +\endqml + +If you check the output of that program, you will see that the \l{NumberAnimation} will animate +from 55 to 50, which would be easier to read with following code: +\qml +import QtQuick + +Rectangle { + NumberAnimation on x { from: 55; to: 50; duration: 1000 } // ok: intentions are clearer now! +} +\endqml + +*/ + |