use-proper-function.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 Use proper function
\brief [use-proper-function] Calling something that might not be a function.
 
\qmllintwarningcategory use-proper-function
 
\section1 Property is a variant property: It may or may not be a method
 
\section2 What happened?
You used a property of the type \l[Qml]{var} as if it were a callable function.
 
\section2 Why is this bad?
 
This affects the readability of the code, the QML engine will error out if the
property does not contain a callable function, and the QML tooling can't apply
specific method optimizations.
 
 
\section2 Example
\qml
import QtQuick
 
Item {
    property var fun: () => console.log("Hello")
    Component.onCompleted: { fun(); }
}
 
\endqml
To fix this warning, declare \c fun as a function to be able to call it:
\qml
import QtQuick
 
Item {
    function fun() {
        console.log("Hello")
    }
 
    Component.onCompleted: { fun(); }
}
 
\endqml
 
\section1 Property is a QJSValue property: It may or may not be a method
 
\section2 What happened?
You used a property of the type \c QJSValue as if it were a callable function.
 
\note Properties of the type QJSValue can only be defined in C++.
 
\section2 Why is this bad?
The property was very probably not meant to be called and will make the QML engine
error out at runtime.
 
\section2 Example
\qml
import QtQuick
 
Rectangle {
    // Rectangle has a property gradient of the type QJSValue
    Component.onCompleted: { console.log(gradient()); }
}
 
\endqml
To fix this warning, remove the call to \c gradient:
\qml
import QtQuick
 
Item {
    // Rectangle has a property gradient of the type QJSValue
    Component.onCompleted: { console.log(gradient); }
}
 
\endqml
 
Alternatively, consider replacing the property definition with a regular
\l{Q_INVOKABLE} method or a \l{signalsandslots.html#slots}{slot} if you are
the author of the property. This is only possible if the function is never
meant to be changed in QML. If you actually need to store a callback, you're
out of luck.
 
\section1 Signal is shadowed by a property
 
\section2 What happened?
You called a signal that is shadowed by a property or you shadowed a signal
with a property.
 
\section2 Why is this bad?
The caller very probably expected to call a signal, and not the shadowing
property. This could make the QML engine error out at runtime.
 
\section2 Example
\qml
import QtQuick
 
Item {
    component Base: Item {
        signal helloSignal
    }
 
    Base {
        property int helloSignal // shadows helloSignal inherited from Base
        Component.onCompleted: { helloSignal(); }
    }
}
 
\endqml
To fix this warning, rename the shadowing property:
\qml
import QtQuick
 
Item {
    component Base: Item {
        signal helloSignal
    }
 
    Base {
        property int helloSignal2 // does not shadow anymore
        Component.onCompleted: { helloSignal(); }
    }
}
 
\endqml
 
\section1 Method is shadowed by a property
 
See \{Signal is shadowed by a property}.
 
\section1 Slot is shadowed by a property
 
See \{Signal is shadowed by a property}.
 
\section1 Property is not a method
 
\section2 What happened?
You used a property of a type other than \l[Qml]{var} or \c QJSValue
as if it were a callable function.
 
\section2 Why is this bad?
The property can't be called and will make the QML engine error out at runtime.
 
\section2 Example
\qml
import QtQuick
 
Item {
    property int hello
    Component.onCompleted: { console.log(hello()); }
}
 
\endqml
To fix this warning, remove the call or make \c hello a function:
\qml
import QtQuick
 
Item {
    property int hello
    Component.onCompleted: { console.log(hello); }
 
    // alternative:
    function helloAlternative() { ... }
    Component.onCompleted: { console.log(helloAlternative()); }
}
 
\endqml
 
*/