missing-property.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
 
/*!
\page qmllint-warnings-and-errors-missing-property.html
\ingroup qmllint-warnings-and-errors
 
\title Missing property
\brief [missing-property] Binding a value to a non-existing property.
 
\qmllintwarningcategory missing-property
 
\section1 Can't assign to non-existent default property
 
\section2 What happened?
You assigned an object to a non-existing default property.
 
\section2 Why is this bad?
The QML engine can't assign this object at runtime.
 
\section2 Example
\qml
import QtQuick
 
Item {
    component MyType: QtObject { property Item myItem; }
 
    MyType {
        Item {}
    }
}
 
\endqml
To fix this warning, specify the property you want to bind to or, if you are the author
of the type, mark a property as default:
\qml
import QtQuick
 
Item {
    component MyType: QtObject { property Item myItem; }
 
    MyType {
        myItem: Item {}
    }
 
    component AlternativeMyType: QtObject { default property Item myItem; }
 
    AlternativeMyType {
        Item {} // bound to myItem via default property
    }
}
\endqml
 
\section1 Property does not exist
 
\section2 What happened?
You assigned an expression to a non-existing property.
 
\section2 Why is this bad?
The QML engine can't assign this expression at runtime.
 
\section2 Example
\qml
import QtQuick
 
Item {
    property int myInt
    myItn: 42
}
 
\endqml
To fix this warning, remove the binding or correct a possible typo:
\qml
import QtQuick
 
Item {
    property int myInt
    myInt: 42
}
 
\endqml
 
\section1 Member not found on type
 
\section2 What happened?
You accessed a member in a field member expression that can't be found by QML tooling.
A field member expression is an expression of the form \c {someId.someProperty}.
 
The member might not exist at all, for example when you made a typo. Or it might exist
at runtime (because it exists on a specific sub-type that is always present), but not
on the statically declared type.
 
\section2 Why is this bad?
The QML tooling can't find this member, meaning that features like go-to-definition and
autocomplete won't work.
At runtime, there might be an error or you might get an incorrect result if the member
really does not exist.
There won't be an error if the member actually always exist at runtime, however in that
case there can still be a performance cost due to missed optimizations.
 
\section2 Example
 
\section3 Property does not actually exist
\qml
import QtQuick
 
Item {
    id: self
    property int myInt
    property int myInt2: 1 + self.myItn
}
 
\endqml
To fix this warning, remove the binding or correct a possible typo:
\qml
import QtQuick
 
Item {
    id: self
    property int myInt
    property int myInt2: 1 + self.myInt
}
\endqml
 
\section3 Property exists, but type is not precise enough
\qml
import QtQuick
import QtQuick.Controls.Basic
 
Item {
  component Message : Item {
    required property string sender
    required property string text
  }
  ListView {
    id: messageView
    delegate: Message {}
  }
 
  Button {
    text: "Reply to %1".arg(messageView.currentItem.sender) // not ok
  }
}
\endqml
 
To fix this warning, \l{Type annotations and assertions}{cast the object} you
read from to the more specific type:
 
\qml
import QtQuick
import QtQuick.Controls.Basic
 
Item {
  component Message : Item {
    required property string sender
    required property string text
  }
  ListView {
    id: messageView
    delegate: Message {}
  }
 
  Button {
    text: "Reply to %1".arg((messageView.currentItem as Message).sender) // now ok
  }
}
\endqml
 
In the example, the declared type of \c{messageView}'s
\l{ListView::}{currentItem} property is \l Item, which has no \c sender
property: \c sender is defined only in \c Message. Here, we know however that
\c currentItem always contains a \c Message, so we can cast \c currentItem to
\c Message using a \l{Type annotations and assertions}{type assertion}.
 
*/