missing-type.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-type.html
\ingroup qmllint-warnings-and-errors
 
\title Missing Type
\brief A type used in a binding or alias was not found.
 
This warning category has multiple warnings:
\list
 \li \l{Cannot Deduce Type of Alias}
 \li \l{No Type Found For Property}
\endlist
 
\section1 Cannot Deduce Type of Alias
 
\section2 What happened?
An alias property points to a property with a C++ type whose QML counterpart was not found. This can
be caused by importing a QML module which do not declare its QML dependencies on other modules.
 
\note If you are importing QML modules with external dependencies, verify that they are
actually installed, and that their modules end up in an
\l{Import Statements#qml-import-path}{import path}.
 
The warning might also indicate that the type of the property referenced by the alias does not have
a QML counterpart. The referenced property type might be missing the
\l{QQmlEngine::}{QML_ELEMENT} macro, for example. Refer to
\l{Defining QML Types from C++} or \l{Overview - QML and C++ Integration} in this case.
 
\section2 Why is this bad?
QML tooling is not able to find the QML counterpart of the C++ type: the
\l{Qt Quick Compiler}{compiler} can't compile this property alias to
C++ and \l{qmllint Reference}{qmllint} as well as \l{\QMLLS Reference}{\QMLLS}
can't analyze this property alias.
 
\section2 Example
Let our QML module have one C++ class with a property \c{myProperty}:
 
\code
#include <QQuickItem>
#include <QtQml/qqmlregistration.h>
#include <QObject>
 
class MyCppObject : public QObject
{
    Q_OBJECT
    QML_ELEMENT
public:
    MyCppObject(QObject *parent = nullptr)
        : QObject(parent)
    {}
 
    Q_PROPERTY(QQuickItem *myProperty READ myProperty WRITE setMyProperty NOTIFY notifyMyProperty)
    QQuickItem *myProperty() { return m_myProperty; }
    void setMyProperty(QQuickItem *item) { emit notifyMyProperty(); m_myProperty = item; }
 
private:
    QQuickItem *m_myProperty;
 
signals:
    void notifyMyProperty();
};
\endcode
 
with following \c{CMakeLists.txt}:
\badcode
project(mymodule VERSION 0.1 LANGUAGES CXX)
 
set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)
qt_standard_project_setup(REQUIRES 6.5)
 
qt_add_executable(appmymodule
    main.cpp
)
 
qt_add_qml_module(appmymodule
    URI mymodule
    VERSION 1.0
    QML_FILES Main.qml HelloWorld.qml
    SOURCES mycppobject.cpp mycppobject.h
)
 
target_link_libraries(appmymodule
 PRIVATE Qt6::Quick
)
\endcode
 
The C++ dependency \c{Quick} was declared, such that this class can compile and the QQuickItem
include can be found. Also, \c{mymodule} does not have any dependency on QtQuick.
 
Now, let's try to use \c{myProperty} in an alias in QML. The program will run but QML tooling like
the \l{Qt Quick Compiler}{compiler}, for example, will complain about the usage of \c{myProperty}:
\qml
import mymodule
 
MyCppObject {
    id: root
 
    property alias myAlias: root.myProperty // not ok: Cannot deduce type of alias [missing-type]
}
\endqml
The reason for the warning message is that in the QML code, the type \c{QQuickItem} of
\c{myProperty} and its QML counterpart \c{Item} are not known, even if you have \c{import QtQuick}
in your QML file. This is because the same type can be exposed multiple times with different
attributes in different modules: \c{mymodule} actually has to be precise about the QML type of
\c{myProperty}.
 
You can fix this warning by adding the dependency in the \c{CMakeLists.txt}:
\badcode
qt_add_qml_module(mymodule
    URI mymodule
    ...
    # declarare QML dependency to QtQuick module
    DEPENDENCIES QtQuick
    ...
)
\endcode
 
Now, the warning should be gone!
 
\b {See also} \l {Declaring module dependencies}.
 
\section1 No Type Found For Property
 
\section2 What happened?
A binding was set on a property whose QML type was not found. This can be caused by a QML module
which does not declare its QML dependencies on other modules.
 
\note If you are importing QML modules with external dependencies, verify that they are
actually installed, and that their modules end up in an
\l{Import Statements#qml-import-path}{import path}.
 
The warning might also indicate that the type of the property does not have
a QML counterpart. The property type might be missing the
\l{QQmlEngine::}{QML_ELEMENT} macro, for example. Refer to
\l{Defining QML Types from C++} or \l{Overview - QML and C++ Integration} in this case.
 
\section2 Why is this bad?
QML tooling is not able to find the QML counterpart of the C++ type: the
\l{Qt Quick Compiler}{compiler} can't compile this property binding to
C++ and \l{qmllint Reference}{qmllint} as well as \l{\QMLLS Reference}{\QMLLS} can't analyze this property binding.
 
\section2 Example
Let our QML module have a C++ class with two properties, \c{myProperty} and \c{myProperty2}:
 
\code
#include <QQuickItem>
#include <QtQml/qqmlregistration.h>
#include <QObject>
 
class MyCppObject : public QObject
{
    Q_OBJECT
    QML_ELEMENT
public:
    MyCppObject(QObject *parent = nullptr)
     : QObject(parent)
    {}
 
    Q_PROPERTY(QQuickItem *myProperty READ myProperty WRITE setMyProperty NOTIFY notifyMyProperty)
    QQuickItem *myProperty() { return m_myProperty; }
    void setMyProperty(QQuickItem *item) { emit notifyMyProperty(); m_myProperty = item; }
 
    Q_PROPERTY(QQuickItem *myProperty2 READ myProperty2 WRITE setMyProperty2 NOTIFY notifyMyProperty2)
    QQuickItem *myProperty2() { return m_myProperty2; }
    void setMyProperty2(QQuickItem *item) { emit notifyMyProperty2(); m_myProperty2 = item; }
 
private:
    QQuickItem *m_myProperty;
    QQuickItem *m_myProperty2;
 
signals:
    void notifyMyProperty();
    void notifyMyProperty2();
};
\endcode
 
with following \c{CMakeLists.txt}:
\badcode
project(mymodule VERSION 0.1 LANGUAGES CXX)
 
set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)
qt_standard_project_setup(REQUIRES 6.5)
 
qt_add_executable(appmymodule
    main.cpp
)
 
qt_add_qml_module(appmymodule
    URI mymodule
    VERSION 1.0
    QML_FILES Main.qml HelloWorld.qml
    SOURCES mycppobject.cpp mycppobject.h
)
 
target_link_libraries(appmymodule
 PRIVATE Qt6::Quick
)
\endcode
 
The C++ dependency \c{Quick} was declared, such that this class can compile and the QQuickItem
include can be found. Also, \c{mymodule} does not have any dependency on QtQuick.
 
Now, let's try to bind \c{myProperty2} to \c{myProperty} in an alias in QML. The program will run
but QML tooling like the \l{Qt Quick Compiler}{compiler}, for example, will complain about the
usage of \c{myProperty}:
 
\qml
import mymodule
 
MyCppObject {
    id: root
 
    myProperty: myProperty2 // not ok: No type found for property "myProperty". [missing-type]
}
\endqml
The reason for the warning message is that in the QML code, the type \c{QQuickItem} of \c{myProperty}
and its QML counterpart \c{Item} are not known: the dependency 'QtQuick' of mymodule was not
declared in the \c{CMakeLists.txt}.
 
You can fix this warning by adding the dependency in the \c{CMakeLists.txt}:
\badcode
qt_add_qml_module(mymodule
    URI mymodule
    ...
    # declarare QML dependency to QtQuick module
    DEPENDENCIES QtQuick
    ...
)
\endcode
 
Now, the warning should be gone!
 
\b {See also} \l {Declaring module dependencies}.
*/