qt6-changes.qdoc

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// Copyright (C) 2020 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page qml-changes-qt6.html
    \title Changes to Qt QML
    \ingroup changes-qt-5-to-6
    \brief Migrate Qt QML to Qt 6.
 
    Qt 6 is a result of the conscious effort to make the framework more
    efficient and easy to use.
 
    We try to maintain binary and source compatibility for all the public
    APIs in each release. But some changes were inevitable in an effort to
    make Qt a better framework.
 
    In this topic we summarize those changes in Qt QML, and provide
    guidance to handle them.
 
    \section1 QML language
 
    \section2 URL resolution
 
    In Qt 5, relative urls were often, albeit inconsistently, directly resolved, especially when
    assigned to an url property. In Qt 6 this is no longer the case.
 
    If you had a QML file stored under "/home/qml/example.qml", and "example.qml" contained
    \code
    property url imageFolder: "./images"
    \endcode
    then the url property would store the URL "/home/qml/images". This made it impossible to use
    relative URLs in QML in this way, so in Qt 6, the URL stays relative, and only gets resolved
    when this is required (e.g. when it is used as the source of an Image component).
    If you depend on the old behavior, you can use \c Qt.resolvedUrl
 
    For example, if you have code like
 
    \code
    property url imageFolder: "./images"
    \endcode
 
    you can rewrite it as
 
    \code
    property url imageFolder: Qt.resolvedUrl("./images")
    \endcode
 
    Qt.resolvedUrl can be used in both Qt 5 and 6.
 
    As a porting aid, the \c{QML_COMPAT_RESOLVE_URLS_ON_ASSIGNMENT} environment variable can be set
    to 1 to obtain the Qt 5 behavior. This is possible since Qt 6.2.2. However, it is recommended
    to only use this to unblock a port, and to use \c Qt.resolvedUrl as explained above.
 
    \section2 Variant Properties
 
    \c variant properties, which have been marked as obsolete since Qt 5, are now treated in exactly
    the same way as \c var properties.
    Code that relied on implicit string conversion triggered on assignment to variant properties
    should be updated to explicitly create an object of the correct type.
 
    For example, if you have code like
 
    \code
    property variant myColor: "red"
    \endcode
 
    you can rewrite it as
 
    \code
    property variant myColor: Qt.color("red")
    \endcode
 
    Implicit conversions were done for strings that could be parsed as
    \list
    \li color (use Qt.color instead),
    \li date (use the Date object instead),
    \li rect (use Qt.rect instead) and
    \li size (use Qt.size instead)
    \endlist
 
    \c variant still remains a deprecated keyword in Qt 6, though new code is strongly encouraged to
    use \c var properties instead.
 
    \note If the type of the property is known not to change, use a property of the concrete type,
    instead of a \c var property.
 
    \note These conversions were also applied to \c QVariant properties of classes registered with
    the engine. As with \c variant properties, code that relied on implicit string conversions need
    to use the corresponding functions of the Qt object.
 
    \section1 Source Incompatible API Changes
 
    \section2 Changed API
 
    \c QQmlListProperty's  \c CountFunction and \c AtFunction have been changed to use \c qsizetype
    instead of \c int to align with the corresponding changes in Qt's containers.
 
    For example, if you have code like
 
    \code
    int myCountFunction(QQmlListProperty<MyType> *);
    MyType *myAtFunction(QQmlListProperty<MyType> *, int);
 
    QQmlListProperty<MyType> myReadOnlyList(containingObject, container, &myCountFunction,
                                            &myAtFunction);
    \endcode
 
    you can rewrite it as
 
    \code
    qsizetype myCountFunction(QQmlListProperty<MyType> *);
    MyType *myAtFunction(QQmlListProperty<MyType> *, qsizetype);
 
    QQmlListProperty<MyType> myReadOnlyList(containingObject, container, &myCountFunction,
                                            &myAtFunction);
    \endcode
 
    Code which needs to supports both Qt 5 and Qt 6 can either use a typedef which is \c int in Qt 5
    and \c qsizetype in Qt 6, or use \c QList::size_type, which already is such a type alias.
 
    \section2 Removed API
 
    Various deprecated functions have been removed.
 
    \list
    \li The QQmlListProperty constructor taking a reference has been removed.
 
    For example, if you have code like
 
    \code
    QQmlListProperty<QObject>(owner, owner->objectList);
    \endcode
 
    you can rewrite it as
 
    \code
    QQmlListProperty<QObject>(owner, &owner->objectList);
    \endcode
 
    \li The functions \c qmlDebug, \c qmlInfo, \c qmlWarning,  \c qmlContext and \c qmlEngine used
    to exist both in the global namespace (or Qt namespace in namespaced builds), and in the \c
    QtQml namespace. These functions now exist only in the global namespace.
 
    For example, if you have code like
 
    \code
    QQmlEngine *engine = QtQml::qmlEngine(qmlObject);
    \endcode
 
    you can rewrite it as
 
    \code
    QQmlEngine *engine = qmlEngine(qmlObject);
    \endcode
 
    \li The \c qmlRegisterType overload taking no arguments has been removed. Use
    \c qmlRegisterAnonymousType instead, or switch to declarative type registration with
    \c QML_ANONYMOUS.
 
    For example, if you have code like
 
    \code
    class AnonymousType : public QObject {
      // ...
    };
 
    qmlRegisterType<AnonymousType>();
    \endcode
 
    you can rewrite it as
 
    \code
    class AnonymousType : public QObject {
      // ...
    };
 
    qmlRegisterAnonymousType<AnonymousType>("MyModule", 1);
    \endcode
 
    Or alternatively
    \code
    class AnonymousType : public QObject {
        QML_ANONYMOUS
        // ...
    };
    \endcode
 
    \li  The overloads of \c qmlRegisterExtendedType and \c qmlRegisterInterface
    which take no version argument have been removed. Use the overloads providing a
    version, or switch to declarative type registration with QML_EXTENDED
    and QML_INTERFACE.
 
    For example, if you have code like
 
    \code
    struct GreetInterface
    {
       virtual ~GreetInterface();
       virtual void greet();
    };
    Q_DECLARE_INTERFACE(GreetInterface, "org.hi.GreetInterface")
 
    qmlRegisterInterface<GreetInterface>("Greeter");
    \endcode
 
    you can rewrite it as
 
    \code
    struct GreetInterface
    {
       virtual ~GreetInterface();
       virtual void greet();
    };
    Q_DECLARE_INTERFACE(GreetInterface, "org.hi.GreetInterface")
 
    qmlRegisterInterface<GreetInterface>("Greeter", 1);
    \endcode
 
    Alternatively
 
    \code
    struct GreetInterface
    {
       QML_INTERFACE(Greeter)
       virtual ~GreetInterface();
       virtual void greet();
    };
    Q_DECLARE_INTERFACE(GreetInterface, "org.hi.GreetInterface")
    \endcode
 
    \note In new code, declarative type registration should be preferred.
 
    \li The function \c QJSValue::engine has been removed. If access to the engine is required, a
    reference to it must be stored instead.
 
    \li \c qmlAttachedPropertiesObjectById and \c qmlAttachedPropertiesObject(int *, const QObject *,
    const QMetaObject *, bool) have been removed.  Use the
    \c qmlAttachedPropertiesObject(QObject *, QQmlAttachedPropertiesFunc, bool) overload of
    \c qmlAttachedPropertiesObject instead.
 
    \li \c QJSEngine::installTranslatorFunctions has been removed. \c QJSEngine::installExtensions
    is available as a replacement.
 
    For example, if you have code like
 
    \code
    QJSEngine engine;
    engine.installTranslatorFunctions();
    \endcode
 
    you can rewrite it as
 
    \code
    QJSEngine engine;
    engine.installExtensions(QJSEngine::TranslationExtension);
    \endcode
 
    \endlist
 
    \sa {Modern QML modules}, {Port QML modules to CMake}
*/