properties.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page properties.html
    \title The Property System
    \brief An overview of Qt's property system.
    \ingroup explanations-basics
    \ingroup qt-basic-concepts
    \keyword Qt's Property System
 
    Qt provides a sophisticated property system similar to the ones
    supplied by some compiler vendors. However, as a compiler- and
    platform-independent library, Qt does not rely on non-standard
    compiler features like \c __property or \c [property]. The Qt
    solution works with \e any standard C++ compiler on every platform
    Qt supports. It is based on the \l {Meta-Object System} that also
    provides inter-object communication via \l{signals and slots}.
 
    \section1 Requirements for Declaring Properties
 
    To declare a property, use the \l {Q_PROPERTY()} {Q_PROPERTY()}
    macro in a class that inherits QObject.
 
    \snippet code/doc_src_properties.cpp 0
 
    Here are some typical examples of property declarations taken from
    class QWidget.
 
    \snippet code/doc_src_properties.cpp 1
 
    Here is an example showing how to export member variables as Qt
    properties using the \c MEMBER keyword.
    Note that a \c NOTIFY signal must be specified to allow QML property bindings.
 
    \snippet code/doc_src_properties.cpp 8
 
    A property behaves like a class data member, but it has additional
    features accessible through the \l {Meta-Object System}.
 
    \list
 
    \li A \c READ accessor function is required if no \c MEMBER variable was
    specified. It is for reading the property value. Ideally, a const function
    is used for this purpose, and it must return either the property's type or a
    const reference to that type. e.g., QWidget::focus is a read-only
    property with \c READ function, QWidget::hasFocus(). If a \c BINDABLE is
    specified, you can write \c{READ default} to have the \c READ accessor
    generated from the \c BINDABLE.
 
    \li A \c WRITE accessor function is optional. It is for setting the
    property value. It must return void and must take exactly one
    argument, either of the property's type or a pointer or reference
    to that type. e.g., QWidget::enabled has the \c WRITE function
    QWidget::setEnabled().  Read-only properties do not need \c WRITE
    functions. e.g., QWidget::focus has no \c WRITE function. If you specify
    both a \c BINDABLE and \c{WRITE default}, a \c WRITE accessor will be
    generated from the \c BINDABLE. The generated \c WRITE accessor will \e not
    explicitly emit any signal declared with \c NOTIFY. You should register
    the signal as change handler to the \c BINDABLE, for example using
    \l{Q_OBJECT_BINDABLE_PROPERTY}.
 
    \li A \c MEMBER variable association is required if no \c READ accessor
    function is specified. This makes the given member variable
    readable and writable without the need of creating \c READ and \c WRITE accessor
    functions. It's still possible to use \c READ or \c WRITE accessor functions in
    addition to \c MEMBER variable association (but not both), if you need to
    control the variable access.
 
    \li A \c RESET function is optional. It is for setting the property
    back to its context specific default value. e.g., QWidget::cursor
    has the typical \c READ and \c WRITE functions, QWidget::cursor()
    and QWidget::setCursor(), and it also has a \c RESET function,
    QWidget::unsetCursor(), since no call to QWidget::setCursor() can
    mean \e {reset to the context specific cursor}. The \c RESET
    function must return void and take no parameters.
 
    \li A \c NOTIFY signal is optional. If defined, it should specify one
    existing signal in that class that is emitted whenever the value
    of the property changes.
    \c NOTIFY signals for \c MEMBER variables must take zero or one parameter,
    which must be of the same type as the property. The parameter will take the
    new value of the property. The \c NOTIFY signal should only be emitted when
    the property has really been changed, to avoid bindings being unnecessarily
    re-evaluated in QML, for example. The signal is emitted automatically when
    the property is changed via the Qt API (QObject::setProperty,
    QMetaProperty, etc.), but not when the MEMBER is changed directly.
 
    \li A \c REVISION number or \c REVISION() macro is optional. If included,
    it defines the property and its notifier signal to be used in a particular
    revision of the API (usually for exposure to QML). If not included, it
    defaults to 0.
 
    \li The \c DESIGNABLE attribute indicates whether the property
    should be visible in the property editor of GUI design tool (e.g.,
    \l {Qt Widgets Designer Manual}{\QD}). Most properties are \c DESIGNABLE
    (default true). Valid values are true and false.
 
    \li The \c SCRIPTABLE attribute indicates whether this property
    should be accessible by a scripting engine (default true).
    Valid values are true and false.
 
    \li The \c STORED attribute indicates whether the property should
    be thought of as existing on its own or as depending on other
    values. It also indicates whether the property value must be saved
    when storing the object's state. Most properties are \c STORED
    (default true), but e.g., QWidget::minimumWidth() has \c STORED
    false, because its value is just taken from the width component
    of property QWidget::minimumSize(), which is a QSize.
 
    \li The \c USER attribute indicates whether the property is
    designated as the user-facing or user-editable property for the
    class. Normally, there is only one \c USER property per class
    (default false). e.g., QAbstractButton::checked is the user
    editable property for (checkable) buttons. Note that QItemDelegate
    gets and sets a widget's \c USER property.
 
    \li The \c {BINDABLE bindableProperty} attribute indicates that the
    property supports \l {Qt Bindable Properties}{bindings},
    and that it is possible to set and inspect
    bindings to this property via the meta object system (QMetaProperty).
    \c bindableProperty names a class member of type QBindable<T>, where T
    is the property type. This attribute was introduced in Qt 6.0.
 
    \li The presence of the \c CONSTANT attribute indicates that the property
    value is constant.  For a given object instance, the READ method of a
    constant property must return the same value every time it is called.  This
    constant value may be different for different instances of the object.  A
    constant property cannot have a WRITE method or a NOTIFY signal.
 
    \li \c FINAL, \c VIRTUAL, \c OVERRIDE modifiers mirror the semantics of their C++ and
    \l {Override Semantics}{QML counterparts}, allowing to make property overriding explicit at the
    meta-object level.
 
    \note At present, these modifiers are not enforced by moc.
    They are recognized syntactically and are primarily used for QML runtime enforcement and tooling
    diagnostics. Future versions may introduce stricter compile-time validation and warnings for
    invalid overrides across modules.
 
    \note If you want to change accessing behaviour for a property, use the
    polymorphism provided by C++.
 
 
    \li The presence of the \c REQUIRED attribute indicates that the property
    should be set by a user of the class. This is not enforced by moc, and is
    mostly useful for classes exposed to QML. In QML, classes with REQUIRED
    properties cannot be instantiated unless all REQUIRED properties have
    been set.
 
    \endlist
 
    The \c READ, \c WRITE, and \c RESET functions can be inherited.
    They can also be virtual. When they are inherited in classes where
    multiple inheritance is used, they must come from the first
    inherited class.
 
    The property type can be any type supported by QVariant, or it can
    be a user-defined type. In this example, class QDate is considered
    to be a user-defined type.
 
    \snippet code/doc_src_properties.cpp 2
 
    Because QDate is user-defined, you must include the \c{<QDate>}
    header file with the property declaration.
 
    For historical reasons, \a QMap and \a QList as property types
    are synonym of \a QVariantMap and \a QVariantList.
 
    \section1 Reading and Writing Properties with the Meta-Object System
 
    A property can be read and written using the generic functions
    QObject::property() and QObject::setProperty(), without knowing
    anything about the owning class except the property's name.  In
    the code snippet below, the call to QAbstractButton::setDown() and
    the call to QObject::setProperty() both set property "down".
 
    \snippet code/doc_src_properties.cpp 3
 
    Accessing a property through its \c WRITE accessor is the better
    of the two, because it is faster and gives better diagnostics at
    compile time, but setting the property this way requires that you
    know about the class at compile time. Accessing properties by name
    lets you access classes you don't know about at compile time. You
    can \e discover a class's properties at run time by querying its
    QObject, QMetaObject, and \l {QMetaProperty} {QMetaProperties}.
 
    \snippet code/doc_src_properties.cpp 4
 
    In the above snippet, QMetaObject::property() is used to get \l
    {QMetaProperty} {metadata} about each property defined in some
    unknown class. The property name is fetched from the metadata and
    passed to QObject::property() to get the \l {QVariant} {value} of
    the property in the current \l {QObject}{object}.
 
    \section1 A Simple Example
 
    Suppose we have a class \c MyClass, which is derived from QObject and which
    uses the Q_OBJECT macro. We want to declare a property in \c MyClass to keep
    track of a priority value. The name of the property will be \c priority, and
    its type will be an enumeration type named \c Priority, which is defined in
    \c MyClass.
 
    We declare the property with the Q_PROPERTY() macro in the private
    section of the class. The required \c READ function is named \c
    priority, and we include a \c WRITE function named \c setPriority.
    The enumeration type must be registered with the \l {Meta-Object
    System} using the Q_ENUM() macro. Registering an enumeration type
    makes the enumerator names available for use in calls to
    QObject::setProperty(). We must also provide our own declarations
    for the \c READ and \c WRITE functions. The declaration of \c MyClass
    then might look like this:
 
    \snippet code/doc_src_properties.cpp 5
 
    The \c READ function is const and returns the property type. The
    \c WRITE function returns void and has exactly one parameter of
    the property type. The meta-object compiler enforces these
    requirements. The equality check in the \c WRITE function, while not
    mandatory, is good practice as there is no point in notifying and
    potentially forcing re-evaluation in other places if nothing has
    changed.
 
    Given a pointer to an instance of \c MyClass or a pointer to a
    QObject that is an instance of \c MyClass, we have two ways to set
    its priority property:
 
    \snippet code/doc_src_properties.cpp 6
 
    In the example, the enumeration type that is the property type is
    declared in \c MyClass and registered with the \l{Meta-Object System}
    using the Q_ENUM() macro. This makes the enumeration values
    available as strings for use as in the call to \l{QObject::}{setProperty()}.
    Had the enumeration type been declared in another class, its fully
    qualified name (i.e., OtherClass::Priority) would be required, and
    that other class would also have to inherit QObject and register
    the enumeration type there using the Q_ENUM() macro.
 
    A similar macro, Q_FLAG(), is also available. Like Q_ENUM(), it
    registers an enumeration type, but it marks the type as being a
    set of \e flags, i.e., values that can be OR'd together. An I/O
    class might have enumeration values \c Read and \c Write and then
    QObject::setProperty() could accept \c{Read | Write}. Q_FLAG()
    should be used to register this enumeration type.
 
    \section1 Dynamic Properties
 
    QObject::setProperty() can also be used to add \e new properties
    to an instance of a class at runtime. When it is called with a
    name and a value, if a property with the given name exists in the
    QObject, and if the given value is compatible with the property's
    type, the value is stored in the property, and true is returned.
    If the value is \e not compatible with the property's type, the
    property is \e not changed, and false is returned. But if the
    property with the given name doesn't exist in the QObject (i.e.,
    if it wasn't declared with Q_PROPERTY()), a new property with the
    given name and value is automatically added to the QObject, but
    false is still returned. This means that a return of false can't
    be used to determine whether a particular property was actually
    set, unless you know in advance that the property already exists
    in the QObject.
 
    Note that \e dynamic properties are added on a per instance basis,
    i.e., they are added to QObject, not QMetaObject. A property can
    be removed from an instance by passing the property name and an
    invalid QVariant value to QObject::setProperty(). The default
    constructor for QVariant constructs an invalid QVariant.
 
    Dynamic properties can be queried with QObject::property(), just
    like properties declared at compile time with Q_PROPERTY().
 
    \sa {Meta-Object System}, {Signals and Slots}
 
    \section1 Properties and Custom Types
 
    Custom types used by properties need to be registered using the
    Q_DECLARE_METATYPE() macro so that their values can be stored in
    QVariant objects. This makes them suitable for use with both
    static properties declared using the Q_PROPERTY() macro in class
    definitions and dynamic properties created at run-time.
 
    \sa Q_DECLARE_METATYPE(), QMetaType, QVariant
 
    \section1 Adding Additional Information to a Class
 
    Connected to the property system is an additional macro,
    Q_CLASSINFO(), that can be used to attach additional
    \e{name}--\e{value} pairs to a class's meta-object. This is
    used for instance to mark a property as the \e default one
    in the context of \l{QML Object Types}:
 
    \snippet code/doc_src_properties.cpp 7
 
    Like other meta-data, class information is accessible at run-time
    through the meta-object; see QMetaObject::classInfo() for details.
 
    \section1 Using Bindable Properties
 
    Three different types can be used to implement bindable properties:
    \list
        \li \l QProperty
        \li \l QObjectBindableProperty
        \li \l QObjectComputedProperty.
    \endlist
    The first one is a general class for bindable properties. The latter
    two can only be used inside a \l QObject.
 
    For more information, including examples, see the classes mentioned above
    and the general tips on implementing and using
    \l {Qt Bindable Properties}{bindable properties}.
 
    \sa {Qt Bindable Properties}, {Defining QML Types from C++}
*/