path: root/doc/src/properties.qdoc

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/*!
    \page properties.html
    \title Qt's Property System
    \ingroup architecture
    \brief An overview of 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 doc/src/snippets/code/doc_src_properties.qdoc 0

    Here are some typical examples of property declarations taken from
    class QWidget.

    \snippet doc/src/snippets/code/doc_src_properties.qdoc 1

    A property behaves like a class data member, but it has additional
    features accessible through the \l {Meta-Object System}.

    \list 

    \o A \c READ accessor function is required. 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 pointer or
    reference to that type. e.g., QWidget::focus is a read-only property
    with \c READ function, QWidget::hasFocus().

    \o 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.

    \o 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.

    \o A \c NOTIFY signal is optional. If defined, the signal will be
    emitted whenever the value of the property changes. The signal must
    take one parameter, which must be of the same type as the property; the
    parameter will take the new value of the property.

    \o The \c DESIGNABLE attribute indicates whether the property
    should be visible in the property editor of GUI design tool (e.g.,
    \l {Qt Designer}). Most properties are \c DESIGNABLE (default
    true). Instead of true or false, you can specify a boolean
    member function.

    \o The \c SCRIPTABLE attribute indicates whether this property
    should be accessible by a scripting engine (default true).
    Instead of true or false, you can specify a boolean member
    function.

    \o 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.

    \o 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.

    \o The presence of the \c CONSTANT attibute 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.

    \o The presence of the \c FINAL attribute indicates that the property
    will not be overridden by a derived class.  This can be used for performance
    optimizations in some cases, but is not enforced by moc.  Care must be taken
    never to override a \c FINAL property.

    \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 doc/src/snippets/code/doc_src_properties.qdoc 2

    Because QDate is user-defined, you must include the \c{<QDate>}
    header file with the property declaration.

    For QMap, QList, and QValueList properties, the property value is
    a QVariant whose value is the entire list or map.  Note that the
    Q_PROPERTY string cannot contain commas, because commas separate
    macro arguments. Therefore, you must use \c QMap as the property
    type instead of \c QMap<QString,QVariant>. For consistency, also
    use \c QList and \c QValueList instead of \c QList<QVariant> and
    \c QValueList<QVariant>.

    \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 doc/src/snippets/code/doc_src_properties.qdoc 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 doc/src/snippets/code/doc_src_properties.qdoc 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 MyClass, which is derived from QObject and
    which uses the Q_OBJECT macro in its private section. We want to
    declare a property in MyClass to keep track of a priorty
    value. The name of the property will be \e priority, and its type
    will be an enumeration type named \e Priority, which is defined in
    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_ENUMS() 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 MyClass
    then might look like this:

    \snippet doc/src/snippets/code/doc_src_properties.qdoc 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.

    Given a pointer to an instance of MyClass or a pointer to an
    instance of QObject that happens to be an instance of MyClass, we
    have two ways to set its priority property.

    \snippet doc/src/snippets/code/doc_src_properties.qdoc 6

    In the example, the enumeration type used for the property type
    was locally declared in MyClass. Had it been declared in another
    class, its fully qualified name (i.e., OtherClass::Priority) would
    be required.  In addition, that other class must also inherit
    QObject and register the enum type using Q_ENUMS().

    A similar macro, Q_FLAGS(), is also available. Like Q_ENUMS(), 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_FLAGS()
    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, for example:

    \snippet doc/src/snippets/code/doc_src_properties.qdoc 7

    Like other meta-data, class information is accessible at run-time
    through the meta-object; see QMetaObject::classInfo() for details.
*/