/****************************************************************************

**

** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).

** All rights reserved.

** Contact: Nokia Corporation (qt-info@nokia.com)

**

** This file is part of the documentation of the Qt Toolkit.

**

** $QT_BEGIN_LICENSE:LGPL$

** No Commercial Usage

** This file contains pre-release code and may not be distributed.

** You may use this file in accordance with the terms and conditions

** contained in the Technology Preview License Agreement accompanying

** this package.

**

** GNU Lesser General Public License Usage

** Alternatively, this file may be used under the terms of the GNU Lesser

** General Public License version 2.1 as published by the Free Software

** Foundation and appearing in the file LICENSE.LGPL included in the

** packaging of this file.  Please review the following information to

** ensure the GNU Lesser General Public License version 2.1 requirements

** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.

**

** In addition, as a special exception, Nokia gives you certain additional

** rights.  These rights are described in the Nokia Qt LGPL Exception

** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.

**

** If you have questions regarding the use of this file, please contact

** Nokia at qt-info@nokia.com.

**

**

**

**

**

**

**

**

** $QT_END_LICENSE$

**

****************************************************************************/

/*!

    \page properties.html

    \title Qt's Property System

    \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 a

    QObject that is 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 that is the property type is

    declared in MyClass and registered with the \l{Meta-Object System}

    using the Q_ENUMS() macro. This makes the enumeration values

    available as strings for use as in the call to 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_ENUMS() macro.

    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.

*/