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/*!
    \example containerextension
    \ingroup examples-designer
    \title Container Extension Example

    \brief Creating a custom multi-page plugin for \QD.

    The Container Extension example shows how to create a custom multi-page
    plugin for \QD using the
    QDesignerContainerExtension class.

    \image containerextension-example.png

    To provide a custom widget that can be used with \QD, we need to
    supply a self-contained implementation. In this example we use a
    custom multi-page widget designed to show the container extension
    feature.

    An extension is an object which modifies the behavior of \QD. The
    QDesignerContainerExtension enables \QD to manage and manipulate a
    custom multi-page widget, i.e. adding and deleting pages to the
    widget.

    There are four available types of extensions in \QD:

    \list
        \li QDesignerMemberSheetExtension  provides an extension that allows
           you to manipulate a widget's member functions which is displayed
           when configuring connections using Qt Designer's mode for editing
           signals and slots.
        \li QDesignerPropertySheetExtension provides an extension that
           allows you to manipulate a widget's properties which is displayed
           in Qt Designer's property editor.
        \li QDesignerTaskMenuExtension provides an extension that allows
           you to add custom menu entries to \QD's task menu.
        \li QDesignerContainerExtension provides an extension that allows
           you to add (and delete) pages to a multi-page container plugin
           in \QD.
    \endlist

    You can use all the extensions following the same pattern as in
    this example, only replacing the respective extension base
    class. For more information, see \l{Qt Designer C++ Classes}.

    The Container Extension example consists of four classes:

    \list
    \li \c MultiPageWidget is a custom container widget that lets the user
       manipulate and populate its pages, and navigate among these
       using a combobox.
    \li \c MultiPageWidgetPlugin exposes the \c MultiPageWidget class
       to \QD.
    \li \c MultiPageWidgetExtensionFactory creates a
       \c MultiPageWidgetContainerExtension object.
    \li \c MultiPageWidgetContainerExtension provides the container
       extension.
    \endlist

    The project file for custom widget plugins needs some additional
    information to ensure that they will work within \QD. For example,
    custom widget plugins rely on components supplied with \QD, and
    this must be specified in the project file that we use. We will
    first take a look at the plugin's project file.

    Then we will continue by reviewing the \c MultiPageWidgetPlugin
    class, and take a look at the \c MultiPageWidgetExtensionFactory
    and \c MultiPageWidgetContainerExtension classes. Finally, we will
    take a quick look at the \c MultiPageWidget class definition.

    \section1 The Project File: containerextension.pro

    The project file must contain some additional information to
    ensure that the plugin will work as expected:

    \snippet containerextension/containerextension.pro 0
    \snippet containerextension/containerextension.pro 1

    The \c TEMPLATE variable's value makes \c qmake create the custom
    widget as a library. Later, we will ensure that the widget will be
    recognized as a plugin by Qt by using the Q_PLUGIN_METADATA() macro
    to export the relevant widget information.

    The \c CONFIG variable is set to \c plugin, which ensures that \c qmake
    considers the custom widget a plugin library.

    The \c QT variable contains the value \c designer. Since the plugin uses
    components supplied with \QD that require linkage, this value ensures
    that our plugin links against \QD's library (\c libQtDesigner.so).

    The header and source files for the widget are declared in the
    usual way:

    \snippet containerextension/containerextension.pro 2

    We provide an implementation of the plugin interface so that \QD
    can use the custom widget. In this particular example we also
    provide implementations of the container extension interface and
    the extension factory.

    It is important to ensure that the plugin is installed in a
    location that is searched by \QD. We do this by specifying a
    target path for the project and adding it to the list of items to
    install:

    \snippet doc/snippets/doc_src_examples_containerextension.pro 0

    The container extension is created as a library, and will be
    installed alongside the other \QD plugins when the project is
    installed (using \c{make install} or an equivalent installation
    procedure).

    Note that if you want the plugins to appear in a Visual Studio
    integration, the plugins must be built in release mode and their
    libraries must be copied into the plugin directory in the install
    path of the integration (for an example, see \c {C:/program
    files/trolltech as/visual studio integration/plugins}).

    For more information about plugins, see the \l {How to Create Qt
    Plugins} documentation.

    \section1 MultiPageWidgetPlugin Class Definition

    The \c MultiPageWidgetPlugin class exposes the \c MultiPageWidget
    class to \QD. Its definition is similar to the \l
    {customwidgetplugin}{Custom Widget Plugin} example's
    plugin class which is explained in detail. The parts of the class
    definition that is specific to this particular custom widget is
    the class name and a couple of private slots:

    \snippet containerextension/multipagewidgetplugin.h 0

    The plugin class provides \QD with basic information about our
    plugin, such as its class name and its include file. Furthermore
    it knows how to create instances of the \c MultiPageWidget widget.
    \c MultiPageWidgetPlugin also defines the \l
    {QDesignerCustomWidgetInterface::initialize()}{initialize()}
    function which is called after the plugin is loaded into \QD. The
    function's QDesignerFormEditorInterface parameter provides the
    plugin with a gateway to all of \QD's API's.

    In the case of a multipage widget such as ours, we must also implement
    two private slots, currentIndexChanged() and pageTitleChanged(),
    to be able to update \QD's property editor whenever the user views
    another page or changes one of the page titles. To be able to give
    each page their own title, we have chosen to use the
    QWidget::windowTitle property to store the page title (for more
    information see the MultiPageWidget class implementation in
    \e containerextension/multipagewidget.cpp. Note that currently there
    is no way of adding a custom property (for example, a page title) to
    the pages without using a predefined property as placeholder.

    The \c MultiPageWidgetPlugin class inherits from both QObject and
    QDesignerCustomWidgetInterface. It is important to remember, when
    using multiple inheritance, to ensure that all the interfaces
    (i.e. the classes that doesn't inherit Q_OBJECT) are made known to
    the meta object system using the Q_INTERFACES() macro. This
    enables \QD to use \l {qobject_cast(QObject * object)} to query for supported
    interfaces using nothing but a QObject pointer.

    \section1 MultiPageWidgetPlugin Class Implementation

    The MultiPageWidgetPlugin class implementation is in most parts
    equivalent to the \l {customwidgetplugin}{Custom Widget
    Plugin} example's plugin class:

    \snippet containerextension/multipagewidgetplugin.cpp 0
    \codeline
    \snippet containerextension/multipagewidgetplugin.cpp 3

    One of the functions that differ is the isContainer() function
    which returns true in this example since our custom widget is
    intended to be used as a container.

    \snippet containerextension/multipagewidgetplugin.cpp 1

    Another function that differ is the function creating our custom widget:

    \snippet containerextension/multipagewidgetplugin.cpp 2

    In addition to create and return the widget, we connect our custom
    container widget's currentIndexChanged() signal to the plugin's
    currentIndexChanged() slot to ensure that \QD's property editor is
    updated whenever the user views another page. We also connect the
    widget's pageTitleChanged() signal to the plugin's
    pageTitleChanged() slot.

    The currentIndexChanged() slot is called whenever our custom
    widget's currentIndexChanged() \e signal is emitted, i.e. whenever
    the user views another page:

    \snippet containerextension/multipagewidgetplugin.cpp 8

    First, we retrieve the object emitting the signal using the
    QObject::sender() and qobject_cast() functions. If it's called in
    a slot activated by a signal, QObject::sender() returns a pointer
    to the object that sent the signal; otherwise it returns 0.

    \snippet containerextension/multipagewidgetplugin.cpp 9

    Once we have the widget we can update the property editor. \QD
    uses the QDesignerPropertySheetExtension class to feed its
    property editor, and whenever a widget is selected in its
    workspace, Qt Designer will query for the widget's property sheet
    extension and update the property editor.

    So what we want to achieve is to notify \QD that our widget's \e
    internal selection has changed: First we use the static
    QDesignerFormWindowInterface::findFormWindow() function to
    retrieve the QDesignerFormWindowInterface object containing the
    widget. The QDesignerFormWindowInterface class allows you to query
    and manipulate form windows appearing in Qt Designer's
    workspace. Then, all we have to do is to emit its \l
    {QDesignerFormWindowInterface::emitSelectionChanged()}{emitSelectionChanged()}
    signal, forcing an update of the property editor.

    When changing a page title a generic refresh of the property
    editor is not enough because it is actually the page's property
    extension that needs to be updated. For that reason we need to
    access the QDesignerPropertySheetExtension object for the page
    which title we want to change. The QDesignerPropertySheetExtension
    class also allows you to manipulate a widget's properties, but to
    get hold of the extension we must first retrieve access to \QD's
    extension manager:

    \snippet containerextension/multipagewidgetplugin.cpp 10
    \snippet containerextension/multipagewidgetplugin.cpp 11

    Again we first retrieve the widget emitting the signal, using the
    QObject::sender() and qobject_cast() functions. Then we retrieve
    the current page from the widget that emitted the signal, and we
    use the static QDesignerFormWindowInterface::findFormWindow()
    function to retrieve the form containing our widget.

    \snippet containerextension/multipagewidgetplugin.cpp 12

    Now that we have the form window, the QDesignerFormWindowInterface
    class provides the \l
    {QDesignerFormWindowInterface::core()}{core()} function which
    returns the current QDesignerFormEditorInterface object. The
    QDesignerFormEditorInterface class allows you to access Qt
    Designer's various components. In particular, the
    QDesignerFormEditorInterface::extensionManager() function returns
    a reference to the current extension manager.

    \snippet containerextension/multipagewidgetplugin.cpp 13

    Once we have the extension manager we can update the extension
    sheet: First we retrieve the property extension for the page which
    title we want to change, using the qt_extension() function.  Then
    we retrieve the index for the page title using the
    QDesignerPropertySheetExtension::indexOf() function. As previously
    mentioned, we have chosen to use the QWidget::windowTitle property
    to store the page title (for more information see the
    MultiPageWidget class implementation in
    \e containerextension/multipagewidget.cpp.
    Finally, we implicitly force an update of the page's property
    sheet by calling the
    QDesignerPropertySheetExtension::setChanged() function.

    \snippet containerextension/multipagewidgetplugin.cpp 4

    Note also the initialize() function: The \c initialize() function
    takes a QDesignerFormEditorInterface object as argument.

    \snippet containerextension/multipagewidgetplugin.cpp 5

    When creating extensions associated with custom widget plugins, we
    need to access \QD's current extension manager which we retrieve
    from the QDesignerFormEditorInterface parameter.

    In addition to allowing you to manipulate a widget's properties,
    the QExtensionManager class provides extension management
    facilities for \QD. Using \QD's current extension manager you can
    retrieve the extension for a given object. You can also register
    and unregister an extension for a given object. Remember that an
    extension is an object which modifies the behavior of \QD.

    When registrering an extension, it is actually the associated
    extension factory that is registered. In \QD, extension factories
    are used to look up and create named extensions as they are
    required. So, in this example, the container extension itself is
    not created until \QD must know whether the associated widget is a
    container, or not.

    \snippet containerextension/multipagewidgetplugin.cpp 6

    We create a \c MultiPageWidgetExtensionFactory object that we
    register using \QD's current \l {QExtensionManager}{extension
    manager} retrieved from the QDesignerFormEditorInterface
    parameter. The first argument is the newly created factory and the
    second argument is an extension identifier which is a string. The
    \c Q_TYPEID() macro simply convert the string into a
    QLatin1String.

    The \c MultiPageWidgetExtensionFactory class is a subclass of
    QExtensionFactory. When \QD must know whether a widget is a
    container, or not, \QD's extension manager will run through all
    its registered factories invoking the first one which is able to
    create a container extension for that widget. This factory will in
    turn create a \c MultiPageWidgetExtension object.

    \snippet containerextension/multipagewidgetplugin.cpp 7

    Finally, take a look at the \c domXml() function. This function
    includes default settings for the widget in the standard XML
    format used by \QD. In this case, we specify the container's first
    page; any inital pages of a multi-page widget must be specified
    within this function.

    \snippet containerextension/multipagewidgetplugin.h 1

    Remember to use the Q_PLUGIN_METADATA() macro to export the
    MultiPageWidgetPlugin class for use with Qt's plugin handling
    classes: This macro ensures that \QD can access and construct the
    custom widget. Without this macro, there is no way for \QD to use
    the widget.

    \section1 MultiPageWidgetExtensionFactory Class Definition

    The \c MultiPageWidgetExtensionFactory class inherits QExtensionFactory
    which provides a standard extension factory for \QD.

    \snippet containerextension/multipagewidgetextensionfactory.h 0

    The subclass's purpose is to reimplement the
    QExtensionFactory::createExtension() function, making it able to
    create a \c MultiPageWidget container extension.


    \section1 MultiPageWidgetExtensionFactory Class Implementation

    The class constructor simply calls the QExtensionFactory base
    class constructor:

    \snippet containerextension/multipagewidgetextensionfactory.cpp 0

    As described above, the factory is invoked when \QD must know
    whether the associated widget is a container, or not.

    \snippet containerextension/multipagewidgetextensionfactory.cpp 1

    \QD's behavior is the same whether the requested extension is
    associated with a container, a member sheet, a property sheet or a
    task menu: Its extension manager runs through all its registered
    extension factories calling \c createExtension() for each until
    one responds by creating the requested extension.

    So the first thing we do in \c
    MultiPageWidgetExtensionFactory::createExtension() is to check if
    the QObject, for which the extension is requested, is in fact a \c
    MultiPageWidget object. Then we check if the requested extension
    is a container extension.

    If the object is a MultiPageWidget requesting a container
    extension, we create and return a \c MultiPageWidgetExtension
    object. Otherwise, we simply return a null pointer, allowing \QD's
    extension manager to continue its search through the registered
    factories.


    \section1 MultiPageWidgetContainerExtension Class Definition

    The \c MultiPageWidgetContainerExtension class inherits
    QDesignerContainerExtension which allows you to add (and delete)
    pages to a multi-page container plugin in \QD.

    \snippet containerextension/multipagewidgetcontainerextension.h 0

    It is important to recognize that the QDesignerContainerExtension
    class only is intended to provide \QD access to your custom
    multi-page widget's functionality; your custom multi-page widget
    must implement functionality corresponding to the extension's
    functions.

    Note also that we implement a constructor that takes \e two
    arguments: the parent widget, and the \c MultiPageWidget object
    for which the task menu is requested.

    QDesignerContainerExtension provides a couple of menu entries in
    \QD's task menu by default, enabling the user to add or delete
    pages to the associated custom multi-page widget in \QD's
    workspace.

    \section1 MultiPageWidgetContainerExtension Class Implementation

    In the constructor we save the reference to the \c MultiPageWidget
    object sent as parameter, i.e the widget associated with the
    extension. We will need this later to access the custom multi-page
    widget performing the requested actions.

    \snippet containerextension/multipagewidgetcontainerextension.cpp 0

    To fully enable \QD to manage and manipulate your custom
    multi-page widget, you must reimplement all the functions of
    QDesignerContainerExtension:

    \snippet containerextension/multipagewidgetcontainerextension.cpp 1
    \codeline
    \snippet containerextension/multipagewidgetcontainerextension.cpp 2
    \codeline
    \snippet containerextension/multipagewidgetcontainerextension.cpp 3

    You must reimplement \l
    {QDesignerContainerExtension::addWidget()}{addWidget()} adding a
    given page to the container, \l
    {QDesignerContainerExtension::count()}{count()} returning the
    number of pages in the container, and \l
    {QDesignerContainerExtension::currentIndex()}{currentIndex()}
    returning the index of the currently selected page.

    \snippet containerextension/multipagewidgetcontainerextension.cpp 4
    \codeline
    \snippet containerextension/multipagewidgetcontainerextension.cpp 5
    \codeline
    \snippet containerextension/multipagewidgetcontainerextension.cpp 6
    \codeline
    \snippet containerextension/multipagewidgetcontainerextension.cpp 7

    You must reimplement \l
    {QDesignerContainerExtension::insertWidget()}{insertWidget()}
    adding a given page to the container at a given index, \l
    {QDesignerContainerExtension::remove()}{remove()} deleting the
    page at a given index, \l
    {QDesignerContainerExtension::setCurrentIndex()}{setCurrentIndex()}
    setting the index of the currently selected page, and finally \l
    {QDesignerContainerExtension::widget()}{widget()} returning the
    page at a given index.

    \section1 MultiPageWidget Class Definition

    The MultiPageWidget class is a custom container widget that lets
    the user manipulate and populate its pages, and navigate among
    these using a combobox.

    \snippet containerextension/multipagewidget.h 0

    The main detail to observe is that your custom multi-page widget
    must implement functionality corresponding to the
    QDesignerContainerExtension's member functions since the
    QDesignerContainerExtension class only is intended to provide Qt
    Designer access to your custom multi-page widget's functionality.

    In addition, we declare the \c currentIndex and \c pageTitle
    properties, and their associated set and get functions. By
    declaring these attributes as properties, we allow \QD to manage
    them in the same way it manages the properties the MultiPageWidget
    widget inherits from QWidget and QObject, for example featuring
    the property editor.

    Note the \c STORED attribute in the declaration of the \c
    pageTitle property: The \c STORED attribute indicates persistence,
    i.e. it declares whether the property's value must be remembered
    when storing an object's state. As mentioned above, we have chosen
    to store the page title using the QWidget::windowTitle property to
    be able to give each page their own title. For that reason the \c
    pageTitle property is a "fake" property, provided for editing
    purposes, and doesn't need to be stored.

    We must also implement and emit the currentIndexChanged() and
    pageTitleChanged() signals to ensure that \QD's property editor is
    updated whenever the user views another page or changes one of the
    page titles.

    See the MultiPageWidget class implementation in
    \e containerextension/multipagewidget.cpp for more details.
*/