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/*!
    \class QDesignerContainerExtension
    \brief The QDesignerContainerExtension class allows you to add pages to
    a custom multi-page container in Qt Designer's workspace.
    \inmodule QtDesigner

    QDesignerContainerExtension provide an interface for creating
    custom container extensions. A container extension consists of a
    collection of functions that \QD needs to manage a multi-page
    container plugin, and a list of the container's pages.

    \image containerextension-example.png

    \warning This is \e not an extension for container plugins in
    general, only custom \e multi-page containers.

    To create a container extension, your extension class must inherit
    from both QObject and QDesignerContainerExtension. For example:

    \snippet doc/src/snippets/code/doc_src_qtdesigner.cpp 6

    Since we are implementing an interface, we must ensure that it's
    made known to the meta object system using the Q_INTERFACES()
    macro. This enables \QD to use the qobject_cast() function to
    query for supported interfaces using nothing but a QObject
    pointer.

    You must reimplement several functions to enable \QD to manage a
    custom multi-page container widget: \QD uses count() to keep track
    of the number pages in your container, widget() to return the page
    at a given index in the list of the container's pages, and
    currentIndex() to return the list index of the selected page. \QD
    uses the addWidget() function to add a given page to the
    container, expecting it to be appended to the list of pages, while
    it expects the insertWidget() function to add a given page to the
    container by inserting it at a given index.

    In \QD the extensions are not created until they are
    required. For that reason you must also create a
    QExtensionFactory, i.e a class that is able to make an instance of
    your extension, and register it using \QD's \l
    {QExtensionManager}{extension manager}.

    When a container extension is required, \QD's \l
    {QExtensionManager}{extension manager} will run through all its
    registered factories calling QExtensionFactory::createExtension()
    for each until the first one that is able to create a container
    extension, is found. This factory will then create the extension
    for the plugin.

    There are four available types of extensions in \QD:
    QDesignerContainerExtension , QDesignerMemberSheetExtension,
    QDesignerPropertySheetExtension and QDesignerTaskMenuExtension.
    \QD's behavior is the same whether the requested extension is
    associated with a multi page container, a member sheet, a property
    sheet or a task menu.

    The QExtensionFactory class provides a standard extension factory,
    and can also be used as an interface for custom extension
    factories. You can either create a new QExtensionFactory and
    reimplement the QExtensionFactory::createExtension() function. For
    example:

    \snippet doc/src/snippets/code/doc_src_qtdesigner.cpp 7

    Or you can use an existing factory, expanding the
    QExtensionFactory::createExtension() function to make the factory
    able to create a container extension as well. For example:

    \snippet doc/src/snippets/code/doc_src_qtdesigner.cpp 8

    For a complete example using the QDesignerContainerExtension
    class, see the \l {designer/containerextension}{Container
    Extension example}. The example shows how to create a custom
    multi-page plugin for \QD.

    \sa QExtensionFactory, QExtensionManager, {Creating Custom Widget
    Extensions}
*/

/*!
    \fn QDesignerContainerExtension::~QDesignerContainerExtension()

    Destroys the extension.
*/

/*!
    \fn int QDesignerContainerExtension::count() const

    Returns the number of pages in the container.
*/

/*!
    \fn QWidget *QDesignerContainerExtension::widget(int index) const

    Returns the page at the given \a index in the extension's list of
    pages.

    \sa addWidget(), insertWidget()
*/

/*!
    \fn int QDesignerContainerExtension::currentIndex() const

    Returns the index of the currently selected page in the
    container.

    \sa setCurrentIndex()
*/

/*!
    \fn void QDesignerContainerExtension::setCurrentIndex(int index)

    Sets the currently selected page in the container to be the
    page at the given \a index in the extension's list of pages.

    \sa currentIndex()
*/

/*!
    \fn void QDesignerContainerExtension::addWidget(QWidget *page)

    Adds the given \a page to the container by appending it to the
    extension's list of pages.

    \sa insertWidget(), remove(), widget()
*/

/*!
    \fn void QDesignerContainerExtension::insertWidget(int index, QWidget *page)

    Adds the given \a page to the container by inserting it at the
    given \a index in the extension's list of pages.

    \sa addWidget(), remove(), widget()
*/

/*!
    \fn void QDesignerContainerExtension::remove(int index)

    Removes the page at the given \a index from the extension's list
    of pages.

    \sa addWidget(), insertWidget()
*/

/*!
    \fn bool QDesignerContainerExtension::canAddWidget() const

    Returns whether a widget can be added. This determines whether
    the context menu options to add or insert pages are enabled.

    This should return false for containers that have a single, fixed
    page, for example QScrollArea or QDockWidget.

    \since 5.0
    \sa addWidget(), canRemove()
*/

/*!
    \fn bool QDesignerContainerExtension::canRemove(int index) const

    Returns whether the widget at the given \a index can be removed.
    This determines whether the context menu option to remove the current
    page is enabled.

    This should return false for containers that have a single, fixed
    page, for example QScrollArea or QDockWidget.

    \since 5.0
    \sa remove(), canAddWidget()
*/