QDesignerContainerExtension Class

Detailed Description

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

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

 class MyContainerExtension : public QObject,
        public QDesignerContainerExtension
 {
     Q_OBJECT
     Q_INTERFACES(QDesignerContainerExtension)

 public:
     MyContainerExtension(MyCustomWidget *widget,
                          QObject *parent = 0);
     int count() const;
     QWidget *widget(int index) const;
     int currentIndex() const;
     void setCurrentIndex(int index);
     void addWidget(QWidget *widget);
     void insertWidget(int index, QWidget *widget);
     void remove(int index);

 private:
     MyCustomWidget *myWidget;
 };

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 Qt Designer to use the qobject_cast() function to query for supported interfaces using nothing but a QObject pointer.

You must reimplement several functions to enable Qt Designer to manage a custom multi-page container widget: Qt Designer 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. Qt Designer 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 Qt Designer 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 Qt Designer's extension manager.

When a container extension is required, Qt Designer's 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 Qt Designer: QDesignerContainerExtension , QDesignerMemberSheetExtension, QDesignerPropertySheetExtension and QDesignerTaskMenuExtension. Qt Designer'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:

 QObject *ANewExtensionFactory::createExtension(QObject *object,
         const QString &iid, QObject *parent) const
 {
     if (iid != Q_TYPEID(QDesignerContainerExtension))
         return 0;

     if (MyCustomWidget *widget = qobject_cast<MyCustomWidget*>
            (object))
         return new MyContainerExtension(widget, parent);

     return 0;
 }

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:

 QObject *AGeneralExtensionFactory::createExtension(QObject *object,
         const QString &iid, QObject *parent) const
 {
     MyCustomWidget *widget = qobject_cast<MyCustomWidget*>(object);

     if (widget && (iid == Q_TYPEID(QDesignerTaskMenuExtension))) {
         return new MyTaskMenuExtension(widget, parent);

     } else if (widget && (iid == Q_TYPEID(QDesignerContainerExtension))) {
         return new MyContainerExtension(widget, parent);

     } else {
         return 0;
     }
 }

For a complete example using the QDesignerContainerExtension class, see the Container Extension example. The example shows how to create a custom multi-page plugin for Qt Designer.