Detailed Description

In Qt Designer the extensions are not created until they are required. For that reason, when implementing an extension, 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.

The registration of an extension factory is typically made in the QDesignerCustomWidgetInterface::initialize() function:

        void MyPlugin::initialize(QDesignerFormEditorInterface *formEditor)
        {
            if (initialized)
                return;

            QExtensionManager *manager = formEditor->extensionManager();
            Q_ASSERT(manager != 0);

            manager->registerExtensions(new MyExtensionFactory(manager),
                                        Q_TYPEID(QDesignerTaskMenuExtension));

            initialized = true;
        }

The QExtensionManager is not intended to be instantiated directly. You can retrieve an interface to Qt Designer's extension manager using the QDesignerFormEditorInterface::extensionManager() function. A pointer to Qt Designer's current QDesignerFormEditorInterface object (formEditor in the example above) is provided by the QDesignerCustomWidgetInterface::initialize() function's parameter. When implementing a custom widget plugin, you must subclass the QDesignerCustomWidgetInterface to expose your plugin to Qt Designer.

Then, when an 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 the requested extension for the selected object, is found. This factory will then make an instance of the extension.

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 container, a member sheet, a property sheet or a task menu.

For a complete example using the QExtensionManager class, see the Task Menu Extension example. The example shows how to create a custom widget plugin for Qt Designer, and how to to use the QDesignerTaskMenuExtension class to add custom items to Qt Designer's task menu.