QExtensionFactory Class

Detailed Description

In Qt Widgets Designer the extensions are not created until they are required. For that reason, when implementing a custom 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 Widgets Designer's extension manager.

The QExtensionManager class provides extension management facilities for Qt Widgets Designer. 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 a 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 Widgets 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.

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 nullptr;

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

             return nullptr;
         }

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

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

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

             if (iid == Q_TYPEID(QDesignerContainerExtension))
                 return new MyContainerExtension(widget, parent);

             return nullptr;
         }

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