QDesignerPropertySheetExtension Class

Detailed Description

QDesignerPropertySheetExtension provides a collection of functions that are typically used to query a widget's properties, and to manipulate the properties' appearance in the property editor. For example:

 QDesignerPropertySheetExtension *propertySheet = nullptr;
 QExtensionManager manager = formEditor->extensionManager();

 propertySheet = qt_extension<QDesignerPropertySheetExtension*>(manager, widget);
 int index = propertySheet->indexOf(u"margin"_s);

 propertySheet->setProperty(index, 10);
 propertySheet->setChanged(index, true);

 delete propertySheet;

Note that if you change the value of a property using the QDesignerPropertySheetExtension::setProperty() function, the undo stack is not updated. To ensure that a property's value can be reverted using the undo stack, you must use the QDesignerFormWindowCursorInterface::setProperty() function, or its buddy setWidgetProperty(), instead.

When implementing a custom widget plugin, a pointer to Qt Designer's current QDesignerFormEditorInterface object (formEditor in the example above) is provided by the QDesignerCustomWidgetInterface::initialize() function's parameter.

The property sheet, or any other extension, can be retrieved by querying Qt Designer's extension manager using the qt_extension() function. When you want to release the extension, you only need to delete the pointer.

All widgets have a default property sheet which populates Qt Designer's property editor with the widget's properties (i.e the ones defined with the Q_PROPERTY() macro). But QDesignerPropertySheetExtension also provides an interface for creating custom property sheet extensions.

Keep the following limitations in mind:

• Qt Designer uses the QDesignerPropertySheetExtension to feed its property editor. Whenever a widget is selected in its workspace, Qt Designer will query for the widget's property sheet extension. If the selected widget has an implemented property sheet extension, this extension will override the default property sheet.
• The data types used by the property sheet for some properties are opaque custom QVariant types containing additional information instead of plain Qt data types. For example, this is the case for enumerations, flags, icons, pixmaps and strings.
• Qt Designer's property editor has no implementation for handling Q_PROPERTY types for custom types that have been declared with Q_DECLARE_METATYPE().

To create a property sheet extension, your extension class must inherit from both QObject and QDesignerPropertySheetExtension. Then, since we are implementing an interface, we must ensure that it's made known to the meta object system using the Q_INTERFACES() macro:

 class MyPropertySheetExtension : public QObject,
         public QDesignerPropertySheetExtension
 {
     Q_OBJECT
     Q_INTERFACES(QDesignerPropertySheetExtension)

 public:
     ...
 }

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

In Qt Designer the extensions are not created until they are required. For that reason, when implementing a property sheet 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.

When a property sheet 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 property sheet extension for the selected widget, is found. This factory will then make an instance of the extension. If no such factory can be found, Qt Designer will use the default property sheet.

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(QDesignerPropertySheetExtension))
         return 0;

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

     return 0;
 }

Or you can use an existing factory, expanding the QExtensionFactory::createExtension() function to make the factory able to create a property sheet 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(QDesignerPropertySheetExtension))) {
         return new MyPropertySheetExtension(widget, parent);

     } else {
         return 0;
     }
 }

For a complete example using an extension 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 Designer's task menu.