Qt 4.3: Creating Custom Widgets for Qt Designer

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# Creating Custom Widgets for Qt Designer

Qt Designer's plugin-based architecture allows user-defined and third party custom widgets to be edited in the same way as standard Qt widgets. All the features of the custom widgets are made available to Qt Designer, including widget properties, signals, and slots. Since Qt Designer uses real widgets during the form design process, custom widgets will appear the same as they do when previewed.

The ability to create custom widgets in Qt Designer is one of the features provided by the QtDesigner module.

- Getting Started
- Providing an Interface Description
- Plugin Requirements
- Creating Well Behaved Widgets
- Building and Installing the Plugin
- Related Examples

## Getting Started

The process of integrating an existing custom widget with Qt Designer usually just requires a suitable description for the widget and an appropriate project file.

## Providing an Interface Description

To inform Qt Designer about the type of widget we want to provide, we must create a subclass of QDesignerCustomWidgetInterface that describes the various properties it exposes. Most of these are supplied by functions that are pure virtual in the base class because it only makes sense for the author of the plugin to provide this information.

Function | Description of the return value
--- | ---
`name()` | The name of the class that provides the widget.
`group()` | The group in Qt Designer's widget box that the widget belongs to.
`toolTip()` | A short description to help users identify the widget in Qt Designer.
`whatsThis()` | A longer description of the widget for users of Qt Designer.
`includeFile()` | The header file that must be included in applications that use this widget. This information is stored in .ui files and will be used by `uic` to create a suitable `#includes` statement in the code it generates for the form containing the custom widget.
`icon()` | An icon that can be used to represent the widget in Qt Designer's widget box.
`isContainer()` | True if the widget will be used to hold child widgets; otherwise false.
`createWidget()` | A QWidget pointer to an instance of the custom widget, constructed with the parent supplied.
`domXml()` | A description of the widget's properties, such as its object name, size hint, and other standard QWidget properties.
`codeTemplate()` | This function is reserved for future use by Qt Designer.

Two other virtual functions can also be reimplemented:

`initialize()` | Sets up extensions and other features for custom widgets. Custom container extensions (see QDesignerContainerExtension) and task menu extensions (see QDesignerTaskMenuExtension) should be set up in this function.
`isInitialized()` | Returns true if the widget has been initialized; otherwise returns false. Reimplementations usually check whether the initialize() function has been called and return the result of this test.

If the custom widget does not provide a reasonable size hint, it is necessary to specify a default geometry in the string returned by the `domXml()` function in your subclass. For example, the `AnalogClockPlugin` provided by the Custom Widget Plugin example, defines a default widgetgeometry in the following way:

         ...
                " <property name=\"geometry\">\n"
                "  <rect>\n"
                "   <x>0</x>\n"
                "   <y>0</y>\n"
                "   <width>100</width>\n"
                "   <height>100</height>\n"
                "  </rect>\n"
                " </property>\n"
         ...

An additional feature of the `domXml()` function is that, if it returns an empty string, the widget will not be installed in Qt Designer's widget box, but it can still be used by other widgets in the form. This feature is used to hide widgets that should not be explicitly created by the user, but which are required by other widgets.

## Plugin Requirements

In order for plugins to work correctly on all platforms, you need to ensure that they export the symbols that are needed by Qt Designer.

First of all, the plugin class must be exported in order for the plugin to be loaded by Qt Designer. Use the Q_EXPORT_PLUGIN2() macro to do this.

Additionally, each custom widget class in a plugin that you want to be instantiated by Qt Designer must be defined using the QDESIGNER_WIDGET_EXPORT macro.

## Creating Well Behaved Widgets

Some custom widgets have special user interface features that may make them behave differently to many of the standard widgets found in Qt Designer. Specifically, if a custom widget grabs the keyboard as a result of a call to QWidget::grabKeyboard(), the operation of Qt Designer will be affected.

To give custom widgets special behavior in Qt Designer, provide an implementation of the initialize() function to configure the widget construction process for Qt Designer specific behavior. This function will be called for the first time before any calls to createWidget() and could perhaps set an internal flag that can be tested later when Qt Designer calls the plugin's createWidget() function.

## Building and Installing the Plugin

The project file for a plugin must specify the headers and sources for both the custom widget and the plugin interface. Typically, the project file only needs to specify that the plugin's project is to be built as a library, but with specific plugin support for Qt Designer. This is done with the following declarations:

     CONFIG      += designer plugin debug_and_release
     TARGET      = $$qtLibraryTarget($$TARGET)
     TEMPLATE    = lib

When Qt is configured to build in both debug and release modes, Qt Designer will be built in release mode. When this occurs, it is necessary to ensure that plugins are also built in release mode. To do this, include the following declaration in the plugin's project file:

     CONFIG += release

If plugins are built in a mode that is incompatible with Qt Designer, they won't be loaded and installed. For more information about plugins, see the Plugins HOWTO document.

It is also necessary to ensure that the plugin is installed alongside the other Qt Designer widget plugins:

     target.path = $$[QT_INSTALL_PLUGINS]/designer
     INSTALLS += target

The `$[QT_INSTALL_PLUGINS]` variable is a placeholder to the location of the installed Qt plugins. You can configure Qt Designer to look for plugins in other locations by setting the `QT_PLUGIN_PATH` environment variable before running the application. Note that Qt Designer will look for a `designer` subdirectory on each path supplied.

See QCoreApplication::libraryPaths() for more information about customizing paths for libraries and plugins with Qt applications.

## Related Examples

Please see the Custom Widget Plugin and World Time Clock Plugin examples for more information about using custom widgets in Qt Designer.

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