Qt 4.0: World Time Clock Plugin Example

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# World Time Clock Plugin Example

Files:

- designer/worldtimeclockplugin/worldtimeclock.cpp
- designer/worldtimeclockplugin/worldtimeclock.h
- designer/worldtimeclockplugin/worldtimeclockplugin.cpp
- designer/worldtimeclockplugin/worldtimeclockplugin.h

The World Time Clock Plugin example shows how to create a custom widget plugin for Qt Designer that uses signals and slots.

In this example, the custom widget used is based on the Analog Clock example, and does not provide any custom signals or slots.

## Preparation

To provide a custom widget that can be used with Qt Designer, we need to supply a self-contained implementation and provide a plugin interface. In this example, we reuse the Analog Clock example for convenience.

Since custom widgets plugins rely on components supplied with Qt Designer, the project file that we use needs to contain information about Qt Designer's library components:

        CONFIG      += designer plugin
        TEMPLATE    = lib
        DESTDIR     = $$QT_BUILD_TREE/plugins/designer

The header and source files for the widget are declared in the usual way, and we provide an implementation of the plugin interface so that Qt Designer can use the custom widget.

        HEADERS     = analogclock.h \
                      customwidgetplugin.h
        SOURCES     = analogclock.cpp \
                      customwidgetplugin.cpp

The custom widget is created as a library.

## AnalogClock Class Definition and Implementation

The `AnalogClock` class is defined and implemented in exactly the same way as described in the Analog Clock example. Since the class is self-contained, and does not require any external configuration, it can be used without modification as a custom widget in Qt Designer.

## AnalogClockPlugin Class Definition

The `AnalogClock` class is exposed to Qt Designer through the `AnalogClockPlugin` class. This class inherits from both QObject and the `ICustomWidget` class, and implements an interface defined by `ICustomWidget`:

        class AnalogClockPlugin : public QObject, public QDesignerCustomWidgetInterface
        {
            Q_OBJECT
            Q_INTERFACES(QDesignerCustomWidgetInterface)
        public:
            AnalogClockPlugin(QObject *parent = 0);
            bool isContainer() const;
            bool isInitialized() const;
            QIcon icon() const;
            QString codeTemplate() const;
            QString domXml() const;
            QString group() const;
            QString includeFile() const;
            QString name() const;
            QString toolTip() const;
            QString whatsThis() const;
            QWidget *createWidget(QWidget *parent);
            void initialize(QDesignerFormEditorInterface *core);

        private:
            bool initialized;
        };

The functions provide information about the widget that Qt Designer can use in the widget box. The `initialized` private member variable is used to record whether the plugin has been initialized by Qt Designer.

Note that the only part of the class definition that is specific to this particular custom widget is the class name.

## AnalogClockPlugin Implementation

The class constructor simply calls the QObject base class constructor and sets the `initialized` variable to `false`.

        AnalogClockPlugin::AnalogClockPlugin(QObject *parent)
            : QObject(parent)
        {
            initialized = false;
        }

Qt Designer will initialize the plugin when it is required by calling the `initialize()` function:

        void AnalogClockPlugin::initialize(QDesignerFormEditorInterface * /*core*/)
        {
            if (initialized)
                return;

            initialized = true;
        }

In this example, the `initialized` private variable is tested, and only set to `true` if the plugin is not already initialized. Although, this plugin does not require any special code to be executed when it is initialized, we could include such code after the test for initialization.

The `isInitialized()` function lets Qt Designer know whether the plugin is ready for use:

        bool AnalogClockPlugin::isInitialized() const
        {
            return initialized;
        }

Instances of the custom widget are supplied by the `createWidget()` function. The implementation for the analog clock is straightforward:

        QWidget *AnalogClockPlugin::createWidget(QWidget *parent)
        {
            return new AnalogClock(parent);
        }

In this case, the custom widget only requires a `parent` to be specified. If other arguments need to be supplied to the widget, they can be introduced here.

The following functions provide information for Qt Designer to use to represent the widget in the widget box. The `name()` function returns the name of class that provides the custom widget:

        QString AnalogClockPlugin::name() const
        {
            return QLatin1String("AnalogClock");
        }

The `group()` function is used to describe the type of widget that the custom widget belongs to:

        QString AnalogClockPlugin::group() const
        {
            return QLatin1String("Display Widgets [Examples]");
        }

Note that all widget plugins will be placed in the "Custom Widgets" section of Qt Designer's widget box. The icon used to represent the widget in the widget box is returned by the `icon()` function:

        QIcon AnalogClockPlugin::icon() const
        {
            return QIcon();
        }

In this case, we return a null icon to indicate that we have no icon that can be used to represent the widget.

A tool tip and "What's This?" help can be supplied for the custom widget's entry in the widget box. The `toolTip()` function should return a short message describing the widget:

        QString AnalogClockPlugin::toolTip() const
        {
            return QString();
        }

The `whatsThis()` function can return a longer description:

        QString AnalogClockPlugin::whatsThis() const
        {
            return QString();
        }

The `isContainer()` and `isForm()` functions provide important information about the way that the widget is to be handled by Qt Designer.

        bool AnalogClockPlugin::isContainer() const
        {
            return false;
        }

Most widgets in Qt can contain child widgets, but it only makes sense to use dedicated container widgets for this purpose in Qt Designer. By returning `false`, we indicate that the custom widget cannot hold other widgets; if we returned true, Qt Designer would allow other widgets to be placed inside the analog clock and a layout to be defined.

The `domXml()` function provides a way to include default settings for the widget in the standard XML format used by Qt Designer. In this case, we only specify the widget's geometry:

        QString AnalogClockPlugin::domXml() const
        {
            return QLatin1String("<widget class=\"AnalogClock\" name=\"analogClock\">\n"
                                 " <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"
                                 " <property name=\"toolTip\" >\n"
                                 "  <string>The current time</string>\n"
                                 " </property>\n"
                                 " <property name=\"whatsThis\" >\n"
                                 "  <string>The analog clock widget displays "
                                 "the current time.</string>\n"
                                 " </property>\n"
                                 "</widget>\n");
        }

To make the analog clock widget usable by applications, we implement the `includeFile()` function to return the name of the header file containing the custom widget class definition:

        QString AnalogClockPlugin::includeFile() const
        {
            return QLatin1String("analogclock.h");
        }

We can provide a sample code template for the custom widget with the `codeTemplate()` function:

        QString AnalogClockPlugin::codeTemplate() const
        {
            return QString();
        }

The code template is optionally used to specify how the widget is used in application code.

Finally, we use the `Q_EXPORT_PLUGIN` macro to export the `AnalogClockPlugin` class for use with Qt Designer:

        Q_EXPORT_PLUGIN(AnalogClockPlugin)