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/*!

    \example designer/worldtimeclockplugin

    \title World Time Clock Plugin Example

    The World Time Clock Plugin example shows how to create a custom

    widget plugin for \QD that uses signals and slots.

    \image worldtimeclockplugin-example.png

    In this example, we simply extend the \l

    {designer/customwidgetplugin}{Custom Widget Plugin} example and

    its custom widget (based on the \l{widgets/analogclock}{Analog

    Clock} example), by introducing the concept of signals and slots.

    The World Time Clock Plugin example consists of two classes:

    \list

    \o \c WorldTimeClock is a custom clock widget with hour and

       minute hands that is automatically updated every few seconds.

    \o \c WorldTimeClockPlugin exposes the \c WorldTimeClock class to \QD.

    \endlist

    First we will take a look at the \c WorldTimeClock class which

    extends the \l {designer/customwidgetplugin}{Custom Widget Plugin}

    example's \c AnalogClock class by providing a signal and a

    slot. Then we will take a quick look at the \c

    WorldTimeClockPlugin class, but this class is in most parts

    identical to the \l {designer/customwidgetplugin}{Custom Widget

    Plugin} example's implementation.

    Finally we take a look at the plugin's project file. The project

    file for custom widget plugins needs some additional information

    to ensure that they will work within \QD. This is also covered in

    the \l {designer/customwidgetplugin}{Custom Widget Plugin} example,

    but due to its importance (custom widget plugins rely on

    components supplied with \QD which must be specified in the

    project file that we use) we will repeat it here.

    \section1 WorldTimeClock Class

    The \c WorldTimeClock class inherits QWidget, and is a custom

    clock widget with hour and minute hands that is automatically

    updated every few seconds. What makes this example different from

    the \l {designer/customwidgetplugin}{Custom Widget Plugin}

    example, is the introduction of the signal and slot in the custom

    widget class:

    \snippet examples/designer/worldtimeclockplugin/worldtimeclock.h 1

    Note the use of the QDESIGNER_WIDGET_EXPORT macro. This is needed

    to ensure that \QD can create instances of the widget on some

    platforms, but it is a good idea to use it on all platforms.

    We declare the \c setTimeZone() slot with an associated \c

    timeZoneOffset variable, and we declare an \c updated() signal

    which takes the current time as argument and is emitted whenever

    the widget is repainted.

    \image worldtimeclock-connection.png

    In \QD's workspace we can then, for example, connect the \c

    WorldTimeClock widget's \c updated() signal to a QTimeEdit's \l

    {QDateTimeEdit::setTime()}{setTime()} slot using \QD's mode

    for editing signal and slots.

    \image worldtimeclock-signalandslot.png

    We can also connect a QSpinBox's \l

    {QSpinBox::valueChanged()}{valueChanged()} signal to the \c

    WorldTimeClock's \c setTimeZone() slot.

    \section1 WorldTimeClockPlugin Class

    The \c WorldTimeClockPlugin class exposes the \c WorldTimeClock

    class to \QD. Its definition is equivalent to the \l

    {designer/customwidgetplugin}{Custom Widget Plugin} example's

    plugin class which is explained in detail. The only part of the

    class definition that is specific to this particular custom widget

    is the class name:

    \snippet examples/designer/worldtimeclockplugin/worldtimeclockplugin.h 0

    The plugin class provides \QD with basic information about our

    plugin, such as its class name and its include file. Furthermore

    it knows how to create instances of the \c WorldTimeClockPlugin

    widget.  \c WorldTimeClockPlugin also defines the \l

    {QDesignerCustomWidgetInterface::initialize()}{initialize()}

    function which is called after the plugin is loaded into \QD. The

    function's QDesignerFormEditorInterface parameter provides the

    plugin with a gateway to all of \QD's API's.

    The \c WorldTimeClockPlugin class inherits from both QObject and

    QDesignerCustomWidgetInterface. It is important to remember, when

    using multiple inheritance, to ensure that all the interfaces

    (i.e. the classes that doesn't inherit Q_OBJECT) are made known to

    the meta object system using the Q_INTERFACES() macro. This

    enables \QD to use \l qobject_cast() to query for supported

    interfaces using nothing but a QObject pointer.

    The implementation of the \c WorldTimeClockPlugin is also

    equivalent to the plugin interface implementation in the \l

    {designer/customwidgetplugin}{Custom Widget Plugin} example (only

    the class name and the implementation of

    QDesignerCustomWidgetInterface::domXml() differ). The main thing

    to remember is to use the Q_EXPORT_PLUGIN2() macro to export the \c

    WorldTimeClockPlugin class for use with \QD:

    \snippet examples/designer/worldtimeclockplugin/worldtimeclockplugin.cpp 0

    Without this macro, there is no way for Qt Designer to use the

    widget.

    \section1 The Project File: worldtimeclockplugin.pro

    The project file for custom widget plugins needs some additional

    information to ensure that they will work as expected within \QD:

    \snippet examples/designer/worldtimeclockplugin/worldtimeclockplugin.pro 0

    \snippet examples/designer/worldtimeclockplugin/worldtimeclockplugin.pro 1

    The \c TEMPLATE variable's value make \c qmake create the custom

    widget as a library. The \c CONFIG variable contains two values,

    \c designer and \c plugin:

    \list

        \o \c designer: Since custom widgets plugins rely on components

           supplied with \QD, this value ensures that our plugin links against

           \QD's library (\c libQtDesigner.so).

        \o \c plugin: We also need to ensure that \c qmake considers the

           custom widget a \e plugin library.

    \endlist

    When Qt is configured to build in both debug and release modes,

    \QD will be built in release mode.  When this occurs, it is

    necessary to ensure that plugins are also built in release

    mode. For that reason you might have to add a \c release value to

    your \c CONFIG variable. Otherwise, if a plugin is built in a mode

    that is incompatible with \QD, it won't be loaded and

    installed.

    The header and source files for the widget are declared in the

    usual way, and in addition we provide an implementation of the

    plugin interface so that \QD can use the custom widget.

    \snippet examples/designer/worldtimeclockplugin/worldtimeclockplugin.pro 2

    It is important to ensure that the plugin is installed in a location that

    is searched by \QD. We do this by specifying a target path for the project

    and adding it to the list of items to install:

    \snippet doc/src/snippets/code/doc_src_examples_worldtimeclockplugin.qdoc 0

    The custom widget is created as a library, and will be installed

    alongside the other \QD plugins when the project is installed

    (using \c{make install} or an equivalent installation procedure).

    Later, we will ensure that it is recognized as a plugin by \QD by

    using the Q_EXPORT_PLUGIN2() macro to export the relevant widget

    information.

    Note that if you want the plugins to appear in a Visual Studio

    integration, the plugins must be built in release mode and their

    libraries must be copied into the plugin directory in the install

    path of the integration (for an example, see \c {C:/program

    files/trolltech as/visual studio integration/plugins}).

    For more information about plugins, see the \l {How to Create Qt

    Plugins} document.

*/