// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example customwidgetplugin
    \examplecategory {Desktop}
    \meta tags {widgets,designer}
    \ingroup examples-designer
    \title Custom Widget Plugin

    \brief Creating a custom widget plugin for \QD.

    \image customwidgetplugin-example.webp {Screenshot of an analog clock widget in
           the \QD}

    In this example, the custom widget used is based on the
    \l{widgets/analogclock}{Analog Clock example}, and does not provide any custom
    signals or slots.

    \section1 Preparation

    To provide a custom widget that can be used with \QD, we need to supply a
    self-contained implementation and provide a plugin interface. In this
    example, we reuse the \l{widgets/analogclock}{Analog Clock example} for
    convenience.

    \section1 Project files

    \section2 CMake

    The project files need to state that a plugin linking
    to the \QD libraries is to be built:

    \snippet customwidgetplugin/CMakeLists.txt 0
    \codeline
    \snippet customwidgetplugin/CMakeLists.txt 2

    The link libraries list specifies \c Qt::UiPlugin. This indicates that
    the plugin uses the abstract interfaces QDesignerCustomWidgetInterface
    and QDesignerCustomWidgetCollectionInterface only and has no linkage
    to the \QD libraries. When accessing other interfaces of \QD that have
    linkage, \c Designer should be used instead; this ensures that the plugin
    dynamically links to the \QD libraries and has a run-time dependency on
    them.

    The following example shows how to add the header and source files of the
    widget:

    \snippet customwidgetplugin/CMakeLists.txt 1

    We provide an implementation of the plugin interface so that \QD
    can use the custom widget.

    It is also 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 customwidgetplugin/CMakeLists.txt 3
    \snippet customwidgetplugin/CMakeLists.txt 4

    The custom widget is created as a library. It will be
    installed alongside the other \QD plugins when the project is
    installed (using \c{ninja install} or an equivalent installation
    procedure).

    For more information about plugins, see the
    \l {How to Create Qt Plugins} documentation.

    \section2 qmake

    The following example shows how to link a plugin to the \QD libraries:

    \snippet customwidgetplugin/customwidgetplugin.pro 0
    \codeline
    \snippet customwidgetplugin/customwidgetplugin.pro 1

    The \c QT variable contains the keyword \c uiplugin, which is
    the equivalent of the \c Qt::UiPlugin library.

    The following example shows how to add the header and source files of the
    widget:

    \snippet customwidgetplugin/customwidgetplugin.pro 2

    The following example shows how to install a plugin to the \QD's plugin path:

    \snippet customwidgetplugin/customwidgetplugin.pro 3

    \section1 AnalogClock Class Definition and Implementation

    The \c AnalogClock class is defined and implemented in exactly the same
    way as described in the \l{widgets/analogclock}{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
    \QD.

    \section1 AnalogClockPlugin Class Definition

    The \c AnalogClock class is exposed to \QD through the \c
    AnalogClockPlugin class. This class inherits from both QObject and
    the QDesignerCustomWidgetInterface class, and implements an
    interface defined by QDesignerCustomWidgetInterface.

    To ensure that Qt recognizes the widget as a plugin, export relevant
    information about the widget by adding the \c Q_PLUGIN_METADATA() macro:

    \snippet customwidgetplugin/customwidgetplugin.h 0

    The functions provide information about the widget that \QD can use in
    the \l{Getting to Know Qt Widgets Designer#WidgetBox}{widget box}.
    The \c initialized private member variable is used to record whether
    the plugin has been initialized by \QD.

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

    \section1 AnalogClockPlugin Implementation

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

    \snippet customwidgetplugin/customwidgetplugin.cpp 0

    \QD will initialize the plugin when it is required by calling the
    \c initialize() function:

    \snippet customwidgetplugin/customwidgetplugin.cpp 1

    In this example, the \c initialized private variable is tested, and only
    set to \c 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 \c isInitialized() function lets \QD know whether the plugin is
    ready for use:

    \snippet customwidgetplugin/customwidgetplugin.cpp 2

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

    \snippet customwidgetplugin/customwidgetplugin.cpp 3

    In this case, the custom widget only requires a \c 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 \QD to use to represent
    the widget in the widget box.
    The \c name() function returns the name of class that provides the
    custom widget:

    \snippet customwidgetplugin/customwidgetplugin.cpp 4

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

    \snippet customwidgetplugin/customwidgetplugin.cpp 5

    The widget plugin will be placed in a section identified by its
    group name in \QD's widget box. The icon used to represent the
    widget in the widget box is returned by the \c icon() function:

    \snippet customwidgetplugin/customwidgetplugin.cpp 6

    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 \c toolTip() function should return a short
    message describing the widget:

    \snippet customwidgetplugin/customwidgetplugin.cpp 7

    The \c whatsThis() function can return a longer description:

    \snippet customwidgetplugin/customwidgetplugin.cpp 8

    The \c isContainer() function tells \QD whether the widget is supposed to
    be used as a container for other widgets. If not, \QD will not allow the
    user to place widgets inside it.

    \snippet customwidgetplugin/customwidgetplugin.cpp 9

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

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

    \snippet customwidgetplugin/customwidgetplugin.cpp 10

    If the widget provides a reasonable size hint, it is not necessary to
    define it here. In addition, returning an empty string instead of a
    \c{<widget>} element will tell \QD not to install the widget in the
    widget box.

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

    \snippet customwidgetplugin/customwidgetplugin.cpp 12
*/
