qt5base-lts/examples/widgets/doc/src/styleplugin.qdoc

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/*!
    \example tools/styleplugin
    \title Style Plugin Example
    \ingroup examples-widgets-tools

    \brief This example shows how to create a plugin that extends Qt with a new
    GUI look and feel.

    \image stylepluginexample.png

    A plugin in Qt is a class stored in a shared library that can be
    loaded by a QPluginLoader at run-time. When you create plugins in
    Qt, they either extend a Qt application or Qt itself. Writing a
    plugin that extends Qt itself is achieved by inheriting one of the
    plugin \l{Plugin Classes}{base classes}, reimplementing functions
    from that class, and adding a macro. In this example we extend Qt
    by adding a new GUI look and feel (i.e., making a new QStyle
    available). A high-level introduction to plugins is given in the
    plugin \l{How to Create Qt Plugins}{overview document}.

    Plugins that provide new styles inherit the QStylePlugin base
    class. Style plugins are loaded by Qt and made available through
    QStyleFactory; we will look at this later. We have implemented \c
    SimpleStylePlugin, which provides \c SimpleStyle. The new style
    contributes to widget styling by drawing button backgrounds in
    red - not a major contribution, but it still makes a new style.

    The new style is platform agnostic in the sense that it is not
    based on any specific style implementation, but uses QProxyStyle
    to merely tweak the looks in the current application style that
    defaults to the native system style.

    \note On some platforms, the native style will prevent the button
    from having a red background. In this case, try to run the example
    in another style (e.g., fusion).

    We test the plugin with \c StyleWindow, in which we display a
    QPushButton. The \c SimpleStyle and \c StyleWindow classes do not
    contain any plugin specific functionality and their implementations
    are trivial; we will therefore leap past them and head on to the \c
    SimpleStylePlugin and the \c main() function. After we have looked
    at that, we examine the plugin's profile.


    \section1 SimpleStylePlugin Class Definition

    \c SimpleStylePlugin inherits QStylePlugin and is the plugin
    class.

    \snippet tools/styleplugin/plugin/simplestyleplugin.h 0

    \c keys() returns a list of style names that this plugin can
    create, while \c create() takes such a string and returns the
    QStyle corresponding to the key. Both functions are pure virtual
    functions reimplemented from QStylePlugin. When an application
    requests an instance of the \c SimpleStyle style, which this
    plugin creates, Qt will create it with this plugin.


    \section1 SimpleStylePlugin Class Implementation

    Here is the implementation of \c keys():

    \snippet tools/styleplugin/plugin/simplestyleplugin.cpp 0

    Since this plugin only supports one style, we return a QStringList
    with the class name of that style.

    Here is the \c create() function:

    \snippet tools/styleplugin/plugin/simplestyleplugin.cpp 1

    Note that the key for style plugins are case insensitive.
    The case sensitivity varies from plugin to plugin, so you need to
    check this when implementing new plugins.

    \section1 The \c main() function

    \snippet tools/styleplugin/stylewindow/main.cpp 0

    Qt loads the available style plugins when the QApplication object
    is initialized. The QStyleFactory class knows about all styles and
    produces them with \l{QStyleFactory::}{create()} (it is a
    wrapper around all the style plugins).

    \section1 The Simple Style Plugin Profile

    The \c SimpleStylePlugin lives in its own directory and have
    its own profile:

    \snippet tools/styleplugin/plugin/plugin.pro 0

    In the plugin profile we need to set the lib template as we are
    building a shared library instead of an executable. We must also
    set the config to plugin. We set the library to be stored in the
    styles folder under stylewindow because this is a path in which Qt
    will search for style plugins.

    \section1 Related Articles and Examples

    In addition to the plugin \l{How to Create Qt Plugins}{overview
    document}, we have other examples and articles that concern
    plugins.

    In the \l{Echo Plugin Example}{echo plugin example} we show how to
    implement plugins that extends Qt applications rather than Qt
    itself, which is the case with the style plugin of this example.
    The \l{Plug & Paint Example}{plug & paint} example shows how to
    implement a static plugin as well as being a more involved example
    on plugins that extend applications.
*/