source: trunk/doc/src/frameworks-technologies/plugins-howto.qdoc@ 609

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/*!
    \group plugins
    \title Plugin Classes
    \ingroup groups

    \brief Plugin related classes.

    These classes deal with shared libraries, (e.g. .so and DLL files),
    and with Qt plugins.

    See the \link plugins-howto.html plugins documentation\endlink.

    See also the \l{ActiveQt framework} for Windows.
*/

/*!
    \page plugins-howto.html
    \title How to Create Qt Plugins
    \brief A guide to creating plugins to extend Qt applications and
    functionality provided by Qt.

    \ingroup frameworks-technologies

    \keyword QT_DEBUG_PLUGINS
    \keyword QT_NO_PLUGIN_CHECK

    Qt provides two APIs for creating plugins:

    \list
    \o A higher-level API for writing extensions to Qt itself: custom database
       drivers, image formats, text codecs, custom styles, etc.
    \o A lower-level API for extending Qt applications.
    \endlist

    For example, if you want to write a custom QStyle subclass and
    have Qt applications load it dynamically, you would use the
    higher-level API.

    Since the higher-level API is built on top of the lower-level API,
    some issues are common to both.

    If you want to provide plugins for use with \QD, see the QtDesigner
    module documentation.

    Topics:

    \tableofcontents

    \section1 The Higher-Level API: Writing Qt Extensions

    Writing a plugin that extends Qt itself is achieved by
    subclassing the appropriate plugin base class, implementing a few
    functions, and adding a macro.

    There are several plugin base classes. Derived plugins are stored
    by default in sub-directories of the standard plugin directory. Qt
    will not find plugins if they are not stored in the right
    directory.

    \table
    \header \o Base Class              \o Directory Name                \o Key Case Sensitivity
    \row    \o QAccessibleBridgePlugin \o \c accessiblebridge \o Case Sensitive
    \row    \o QAccessiblePlugin       \o \c accessible       \o Case Sensitive
    \row    \o QDecorationPlugin       \o \c decorations      \o Case Insensitive
    \row    \o QFontEnginePlugin       \o \c fontengines      \o Case Insensitive
    \row    \o QIconEnginePlugin       \o \c iconengines      \o Case Insensitive
    \row    \o QImageIOPlugin          \o \c imageformats     \o Case Sensitive
    \row    \o QInputContextPlugin     \o \c inputmethods     \o Case Sensitive
    \row    \o QKbdDriverPlugin        \o \c kbddrivers       \o Case Insensitive
    \row    \o QMouseDriverPlugin      \o \c mousedrivers     \o Case Insensitive
    \row    \o QScreenDriverPlugin     \o \c gfxdrivers       \o Case Insensitive
    \row    \o QScriptExtensionPlugin  \o \c script           \o Case Sensitive
    \row    \o QSqlDriverPlugin        \o \c sqldrivers       \o Case Sensitive
    \row    \o QStylePlugin            \o \c styles           \o Case Insensitive
    \row    \o QTextCodecPlugin        \o \c codecs           \o Case Sensitive
    \endtable

    Suppose that you have a new style class called \c MyStyle that you
    want to make available as a plugin. The required code is
    straightforward, here is the class definition (\c
    mystyleplugin.h):

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 0

    Ensure that the class implementation is located in a \c .cpp file
    (including the class definition):

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 1

    (Note that QStylePlugin is case insensitive, and the lower-case
    version of the key is used in our
    \l{QStylePlugin::create()}{create()} implementation; most other
    plugins are case sensitive.)

    For database drivers, image formats, text codecs, and most other
    plugin types, no explicit object creation is required. Qt will
    find and create them as required. Styles are an exception, since
    you might want to set a style explicitly in code. To apply a
    style, use code like this:

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 2

    Some plugin classes require additional functions to be
    implemented. See the class documentation for details of the
    virtual functions that must be reimplemented for each type of
    plugin.

    The \l{Style Plugin Example} shows how to implement a plugin
    that extends the QStylePlugin base class.

    \section1 The Lower-Level API: Extending Qt Applications

    Not only Qt itself but also Qt application can be extended
    through plugins. This requires the application to detect and load
    plugins using QPluginLoader. In that context, plugins may provide
    arbitrary functionality and are not limited to database drivers,
    image formats, text codecs, styles, and the other types of plugin
    that extend Qt's functionality.

    Making an application extensible through plugins involves the
    following steps:

    \list 1
    \o Define a set of interfaces (classes with only pure virtual
       functions) used to talk to the plugins.
    \o Use the Q_DECLARE_INTERFACE() macro to tell Qt's
       \l{meta-object system} about the interface.
    \o Use QPluginLoader in the application to load the plugins.
    \o Use qobject_cast() to test whether a plugin implements a given
       interface.
    \endlist

    Writing a plugin involves these steps:

    \list 1
    \o Declare a plugin class that inherits from QObject and from the
       interfaces that the plugin wants to provide.
    \o Use the Q_INTERFACES() macro to tell Qt's \l{meta-object
       system} about the interfaces.
    \o Export the plugin using the Q_EXPORT_PLUGIN2() macro.
    \o Build the plugin using a suitable \c .pro file.
    \endlist

    For example, here's the definition of an interface class:

    \snippet examples/tools/plugandpaint/interfaces.h 2

    Here's the definition of a plugin class that implements that
    interface:

    \snippet examples/tools/plugandpaintplugins/extrafilters/extrafiltersplugin.h 0

    The \l{tools/plugandpaint}{Plug & Paint} example documentation
    explains this process in detail. See also \l{Creating Custom
    Widgets for Qt Designer} for information about issues that are
    specific to \QD. You can also take a look at the \l{Echo Plugin
    Example} is a more trivial example on how to implement a plugin
    that extends Qt applications. Please note that a QCoreApplication
    must have been initialized before plugins can be loaded.

    \section1 Locating Plugins

    Qt applications automatically know which plugins are available,
    because plugins are stored in the standard plugin subdirectories.
    Because of this applications don't require any code to find and load
    plugins, since Qt handles them automatically.

    During development, the directory for plugins is \c{QTDIR/plugins}
    (where \c QTDIR is the directory where Qt is installed), with each
    type of plugin in a subdirectory for that type, e.g. \c styles. If
    you want your applications to use plugins and you don't want to use
    the standard plugins path, have your installation process
    determine the path you want to use for the plugins, and save the
    path, e.g. using QSettings, for the application to read when it
    runs. The application can then call
    QCoreApplication::addLibraryPath() with this path and your
    plugins will be available to the application. Note that the final
    part of the path (e.g., \c styles) cannot be changed.

    If you want the plugin to be loadable then one approach is to
    create a subdirectory under the application and place the plugin
    in that directory. If you distribute any of the plugins that come
    with Qt (the ones located in the \c plugins directory), you must
    copy the sub-directory under \c plugins where the plugin is
    located to your applications root folder (i.e., do not include the
    \c plugins directory).

    \note In Symbian all binaries must be located in the directory \\sys\\bin,
    so each Qt plugin has a stub with the same basename as the plugin dll 
    and suffix ".qtplugin" to make Qt extension plugins work similarly to
    other platforms.
    When trying to locate the plugin, Qt actually looks for the stub
    instead of the plugin binary. While plugin stub files have the
    suffix ".qtplugin", they can still be loaded also by specifying a filename
    with the normal library suffix ".dll" for QPluginLoader, so normally application
    developer doesn't need to care about the different suffix of the stub.
    Because of the way applications can be installed
    on ROM or various other drives in Symbian, Qt looks for the stub from
    the same directory on all available drives if it is not located in the given
    directory when loading a plugin.

    For more information about deployment,
    see the \l {Deploying Qt Applications} and \l {Deploying Plugins}
    documentation.

    \section1 Static Plugins

    The normal and most flexible way to include a plugin with an
    application is to compile it into a dynamic library that is shipped
    separately, and detected and loaded at runtime.

    Plugins can be linked statically against your application. If you
    build the static version of Qt, this is the only option for
    including Qt's predefined plugins. Using static plugins makes the
    deployment less error-prone, but has the disadvantage that no
    functionality from plugins can be added without a complete rebuild
    and redistribution of the application.

    When compiled as a static library, Qt provides the following
    static plugins:

    \table
    \header \o Plugin name                  \o Type               \o Description
    \row    \o \c qtaccessiblecompatwidgets \o Accessibility      \o Accessibility for Qt 3 support widgets
    \row    \o \c qtaccessiblewidgets       \o Accessibility      \o Accessibility for Qt widgets
    \row    \o \c qdecorationdefault        \o Decorations (Qt Extended) \o Default style
    \row    \o \c qdecorationwindows        \o Decorations (Qt Extended) \o Windows style
    \row    \o \c qgif                      \o Image formats      \o GIF
    \row    \o \c qjpeg                     \o Image formats      \o JPEG
    \row    \o \c qmng                      \o Image formats      \o MNG
    \row    \o \c qico                      \o Image formats      \o ICO
    \row    \o \c qsvg                      \o Image formats      \o SVG
    \row    \o \c qtiff                     \o Image formats      \o TIFF
    \row    \o \c qimsw_multi               \o Input methods (Qt Extended) \o Input Method Switcher
    \row    \o \c qwstslibmousehandler      \o Mouse drivers (Qt Extended) \o \c tslib mouse
    \row    \o \c qgfxtransformed           \o Graphic drivers (Qt Extended) \o Transformed screen
    \row    \o \c qgfxvnc                   \o Graphic drivers (Qt Extended) \o VNC
    \row    \o \c qscreenvfb                \o Graphic drivers (Qt Extended) \o Virtual frame buffer
    \row    \o \c qsqldb2                   \o SQL driver         \o IBM DB2    \row    \o \c qsqlibase       \o SQL driver         \o Borland InterBase
    \row    \o \c qsqlite                   \o SQL driver         \o SQLite version 3
    \row    \o \c qsqlite2                  \o SQL driver         \o SQLite version 2
    \row    \o \c qsqlmysql                 \o SQL driver         \o MySQL
    \row    \o \c qsqloci                   \o SQL driver         \o Oracle (OCI)
    \row    \o \c qsqlodbc                  \o SQL driver         \o Open Database Connectivity (ODBC)
    \row    \o \c qsqlpsql                  \o SQL driver         \o PostgreSQL
    \row    \o \c qsqltds                   \o SQL driver         \o Sybase Adaptive Server (TDS)
    \row    \o \c qcncodecs                 \o Text codecs        \o Simplified Chinese (People's Republic of China)
    \row    \o \c qjpcodecs                 \o Text codecs        \o Japanese
    \row    \o \c qkrcodecs                 \o Text codecs        \o Korean
    \row    \o \c qtwcodecs                 \o Text codecs        \o Traditional Chinese (Taiwan)
    \endtable

    To link statically against those plugins, you need to use the
    Q_IMPORT_PLUGIN() macro in your application and you need to add
    the required plugins to your build using \c QTPLUGIN.
    For example, in your \c main.cpp:

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 4

    In the \c .pro file for your application, you need the following
    entry:

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 5

    It is also possible to create your own static plugins, by
    following these steps:

    \list 1
    \o Add \c{CONFIG += static} to your plugin's \c .pro file.
    \o Use the Q_IMPORT_PLUGIN() macro in your application.
    \o Link your application with your plugin library using \c LIBS
       in the \c .pro file.
    \endlist

    See the \l{tools/plugandpaint}{Plug & Paint} example and the
    associated \l{tools/plugandpaintplugins/basictools}{Basic Tools}
    plugin for details on how to do this.

    \note If you are not using qmake to build your application you need
    to make sure that the \c{QT_STATICPLUGIN} preprocessor macro is
    defined.

    \sa QPluginLoader, QLibrary, {Plug & Paint Example}
*/