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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). 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} */