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\page plugins-howto.html
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\title How to Create Qt Plugins
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\brief A guide to creating plugins to extend Qt applications and functionality provided by Qt.
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\keyword QT_DEBUG_PLUGINS
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\keyword QT_NO_PLUGIN_CHECK
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Qt provides two APIs for creating plugins:
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\o A higher-level API for writing extensions to Qt itself: custom database
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drivers, image formats, text codecs, custom styles, etc.
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\o A lower-level API for extending Qt applications.
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For example, if you want to write a custom QStyle subclass and
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have Qt applications load it dynamically, you would use the
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Since the higher-level API is built on top of the lower-level API,
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some issues are common to both.
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If you want to provide plugins for use with \QD, see the QtDesigner
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\section1 The Higher-Level API: Writing Qt Extensions
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Writing a plugin that extends Qt itself is achieved by
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subclassing the appropriate plugin base class, implementing a few
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functions, and adding a macro.
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There are several plugin base classes. Derived plugins are stored
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by default in sub-directories of the standard plugin directory. Qt
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will not find plugins if they are not stored in the right
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\header \o Base Class \o Directory Name \o Key Case Sensitivity
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\row \o QAccessibleBridgePlugin \o \c accessiblebridge \o Case Sensitive
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\row \o QAccessiblePlugin \o \c accessible \o Case Sensitive
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\row \o QDecorationPlugin \o \c decorations \o Case Insensitive
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\row \o QFontEnginePlugin \o \c fontengines \o Case Insensitive
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\row \o QIconEnginePlugin \o \c iconengines \o Case Insensitive
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\row \o QImageIOPlugin \o \c imageformats \o Case Sensitive
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\row \o QInputContextPlugin \o \c inputmethods \o Case Sensitive
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\row \o QKbdDriverPlugin \o \c kbddrivers \o Case Insensitive
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\row \o QMouseDriverPlugin \o \c mousedrivers \o Case Insensitive
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\row \o QPictureFormatPlugin \o \c pictureformats \o Case Sensitive
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\row \o QScreenDriverPlugin \o \c gfxdrivers \o Case Insensitive
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\row \o QScriptExtensionPlugin \o \c script \o Case Sensitive
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\row \o QSqlDriverPlugin \o \c sqldrivers \o Case Sensitive
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\row \o QStylePlugin \o \c styles \o Case Insensitive
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\row \o QTextCodecPlugin \o \c codecs \o Case Sensitive
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But where is the \c{plugins} directory? When the application
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is run, Qt will first treat the application's executable directory
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as the \c{pluginsbase}. For example if the application is in
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\c{C:\Program Files\MyApp} and has a style plugin, Qt will look in
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\c{C:\Program Files\MyApp\styles}. (See
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QCoreApplication::applicationDirPath() for how to find out where
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the application's executable is.) Qt will also look in the
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directory specified by
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QLibraryInfo::location(QLibraryInfo::PluginsPath), which typically
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is located in \c QTDIR/plugins (where \c QTDIR is the directory
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where Qt is installed). If you want Qt to look in additional
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places you can add as many paths as you need with calls to
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QCoreApplication::addLibraryPath(). And if you want to set your
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own path or paths you can use QCoreApplication::setLibraryPaths().
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You can also use a \c qt.conf file to override the hard-coded
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paths that are compiled into the Qt library. For more information,
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see the \l {Using qt.conf} documentation. Yet another possibility
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is to set the \c QT_PLUGIN_PATH environment variable before running
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the application. If set, Qt will look for plugins in the
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paths (separated by the system path separator) specified in the variable.
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Suppose that you have a new style class called \c MyStyle that you
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want to make available as a plugin. The required code is
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straightforward, here is the class definition (\c
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\snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 0
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Ensure that the class implementation is located in a \c .cpp file
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(including the class definition):
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\snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 1
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(Note that QStylePlugin is case insensitive, and the lower-case
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version of the key is used in our
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\l{QStylePlugin::create()}{create()} implementation; most other
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plugins are case sensitive.)
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For database drivers, image formats, text codecs, and most other
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plugin types, no explicit object creation is required. Qt will
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find and create them as required. Styles are an exception, since
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you might want to set a style explicitly in code. To apply a
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style, use code like this:
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\snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 2
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Some plugin classes require additional functions to be
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implemented. See the class documentation for details of the
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virtual functions that must be reimplemented for each type of
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Qt applications automatically know which plugins are available,
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because plugins are stored in the standard plugin subdirectories.
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Because of this applications don't require any code to find and load
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plugins, since Qt handles them automatically.
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The default directory for plugins is \c{QTDIR/plugins} (where \c
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QTDIR is the directory where Qt is installed), with each type of
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plugin in a subdirectory for that type, e.g. \c styles. If you
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want your applications to use plugins and you don't want to use
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the standard plugins path, have your installation process
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determine the path you want to use for the plugins, and save the
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path, e.g. using QSettings, for the application to read when it
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runs. The application can then call
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QCoreApplication::addLibraryPath() with this path and your
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plugins will be available to the application. Note that the final
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part of the path (e.g., \c styles) cannot be changed.
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The normal way to include a plugin with an application is either
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to \l{Static Plugins}{compile it in with the application} or to
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compile it into a dynamic library and use it like any other
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If you want the plugin to be loadable then one approach is to
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create a subdirectory under the application and place the plugin
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in that directory. If you distribute any of the plugins that come
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with Qt (the ones located in the \c plugins directory), you must
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copy the sub-directory under \c plugins where the plugin is
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located to your applications root folder (i.e., do not include the
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\c plugins directory).
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For more information about deployment,
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see the \l {Deploying Qt Applications} documentation.
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The \l{Style Plugin Example} shows how to implement a plugin
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that extends the QStylePlugin base class.
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\section1 The Lower-Level API: Extending Qt Applications
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Not only Qt itself but also Qt application can be extended
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through plugins. This requires the application to detect and load
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plugins using QPluginLoader. In that context, plugins may provide
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arbitrary functionality and are not limited to database drivers,
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image formats, text codecs, styles, and the other types of plugin
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that extend Qt's functionality.
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Making an application extensible through plugins involves the
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\o Define a set of interfaces (classes with only pure virtual
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functions) used to talk to the plugins.
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\o Use the Q_DECLARE_INTERFACE() macro to tell Qt's
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\l{meta-object system} about the interface.
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\o Use QPluginLoader in the application to load the plugins.
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\o Use qobject_cast() to test whether a plugin implements a given
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Writing a plugin involves these steps:
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\o Declare a plugin class that inherits from QObject and from the
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interfaces that the plugin wants to provide.
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\o Use the Q_INTERFACES() macro to tell Qt's \l{meta-object
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system} about the interfaces.
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\o Export the plugin using the Q_EXPORT_PLUGIN2() macro.
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\o Build the plugin using a suitable \c .pro file.
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For example, here's the definition of an interface class:
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\snippet examples/tools/plugandpaint/interfaces.h 2
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Here's the definition of a plugin class that implements that
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\snippet examples/tools/plugandpaintplugins/extrafilters/extrafiltersplugin.h 0
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The \l{tools/plugandpaint}{Plug & Paint} example documentation
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explains this process in detail. See also \l{Creating Custom
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Widgets for Qt Designer} for information about issues that are
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specific to \QD. You can also take a look at the \l{Echo Plugin
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Example} is a more trivial example on how to implement a plugin
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that extends Qt applications. Please note that a QCoreApplication
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must have been initialized before plugins can be loaded.
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\section1 Loading and Verifying Plugins Dynamically
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When loading plugins, the Qt library does some sanity checking to
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determine whether or not the plugin can be loaded and used. This
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provides the ability to have multiple versions and configurations of
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the Qt library installed side by side.
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\o Plugins linked with a Qt library that has a higher version number
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will not be loaded by a library with a lower version number.
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\bold{Example:} Qt 4.3.0 will \e{not} load a plugin built with Qt 4.3.1.
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\o Plugins linked with a Qt library that has a lower major version
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number will not be loaded by a library with a higher major version
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\bold{Example:} Qt 4.3.1 will \e{not} load a plugin built with Qt 3.3.1.
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\bold{Example:} Qt 4.3.1 will load plugins built with Qt 4.3.0 and Qt 4.2.3.
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\o The Qt library and all plugins are built using a \e {build
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key}. The build key in the Qt library is examined against the build
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key in the plugin, and if they match, the plugin is loaded. If the
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build keys do not match, then the Qt library refuses to load the
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\br \bold{Rationale:} See the \l{#The Build Key}{The Build Key} section below.
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When building plugins to extend an application, it is important to ensure
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that the plugin is configured in the same way as the application. This means
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that if the application was built in release mode, plugins should be built
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in release mode, too.
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If you configure Qt to be built in both debug and release modes,
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but only build applications in release mode, you need to ensure that your
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plugins are also built in release mode. By default, if a debug build of Qt is
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available, plugins will \e only be built in debug mode. To force the
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plugins to be built in release mode, add the following line to the plugin's
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\snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 3
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This will ensure that the plugin is compatible with the version of the library
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used in the application.
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\section2 The Build Key
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When loading plugins, Qt checks the build key of each plugin against its
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own configuration to ensure that only compatible plugins are loaded; any
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plugins that are configured differently are not loaded.
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The build key contains the following information:
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\o Architecture, operating system and compiler.
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In cases where different versions of the same compiler do not
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produce binary compatible code, the version of the compiler is
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also present in the build key.
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\o Configuration of the Qt library. The configuration is a list
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of the missing features that affect the available API in the
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Two different configurations of the same version of
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the Qt library are not binary compatible. The Qt library that
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loads the plugin uses the list of (missing) features to
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determine if the plugin is binary compatible.
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\e {Note:} There are cases where a plugin can use features that are
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available in two different configurations. However, the
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developer writing plugins would need to know which features are
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in use, both in their plugin and internally by the utility
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classes in Qt. The Qt library would require complex feature
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and dependency queries and verification when loading plugins.
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Requiring this would place an unnecessary burden on the developer, and
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increase the overhead of loading a plugin. To reduce both
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development time and application runtime costs, a simple string
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comparision of the build keys is used.
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\o Optionally, an extra string may be specified on the configure
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When distributing binaries of the Qt library with an
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application, this provides a way for developers to write
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plugins that can only be loaded by the library with which the
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For debugging purposes, it is possible to override the run-time build key
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checks by configuring Qt with the \c QT_NO_PLUGIN_CHECK preprocessor macro
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\section1 Static Plugins
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Plugins can be linked statically against your application. If you
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build the static version of Qt, this is the only option for
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including Qt's predefined plugins.
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When compiled as a static library, Qt provides the following
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\header \o Plugin name \o Type \o Description
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\row \o \c qtaccessiblecompatwidgets \o Accessibility \o Accessibility for Qt 3 support widgets
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\row \o \c qtaccessiblewidgets \o Accessibility \o Accessibility for Qt widgets
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\row \o \c qdecorationdefault \o Decorations (Qt Extended) \o Default style
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\row \o \c qdecorationwindows \o Decorations (Qt Extended) \o Windows style
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\row \o \c qgif \o Image formats \o GIF
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\row \o \c qjpeg \o Image formats \o JPEG
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\row \o \c qmng \o Image formats \o MNG
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\row \o \c qico \o Image formats \o ICO
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\row \o \c qsvg \o Image formats \o SVG
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\row \o \c qtiff \o Image formats \o TIFF
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\row \o \c qimsw_multi \o Input methods (Qt Extended) \o Input Method Switcher
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\row \o \c qwstslibmousehandler \o Mouse drivers (Qt Extended) \o \c tslib mouse
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\row \o \c qgfxtransformed \o Graphic drivers (Qt Extended) \o Transformed screen
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\row \o \c qgfxvnc \o Graphic drivers (Qt Extended) \o VNC
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\row \o \c qscreenvfb \o Graphic drivers (Qt Extended) \o Virtual frame buffer
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\row \o \c qsqldb2 \o SQL driver \o IBM DB2 \row \o \c qsqlibase \o SQL driver \o Borland InterBase
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\row \o \c qsqlite \o SQL driver \o SQLite version 3
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\row \o \c qsqlite2 \o SQL driver \o SQLite version 2
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\row \o \c qsqlmysql \o SQL driver \o MySQL
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\row \o \c qsqloci \o SQL driver \o Oracle (OCI)
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\row \o \c qsqlodbc \o SQL driver \o Open Database Connectivity (ODBC)
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\row \o \c qsqlpsql \o SQL driver \o PostgreSQL
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\row \o \c qsqltds \o SQL driver \o Sybase Adaptive Server (TDS)
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\row \o \c qcncodecs \o Text codecs \o Simplified Chinese (People's Republic of China)
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\row \o \c qjpcodecs \o Text codecs \o Japanese
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\row \o \c qkrcodecs \o Text codecs \o Korean
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\row \o \c qtwcodecs \o Text codecs \o Traditional Chinese (Taiwan)
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To link statically against those plugins, you need to use the
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Q_IMPORT_PLUGIN() macro in your application and you need to add
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the required plugins to your build using \c QTPLUGIN.
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For example, in your \c main.cpp:
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\snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 4
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In the \c .pro file for your application, you need the following
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\snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 5
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It is also possible to create your own static plugins, by
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following these steps:
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\o Add \c{CONFIG += static} to your plugin's \c .pro file.
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\o Use the Q_IMPORT_PLUGIN() macro in your application.
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\o Link your application with your plugin library using \c LIBS
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See the \l{tools/plugandpaint}{Plug & Paint} example and the
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associated \l{tools/plugandpaintplugins/basictools}{Basic Tools}
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plugin for details on how to do this.
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\note If you are not using qmake to build your application you need
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to make sure that the \c{QT_STATICPLUGIN} preprocessor macro is
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\sa QPluginLoader, QLibrary, {Plug & Paint Example}
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\section1 The Plugin Cache
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In order to speed up loading and validation of plugins, some of
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the information that is collected when plugins are loaded is cached
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through QSettings. This includes information about whether or not
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a plugin was successfully loaded, so that subsequent load operations
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don't try to load an invalid plugin. However, if the "last modified"
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timestamp of a plugin has changed, the plugin's cache entry is
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invalidated and the plugin is reloaded regardless of the values in
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the cache entry, and the cache entry itself is updated with the new
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This also means that the timestamp must be updated each time the
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plugin or any dependent resources (such as a shared library) is
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updated, since the dependent resources might influence the result
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Sometimes, when developing plugins, it is necessary to remove entries
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from the plugin cache. Since Qt uses QSettings to manage the plugin
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cache, the locations of plugins are platform-dependent; see
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\l{QSettings#Platform-Specific Notes}{the QSettings documentation}
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for more information about each platform.
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For example, on Windows the entries are stored in the registry, and the
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paths for each plugin will typically begin with either of these two strings:
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\snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 6
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\section1 Debugging Plugins
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There are a number of issues that may prevent correctly-written plugins from
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working with the applications that are designed to use them. Many of these
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are related to differences in the way that plugins and applications have been
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built, often arising from separate build systems and processes.
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The following table contains descriptions of the common causes of problems
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developers experience when creating plugins:
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\header \o Problem \o Cause \o Solution
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\row \o Plugins sliently fail to load even when opened directly by the
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application. \QD shows the plugin libraries in its
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\gui{Help|About Plugins} dialog, but no plugins are listed under each
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\o The application and its plugins are built in different modes.
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\o Either share the same build information or build the plugins in both
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debug and release modes by appending the \c debug_and_release to
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the \l{qmake Variable Reference#CONFIG}{CONFIG} variable in each of
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\row \o A valid plugin that replaces an invalid (or broken) plugin fails to load.
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\o The entry for the plugin in the plugin cache indicates that the original
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plugin could not be loaded, causing Qt to ignore the replacement.
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\o Either ensure that the plugin's timestamp is updated, or delete the
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entry in the \l{#The Plugin Cache}{plugin cache}.
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You can also use the \c QT_DEBUG_PLUGINS environment variable to obtain
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diagnostic information from Qt about each plugin it tries to load. Set this
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variable to a non-zero value in the environment from which your application is