Only reset the backend pointer after we're done with it

[qt-netbsd.git] / doc / src / plugins-howto.qdoc

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/*!
    \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 howto

    \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 QPictureFormatPlugin    \o \c pictureformats   \o Case Sensitive
    \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

    But where is the \c{plugins} directory? When the application
    is run, Qt will first treat the application's executable directory
    as the \c{pluginsbase}. For example if the application is in
    \c{C:\Program Files\MyApp} and has a style plugin, Qt will look in
    \c{C:\Program Files\MyApp\styles}. (See
    QCoreApplication::applicationDirPath() for how to find out where
    the application's executable is.) Qt will also look in the
    directory specified by
    QLibraryInfo::location(QLibraryInfo::PluginsPath), which typically
    is located in \c QTDIR/plugins (where \c QTDIR is the directory
    where Qt is installed). If you want Qt to look in additional
    places you can add as many paths as you need with calls to
    QCoreApplication::addLibraryPath(). And if you want to set your
    own path or paths you can use QCoreApplication::setLibraryPaths().
    You can also use a \c qt.conf file to override the hard-coded
    paths that are compiled into the Qt library. For more information,
    see the \l {Using qt.conf} documentation. Yet another possibility
    is to set the \c QT_PLUGIN_PATH environment variable before running
    the application. If set, Qt will look for plugins in the
    paths (separated by the system path separator) specified in the variable.

    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.

    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.

    The default 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.

    The normal way to include a plugin with an application is either
    to \l{Static Plugins}{compile it in with the application} or to
    compile it into a dynamic library and use it like any other
    library.

    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} documentation.

    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 Loading and Verifying Plugins Dynamically

    When loading plugins, the Qt library does some sanity checking to
    determine whether or not the plugin can be loaded and used. This
    provides the ability to have multiple versions and configurations of
    the Qt library installed side by side.

    \list
    \o Plugins linked with a Qt library that has a higher version number
       will not be loaded by a library with a lower version number.

      \br
      \bold{Example:} Qt 4.3.0 will \e{not} load a plugin built with Qt 4.3.1.

    \o Plugins linked with a Qt library that has a lower major version
       number will not be loaded by a library with a higher major version
       number.

      \br
      \bold{Example:} Qt 4.3.1 will \e{not} load a plugin built with Qt 3.3.1.
      \br
      \bold{Example:} Qt 4.3.1 will load plugins built with Qt 4.3.0 and Qt 4.2.3.

    \o The Qt library and all plugins are built using a \e {build
       key}. The build key in the Qt library is examined against the build
       key in the plugin, and if they match, the plugin is loaded. If the
       build keys do not match, then the Qt library refuses to load the
       plugin.

      \br \bold{Rationale:} See the \l{#The Build Key}{The Build Key} section below.
    \endlist

    When building plugins to extend an application, it is important to ensure
    that the plugin is configured in the same way as the application. This means
    that if the application was built in release mode, plugins should be built
    in release mode, too.

    If you configure Qt to be built in both debug and release modes,
    but only build applications in release mode, you need to ensure that your
    plugins are also built in release mode. By default, if a debug build of Qt is
    available, plugins will \e only be built in debug mode. To force the
    plugins to be built in release mode, add the following line to the plugin's
    project file:

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 3

    This will ensure that the plugin is compatible with the version of the library
    used in the application.

    \section2 The Build Key

    When loading plugins, Qt checks the build key of each plugin against its
    own configuration to ensure that only compatible plugins are loaded; any
    plugins that are configured differently are not loaded.

    The build key contains the following information:
    \list
    \o Architecture, operating system and compiler.

       \e {Rationale:}
       In cases where different versions of the same compiler do not
       produce binary compatible code, the version of the compiler is
       also present in the build key.

    \o Configuration of the Qt library. The configuration is a list
       of the missing features that affect the available API in the
       library.

       \e {Rationale:}
       Two different configurations of the same version of
       the Qt library are not binary compatible. The Qt library that
       loads the plugin uses the list of (missing) features to
       determine if the plugin is binary compatible.

       \e {Note:} There are cases where a plugin can use features that are
       available in two different configurations. However, the
       developer writing plugins would need to know which features are
       in use, both in their plugin and internally by the utility
       classes in Qt. The Qt library would require complex feature
       and dependency queries and verification when loading plugins.
       Requiring this would place an unnecessary burden on the developer, and
       increase the overhead of loading a plugin. To reduce both
       development time and application runtime costs, a simple string
       comparision of the build keys is used.

    \o Optionally, an extra string may be specified on the configure
       script command line.

       \e {Rationale:}
       When distributing binaries of the Qt library with an
       application, this provides a way for developers to write
       plugins that can only be loaded by the library with which the
       plugins were linked.
    \endlist

    For debugging purposes, it is possible to override the run-time build key
    checks by configuring Qt with the \c QT_NO_PLUGIN_CHECK preprocessor macro
    defined.

    \section1 Static Plugins

    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.

    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}

    \section1 The Plugin Cache

    In order to speed up loading and validation of plugins, some of
    the information that is collected when plugins are loaded is cached
    through QSettings. This includes information about whether or not
    a plugin was successfully loaded, so that subsequent load operations
    don't try to load an invalid plugin. However, if the "last modified"
    timestamp of a plugin has changed, the plugin's cache entry is
    invalidated and the plugin is reloaded regardless of the values in
    the cache entry, and the cache entry itself is updated with the new
    result.

    This also means that the timestamp must be updated each time the
    plugin or any dependent resources (such as a shared library) is
    updated, since the dependent resources might influence the result
    of loading a plugin.

    Sometimes, when developing plugins, it is necessary to remove entries
    from the plugin cache. Since Qt uses QSettings to manage the plugin
    cache, the locations of plugins are platform-dependent; see
    \l{QSettings#Platform-Specific Notes}{the QSettings documentation}
    for more information about each platform.

    For example, on Windows the entries are stored in the registry, and the
    paths for each plugin will typically begin with either of these two strings:

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 6

    \section1 Debugging Plugins

    There are a number of issues that may prevent correctly-written plugins from
    working with the applications that are designed to use them. Many of these
    are related to differences in the way that plugins and applications have been
    built, often arising from separate build systems and processes.

    The following table contains descriptions of the common causes of problems
    developers experience when creating plugins:

    \table
    \header \o Problem \o Cause \o Solution
    \row \o Plugins sliently fail to load even when opened directly by the
            application. \QD shows the plugin libraries in its
            \gui{Help|About Plugins} dialog, but no plugins are listed under each
            of them.
         \o The application and its plugins are built in different modes.
         \o Either share the same build information or build the plugins in both
            debug and release modes by appending the \c debug_and_release to
            the \l{qmake Variable Reference#CONFIG}{CONFIG} variable in each of
            their project files.
    \row \o A valid plugin that replaces an invalid (or broken) plugin fails to load.
         \o The entry for the plugin in the plugin cache indicates that the original
            plugin could not be loaded, causing Qt to ignore the replacement.
         \o Either ensure that the plugin's timestamp is updated, or delete the
            entry in the \l{#The Plugin Cache}{plugin cache}.
    \endtable

    You can also use the \c QT_DEBUG_PLUGINS environment variable to obtain
    diagnostic information from Qt about each plugin it tries to load. Set this
    variable to a non-zero value in the environment from which your application is
    launched.
*/