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Qt Plugins HOWTO

Qt provides a simple plugin interface which makes it easy to create custom database drivers, image formats, text codecs, styles and widgets as stand-alone components.

Writing a plugin is achieved by subclassing the appropriate plugin base clase, implementing a few functions, and adding a macro.

There are five plugin base classes. Derived plugins are stored by default in the standard plugin directory.

Suppose that you have a new style class called 'MyStyle' that you want to make available as a plugin. The required code is straightforward:

    class MyStylePlugin : public QStylePlugin
    {
    public:
        MyStylePlugin() {}
        ~MyStylePlugin() {}

        QStringList keys() const { 
            return QStringList() << "MyStyle"; 
        }

        QStyle* create( const QString& key ) { 
            if ( key == "MyStyle" ) 
                return new MyStyle;
            return 0;
        }
    };

    Q_EXPORT_PLUGIN( MyStylePlugin )

The constructor and destructor do not need to do anything, so are left empty. There are only two virtual functions that must be implemented. The first is keys() which returns a string list of the classes implemented in the plugin. (We've just implemented one class in the example above.) The second is a function that returns an object of the required class (or 0 if the plugin is asked to create an object of a class that it doesn't implement). For QStylePlugin, this second function is called create().

It is possible to implement any number of plugin subclasses in a single plugin, providing they are all derived from the same base class, e.g. QStylePlugin.

For database drivers, image formats, custom widgets and text codecs, 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:

    QApplication::setStyle( QStyleFactory::create( "MyStyle" ) );

Some plugin classes require additional functions to be implemented, see the Qt Designer manual's, 'Creating Custom Widgets' section in the 'Creating Custom Widgets' chapter, for a complete example of a QWidgetPlugin, which implements extra functions to integrate the plugin into Qt Designer. The QWidgetFactory class provides additional information on QWidgetPlugins.

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 $QTDIR/plugins, with each type of plugin in a subdirectory for that type, e.g. 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 QApplication::addLibraryPath() with this path and your plugins will be available to the application. Note that the final part of the path, i.e. styles, widgets, etc. cannot be changed.

Plugins and Threaded Applications

If you want to build a plugin which you want to use with a threaded Qt library (whether or not the plugin itself uses threads) you must use a threaded environment. Specifically, you must link the plugin with a threaded Qt library, and you must build Qt Designer with that library. Your .pro file for your plugin must include the line:

    CONFIG += thread

Warning: Do not mix the normal Qt library and the threaded Qt library in an application. If your application uses the threaded Qt library, you should not link your plugin with the normal Qt library. Nor should you dynamically load the normal Qt library or dynamically load another library, e.g. a plugin, that depends on the normal Qt library. On some systems, mixing threaded and non-threaded libraries or plugins will corrupt the static data used in the Qt library.