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Qt makes it possible to create custom plugins and to load them using QPluginLoader. To ensure that plugins don't get lost, it is also possible to link them statically to the executable. The Plug & Paint example uses plugins to support custom brushes, shapes, and image filters. A single plugin can provide multiple brushes, shapes, and/or filters. If you want to learn how to make your own application extensible through plugins, we recommend that you start by reading this overview, which explains how to make an application use plugins. Afterward, you can read the \l{tools/plugandpaintplugins/basictools}{Basic Tools} and \l{tools/plugandpaintplugins/extrafilters}{Extra Filters} overviews, which show how to implement static and dynamic plugins, respectively. Plug & Paint consists of the following classes: \list \o \c MainWindow is a QMainWindow subclass that provides the menu system and that contains a \c PaintArea as the central widget. \o \c PaintArea is a QWidget that allows the user to draw using a brush and to insert shapes. \o \c PluginDialog is a dialog that shows information about the plugins detected by the application. \o \c BrushInterface, \c ShapeInterface, and \c FilterInterface are abstract base classes that can be implemented by plugins to provide custom brushes, shapes, and image filters. \endlist \section1 The Plugin Interfaces We will start by reviewing the interfaces defined in \c interfaces.h. These interfaces are used by the Plug & Paint application to access extra functionality. They are implemented in the plugins. \snippet examples/tools/plugandpaint/interfaces.h 0 The \c BrushInterface class declares four pure virtual functions. The first pure virtual function, \c brushes(), returns a list of strings that identify the brushes provided by the plugin. By returning a QStringList instead of a QString, we make it possible for a single plugin to provide multiple brushes. The other functions have a \c brush parameter to identify which brush (among those returned by \c brushes()) is used. \c mousePress(), \c mouseMove(), and \c mouseRelease() take a QPainter and one or two \l{QPoint}s, and return a QRect identifying which portion of the image was altered by the brush. The class also has a virtual destructor. Interface classes usually don't need such a destructor (because it would make little sense to \c delete the object that implements the interface through a pointer to the interface), but some compilers emit a warning for classes that declare virtual functions but no virtual destructor. We provide the destructor to keep these compilers happy. \snippet examples/tools/plugandpaint/interfaces.h 1 The \c ShapeInterface class declares a \c shapes() function that works the same as \c{BrushInterface}'s \c brushes() function, and a \c generateShape() function that has a \c shape parameter. Shapes are represented by a QPainterPath, a data type that can represent arbitrary 2D shapes or combinations of shapes. The \c parent parameter can be used by the plugin to pop up a dialog asking the user to specify more information. \snippet examples/tools/plugandpaint/interfaces.h 2 The \c FilterInterface class declares a \c filters() function that returns a list of filter names, and a \c filterImage() function that applies a filter to an image. \snippet examples/tools/plugandpaint/interfaces.h 4 To make it possible to query at run-time whether a plugin implements a given interface, we must use the \c Q_DECLARE_INTERFACE() macro. The first argument is the name of the interface. The second argument is a string identifying the interface in a unique way. By convention, we use a "Java package name" syntax to identify interfaces. If we later change the interfaces, we must use a different string to identify the new interface; otherwise, the application might crash. It is therefore a good idea to include a version number in the string, as we did above. The \l{tools/plugandpaintplugins/basictools}{Basic Tools} plugin and the \l{tools/plugandpaintplugins/extrafilters}{Extra Filters} plugin shows how to derive from \c BrushInterface, \c ShapeInterface, and \c FilterInterface. A note on naming: It might have been tempting to give the \c brushes(), \c shapes(), and \c filters() functions a more generic name, such as \c keys() or \c features(). However, that would have made multiple inheritance impractical. When creating interfaces, we should always try to give unique names to the pure virtual functions. \section1 The MainWindow Class The \c MainWindow class is a standard QMainWindow subclass, as found in many of the other examples (e.g., \l{mainwindows/application}{Application}). Here, we'll concentrate on the parts of the code that are related to plugins. \snippet examples/tools/plugandpaint/mainwindow.cpp 4 The \c loadPlugins() function is called from the \c MainWindow constructor to detect plugins and update the \gui{Brush}, \gui{Shapes}, and \gui{Filters} menus. We start by handling static plugins (available through QPluginLoader::staticInstances()) To the application that uses the plugin, a Qt plugin is simply a QObject. That QObject implements plugin interfaces using multiple inheritance. \snippet examples/tools/plugandpaint/mainwindow.cpp 5 The next step is to load dynamic plugins. We initialize the \c pluginsDir member variable to refer to the \c plugins subdirectory of the Plug & Paint example. On Unix, this is just a matter of initializing the QDir variable with QApplication::applicationDirPath(), the path of the executable file, and to do a \l{QDir::cd()}{cd()}. On Windows and Mac OS X, this file is usually located in a subdirectory, so we need to take this into account. \snippet examples/tools/plugandpaint/mainwindow.cpp 6 \snippet examples/tools/plugandpaint/mainwindow.cpp 7 \snippet examples/tools/plugandpaint/mainwindow.cpp 8 We use QDir::entryList() to get a list of all files in that directory. Then we iterate over the result using \l foreach and try to load the plugin using QPluginLoader. The QObject provided by the plugin is accessible through QPluginLoader::instance(). If the dynamic library isn't a Qt plugin, or if it was compiled against an incompatible version of the Qt library, QPluginLoader::instance() returns a null pointer. If QPluginLoader::instance() is non-null, we add it to the menus. \snippet examples/tools/plugandpaint/mainwindow.cpp 9 At the end, we enable or disable the \gui{Brush}, \gui{Shapes}, and \gui{Filters} menus based on whether they contain any items. \snippet examples/tools/plugandpaint/mainwindow.cpp 10 For each plugin (static or dynamic), we check which interfaces it implements using \l qobject_cast(). First, we try to cast the plugin instance to a \c BrushInterface; if it works, we call the private function \c addToMenu() with the list of brushes returned by \c brushes(). Then we do the same with the \c ShapeInterface and the \c FilterInterface. \snippet examples/tools/plugandpaint/mainwindow.cpp 3 The \c aboutPlugins() slot is called on startup and can be invoked at any time through the \gui{About Plugins} action. It pops up a \c PluginDialog, providing information about the loaded plugins. \image plugandpaint-plugindialog.png Screenshot of the Plugin dialog The \c addToMenu() function is called from \c loadPlugin() to create \l{QAction}s for custom brushes, shapes, or filters and add them to the relevant menu. The QAction is created with the plugin from which it comes from as the parent; this makes it convenient to get access to the plugin later. \snippet examples/tools/plugandpaint/mainwindow.cpp 0 The \c changeBrush() slot is invoked when the user chooses one of the brushes from the \gui{Brush} menu. We start by finding out which action invoked the slot using QObject::sender(). Then we get the \c BrushInterface out of the plugin (which we conveniently passed as the QAction's parent) and we call \c PaintArea::setBrush() with the \c BrushInterface and the string identifying the brush. Next time the user draws on the paint area, \c PaintArea will use this brush. \snippet examples/tools/plugandpaint/mainwindow.cpp 1 The \c insertShape() is invoked when the use chooses one of the shapes from the \gui{Shapes} menu. We retrieve the QAction that invoked the slot, then the \c ShapeInterface associated with that QAction, and finally we call \c ShapeInterface::generateShape() to obtain a QPainterPath. \snippet examples/tools/plugandpaint/mainwindow.cpp 2 The \c applyFilter() slot is similar: We retrieve the QAction that invoked the slot, then the \c FilterInterface associated to that QAction, and finally we call \c FilterInterface::filterImage() to apply the filter onto the current image. \section1 The PaintArea Class The \c PaintArea class contains some code that deals with \c BrushInterface, so we'll review it briefly. \snippet examples/tools/plugandpaint/paintarea.cpp 0 In \c setBrush(), we simply store the \c BrushInterface and the brush that are given to us by \c MainWindow. \snippet examples/tools/plugandpaint/paintarea.cpp 1 In the \l{QWidget::mouseMoveEvent()}{mouse move event handler}, we call the \c BrushInterface::mouseMove() function on the current \c BrushInterface, with the current brush. The mouse press and mouse release handlers are very similar. \section1 The PluginDialog Class The \c PluginDialog class provides information about the loaded plugins to the user. Its constructor takes a path to the plugins and a list of plugin file names. It calls \c findPlugins() to fill the QTreeWdiget with information about the plugins: \snippet examples/tools/plugandpaint/plugindialog.cpp 0 The \c findPlugins() is very similar to \c MainWindow::loadPlugins(). It uses QPluginLoader to access the static and dynamic plugins. Its helper function \c populateTreeWidget() uses \l qobject_cast() to find out which interfaces are implemented by the plugins: \snippet examples/tools/plugandpaint/plugindialog.cpp 1 \section1 Importing Static Plugins The \l{tools/plugandpaintplugins/basictools}{Basic Tools} plugin is built as a static plugin, to ensure that it is always available to the application. This requires using the Q_IMPORT_PLUGIN() macro somewhere in the application (in a \c .cpp file) and specifying the plugin in the \c .pro file. For Plug & Paint, we have chosen to put Q_IMPORT_PLUGIN() in \c main.cpp: \snippet examples/tools/plugandpaint/main.cpp 0 The argument to Q_IMPORT_PLUGIN() is the plugin's name, as specified with Q_EXPORT_PLUGIN2() in the \l{Exporting the Plugin}{plugin}. In the \c .pro file, we need to specify the static library. Here's the project file for building Plug & Paint: \snippet examples/tools/plugandpaint/plugandpaint.pro 0 The \c LIBS line variable specifies the library \c pnp_basictools located in the \c ../plugandpaintplugins/basictools directory. (Although the \c LIBS syntax has a distinct Unix flavor, \c qmake supports it on all platforms.) The \c CONFIG() code at the end is necessary for this example because the example is part of the Qt distribution and Qt can be configured to be built simultaneously in debug and in release modes. You don't need to for your own plugin applications. This completes our review of the Plug & Paint application. At this point, you might want to take a look at the \l{tools/plugandpaintplugins/basictools}{Basic Tools} example plugin. */ /*! \example tools/plugandpaintplugins/basictools \title Plug & Paint Basic Tools Example The Basic Tools example is a static plugin for the \l{tools/plugandpaint}{Plug & Paint} example. It provides a set of basic brushes, shapes, and filters. Through the Basic Tools example, we will review the four steps involved in writing a Qt plugin: \list 1 \o Declare a plugin class. \o Implement the interfaces provided by the plugin. \o Export the plugin using the Q_EXPORT_PLUGIN2() macro. \o Build the plugin using an adequate \c .pro file. \endlist \section1 Declaration of the Plugin Class \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.h 0 We start by including \c interfaces.h, which defines the plugin interfaces for the \l{tools/plugandpaint}{Plug & Paint} application. For the \c #include to work, we need to add an \c INCLUDEPATH entry to the \c .pro file with the path to Qt's \c examples/tools directory. The \c BasicToolsPlugin class is a QObject subclass that implements the \c BrushInterface, the \c ShapeInterface, and the \c FilterInterface. This is done through multiple inheritance. The \c Q_INTERFACES() macro is necessary to tell \l{moc}, Qt's meta-object compiler, that the base classes are plugin interfaces. Without the \c Q_INTERFACES() macro, we couldn't use \l qobject_cast() in the \l{tools/plugandpaint}{Plug & Paint} application to detect interfaces. \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.h 2 In the \c public section of the class, we declare all the functions from the three interfaces. \section1 Implementation of the Brush Interface Let's now review the implementation of the \c BasicToolsPlugin member functions inherited from \c BrushInterface. \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 0 The \c brushes() function returns a list of brushes provided by this plugin. We provide three brushes: \gui{Pencil}, \gui{Air Brush}, and \gui{Random Letters}. \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 1 On a mouse press event, we just call \c mouseMove() to draw the spot where the event occurred. \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 2 In \c mouseMove(), we start by saving the state of the QPainter and we compute a few variables that we'll need later. \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 3 Then comes the brush-dependent part of the code: \list \o If the brush is \gui{Pencil}, we just call QPainter::drawLine() with the current QPen. \o If the brush is \gui{Air Brush}, we start by setting the painter's QBrush to Qt::Dense6Pattern to obtain a dotted pattern. Then we draw a circle filled with that QBrush several times, resulting in a thick line. \o If the brush is \gui{Random Letters}, we draw a random letter at the new cursor position. Most of the code is for setting the font to be bold and larger than the default font and for computing an appropriate bounding rect. \endlist At the end, we restore the painter state to what it was upon entering the function and we return the bounding rectangle. \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 4 When the user releases the mouse, we do nothing and return an empty QRect. \section1 Implementation of the Shape Interface \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 5 The plugin provides three shapes: \gui{Circle}, \gui{Star}, and \gui{Text...}. The three dots after \gui{Text} are there because the shape pops up a dialog asking for more information. We know that the shape names will end up in a menu, so we include the three dots in the shape name. A cleaner but more complicated design would have been to distinguish between the internal shape name and the name used in the user interface. \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 6 The \c generateShape() creates a QPainterPath for the specified shape. If the shape is \gui{Text}, we pop up a QInputDialog to let the user enter some text. \section1 Implementation of the Filter Interface \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 7 The plugin provides three filters: \gui{Invert Pixels}, \gui{Swap RGB}, and \gui{Grayscale}. \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 8 The \c filterImage() function takes a filter name and a QImage as parameters and returns an altered QImage. The first thing we do is to convert the image to a 32-bit RGB format, to ensure that the algorithms will work as expected. For example, QImage::invertPixels(), which is used to implement the \gui{Invert Pixels} filter, gives counterintuitive results for 8-bit images, because they invert the indices into the color table instead of inverting the color table's entries. \section1 Exporting the Plugin Whereas applications have a \c main() function as their entry point, plugins need to contain exactly one occurrence of the Q_EXPORT_PLUGIN2() macro to specify which class provides the plugin: \snippet examples/tools/plugandpaintplugins/basictools/basictoolsplugin.cpp 9 This line may appear in any \c .cpp file that is part of the plugin's source code. \section1 The .pro File Here's the project file for building the Basic Tools plugin: \snippet examples/tools/plugandpaintplugins/basictools/basictools.pro 0 The \c .pro file differs from typical \c .pro files in many respects. First, it starts with a \c TEMPLATE entry specifying \c lib. (The default template is \c app.) It also adds \c plugin to the \c CONFIG variable. This is necessary on some platforms to avoid generating symbolic links with version numbers in the file name, which is appropriate for most dynamic libraries but not for plugins. To make the plugin a static plugin, all that is required is to specify \c static in addition to \c plugin. The \l{tools/plugandpaintplugins/extrafilters}{Extra Filters} plugin, which is compiled as a dynamic plugin, doesn't specify \c static in its \c .pro file. The \c INCLUDEPATH variable sets the search paths for global headers (i.e., header files included using \c{#include <...>}). We add Qt's \c examples/tools directory (strictly speaking, \c{examples/tools/plugandpaintplugins/basictools/../..}) to the list, so that we can include \c <plugandpaint/interfaces.h>. The \c TARGET variable specifies which name we want to give the target library. We use \c pnp_ as the prefix to show that the plugin is designed to work with Plug & Paint. On Unix, \c lib is also prepended to that name. On all platforms, a platform-specific suffix is appended (e.g., \c .dll on Windows, \c .a on Linux). The \c CONFIG() code at the end is necessary for this example because the example is part of the Qt distribution and Qt can be configured to be built simultaneously in debug and in release modes. You don't need to for your own plugins. */ /*! \example tools/plugandpaintplugins/extrafilters \title Plug & Paint Extra Filters Example The Extra Filters example is a plugin for the \l{tools/plugandpaint}{Plug & Paint} example. It provides a set of filters in addition to those provided by the \l{tools/plugandpaintplugins/basictools}{Basic Tools} plugin. Since the approach is identical to \l{tools/plugandpaintplugins/basictools}{Basic Tools}, we won't review the code here. The only part of interest is the \c .pro file, since Extra Filters is a dynamic plugin (\l{tools/plugandpaintplugins/basictools}{Basic Tools} is linked statically into the Plug & Paint executable). Here's the project file for building the Extra Filters plugin: \snippet examples/tools/plugandpaintplugins/extrafilters/extrafilters.pro 0 The \c .pro file differs from typical \c .pro files in many respects. First, it starts with a \c TEMPLATE entry specifying \c lib. (The default template is \c app.) It also adds \c plugin to the \c CONFIG variable. This is necessary on some platforms to avoid generating symbolic links with version numbers in the file name, which is appropriate for most dynamic libraries but not for plugins. The \c INCLUDEPATH variable sets the search paths for global headers (i.e., header files included using \c{#include <...>}). We add Qt's \c examples/tools directory (strictly speaking, \c{examples/tools/plugandpaintplugins/basictools/../..}) to the list, so that we can include \c <plugandpaint/interfaces.h>. The \c TARGET variable specifies which name we want to give the target library. We use \c pnp_ as the prefix to show that the plugin is designed to work with Plug & Paint. On Unix, \c lib is also prepended to that name. On all platforms, a platform-specific suffix is appended (e.g., \c .dll on Windows, \c .so on Linux). The \c DESTDIR variable specifies where we want to install the plugin. We put it in Plug & Paint's \c plugins subdirectory, since that's where the application looks for dynamic plugins. The \c CONFIG() code at the end is necessary for this example because the example is part of the Qt distribution and Qt can be configured to be built simultaneously in debug and in release modes. You don't need to for your own plugins. */