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+/*!
+\example tools/plugandpaint
+\title Plug & Paint Example
+The Plug & Paint example demonstrates how to write Qt
+applications that can be extended through plugins.
+\image plugandpaint.png Screenshot of the Plug & Paint example
+A plugin is a dynamic library that can be loaded at run-time to
+extend an application. 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 interes 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.
+*/