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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