path: root/doc/src/examples/plugandpaint.qdoc

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/*!
    \example tools/plugandpaint
    \title Plug & Paint Example

    \brief 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 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.
*/