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+/****************************************************************************
+**
+** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:FDL$
+** No Commercial Usage
+** This file contains pre-release code and may not be distributed.
+** You may use this file in accordance with the terms and conditions
+** contained in the Technology Preview License Agreement accompanying
+** this package.
+**
+** GNU Free Documentation License
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of this
+** file.
+**
+** If you have questions regarding the use of this file, please contact
+** Nokia at qt-info@nokia.com.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+    \example tools/echoplugin
+    \title Echo Plugin Example
+
+    This example shows how to create a Qt plugin.
+
+    \image echopluginexample.png
+
+    There are two kinds of plugins in Qt: plugins that extend Qt
+    itself and plugins that extend applications written in Qt. In this
+    example, we show the procedure of implementing plugins that extend
+    applications. When you create a plugin you declare an interface,
+    which is a class with only pure virtual functions. This interface
+    is inherited by the class that implements the plugin. The class is
+    stored in a shared library and can therefore be loaded by
+    applications at run-time. When loaded, the plugin is dynamically
+    cast to the interface using Qt's \l{Meta-Object
+    System}{meta-object system}. The plugin \l{How to Create Qt
+    Plugins}{overview document} gives a high-level introduction to
+    plugins.
+    
+    We have implemented a plugin, the \c EchoPlugin, which implements
+    the \c EchoInterface. The interface consists of \c echo(), which
+    takes a QString as argument. The \c EchoPlugin returns the string
+    unaltered (i.e., it works as the familiar echo command found in
+    both Unix and Windows).
+
+    We test the plugin in \c EchoWindow: when you push the QPushButton
+    (as seen in the image above), the application sends the text in
+    the QLineEdit to the plugin, which echoes it back to the
+    application. The answer from the plugin is displayed in the
+    QLabel.
+
+
+    \section1 EchoWindow Class Definition
+
+    The \c EchoWindow class lets us test the \c EchoPlugin through a
+    GUI.
+
+    \snippet examples/tools/echoplugin/echowindow/echowindow.h 0
+
+    We load the plugin in \c loadPlugin() and cast it to \c
+    EchoInterface. When the user clicks the \c button we take the 
+    text in \c lineEdit and call the interface's \c echo() with it. 
+
+
+    \section1 EchoWindow Class Implementation
+
+    We start with a look at the constructor:
+
+    \snippet examples/tools/echoplugin/echowindow/echowindow.cpp 0
+
+    We create the widgets and set a title for the window. We then load
+    the plugin. \c loadPlugin() returns false if the plugin could not
+    be loaded, in which case we disable the widgets. If you wish a
+    more detailed error message, you can use
+    \l{QPluginLoader::}{errorString()}; we will look more closely at
+    QPluginLoader later.
+
+    Here is the implementation of \c sendEcho():
+
+    \snippet examples/tools/echoplugin/echowindow/echowindow.cpp 1
+
+    This slot is called when the user pushes \c button or presses
+    enter in \c lineEdit. We call \c echo() of the echo interface. In
+    our example this is the \c EchoPlugin, but it could be any plugin
+    that inherit the \c EchoInterface. We take the QString returned
+    from \c echo() and display it in the \c label.
+
+    Here is the implementation of \c createGUI():
+
+    \snippet examples/tools/echoplugin/echowindow/echowindow.cpp 2
+
+    We create the widgets and lay them out in a grid layout. We
+    connect the label and line edit to our \c sendEcho() slot.
+
+    Here is the \c loadPlugin() function:
+
+    \snippet examples/tools/echoplugin/echowindow/echowindow.cpp 3
+
+    Access to plugins at run-time is provided by QPluginLoader. You
+    supply it with the filename of the shared library the plugin is
+    stored in and call \l{QPluginLoader::}{instance()}, which loads
+    and returns the root component of the plugin (i.e., it resolves
+    the type of the plugin and creates a QObject instance of it). If
+    the plugin was not successfully loaded, it will be null, so we
+    return false. If it was loaded correctly, we can cast the plugin
+    to our \c EchoInterface and return true. In the case that the
+    plugin loaded does not implement the \c EchoInterface, \c
+    instance() will return null, but this cannot happen in our
+    example. Notice that the location of the plugin is not the same
+    for all platforms.
+
+
+    \section1 EchoInterface Class Definition
+
+    The \c EchoInterface defines the functions that the plugin will
+    provide. An interface is a class that only consists of pure
+    virtual functions. If non virtual functions were present in the
+    class you would get misleading compile errors in the moc files.
+
+    \snippet examples/tools/echoplugin/echowindow/echointerface.h 0
+
+    We declare \c echo(). In our \c EchoPlugin we use this method to
+    return, or echo, \a message.
+
+    We use the Q_DECLARE_INTERFACE macro to let \l{Meta-Object
+    System}{Qt's meta object system} aware of the interface. We do
+    this so that it will be possible to identify plugins that
+    implements the interface at run-time. The second argument is a
+    string that must identify the interface in a unique way. 
+
+
+    \section1 EchoPlugin Class Definition
+
+    We inherit both QObject and \c EchoInterface to make this class a
+    plugin. The Q_INTERFACES macro tells Qt which interfaces the class
+    implements. In our case we only implement the \c EchoInterface. 
+    If a class implements more than one interface, they are given as 
+    a comma separated list.
+
+    \snippet examples/tools/echoplugin/plugin/echoplugin.h 0
+
+
+    \section1 EchoPlugin Class Implementation
+
+    Here is the implementation of \c echo():
+
+    \snippet examples/tools/echoplugin/plugin/echoplugin.cpp 0
+
+    We simply return the functions parameter. 
+
+    \snippet examples/tools/echoplugin/plugin/echoplugin.cpp 1
+
+    We use the Q_EXPORT_PLUGIN2 macro to let Qt know that the \c
+    EchoPlugin class is a plugin. The first parameter is the name of
+    the plugin; it is usual to give the plugin and the library file it
+    is stored in the same name.
+
+    \section1 The \c main() function
+
+    \snippet examples/tools/echoplugin/echowindow/main.cpp 0
+
+    We create an \c EchoWindow and display it as a top-level window.
+
+    \section1 The Profiles
+
+    When creating plugins the profiles need to be adjusted.
+    We show here what changes need to be done. 
+
+    The profile in the echoplugin directory uses the \c subdirs
+    template and simply includes includes to directories in which
+    the echo window and echo plugin lives:
+
+    \snippet examples/tools/echoplugin/echoplugin.pro 0
+
+    The profile for the echo window does not need any plugin specific
+    settings. We move on to the plugin profile:
+
+    \snippet examples/tools/echoplugin/plugin/plugin.pro 0
+
+    We need to set the TEMPLATE as we now want to make a library
+    instead of an executable. We also need to tell qmake that we are
+    creating a plugin. The \c EchoInterface that the plugin implements
+    lives in the \c echowindow directory, so we need to add that
+    directory to the include path. We set the TARGET of the project,
+    which is the name of the library file in which the plugin will be
+    stored; qmake appends the appropriate file extension depending on
+    the platform. By convention the target should have the same name
+    as the plugin (set with Q_EXPORT_PLUGIN2)
+
+    \section1 Further reading and examples
+
+    You can find an overview of the macros needed to create plugins
+    \l{Macros for Defining Plugins}{here}.
+    
+    We give an example of a plugin that extend Qt in the \l{Style
+    Plugin Example}{style plugin} example. The \l{Plug & Paint
+    Example}{plug and paint} example shows how to create static
+    plugins.
+*/