doc: Add documentation for Tank Game example

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diff --git a/doc/src/examples.qdoc b/doc/src/examples.qdoc
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@@ -315,6 +315,7 @@
     \o \l{statemachine/pingpong}{Ping Pong States}\raisedaster
     \o \l{statemachine/trafficlight}{Traffic Light}\raisedaster
     \o \l{statemachine/twowaybutton}{Two-way Button}\raisedaster
+    \o \l{statemachine/tankgame}{Tank Game}\raisedaster
     \endlist
 
     \section1 Threads
diff --git a/doc/src/examples/tankgame.qdoc b/doc/src/examples/tankgame.qdoc
new file mode 100644
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+/****************************************************************************
+**
+** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
+** Contact: Qt Software Information (qt-info@nokia.com)
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** 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 either Technology Preview License Agreement or the
+** Beta Release License Agreement.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain
+** additional rights. These rights are described in the Nokia Qt LGPL
+** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
+** package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3.0 as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU General Public License version 3.0 requirements will be
+** met: http://www.gnu.org/copyleft/gpl.html.
+**
+** If you are unsure which license is appropriate for your use, please
+** contact the sales department at qt-sales@nokia.com.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+    \example statemachine/tankgame
+    \title Tank Game Example
+
+    The Tank Game example is part of the in \l{The State Machine Framework}. It shows how to use 
+    parallel states to implement artificial intelligence controllers that run in parallel, and error 
+    states to handle run-time errors in parts of the state graph created by external plugins.
+
+    \image tankgame-example.png
+
+    In this example we write a simple game. The application runs a state machine with two main 
+    states: A "stopped" state and a "running" state. The user can load plugins from the disk by 
+    selecting the "Add tank" menu item. 
+    
+    When the "Add tank" menu item is selected, the "plugins" subdirectory in the example's 
+    directory is searched for compatible plugins. If any are found, they will be listed in a 
+    dialog box created using QInputDialog::getItem().
+    
+    \snippet examples/statemachine/tankgame/mainwindow.cpp 1
+    
+    If the user selects a plugin, the application will construct a TankItem object, which inherits
+    from QGraphicsItem and QObject, and which implements an agreed-upon interface using the 
+    meta-object mechanism.
+    
+    \snippet examples/statemachine/tankgame/tankitem.h 0
+    
+    The tank item will be passed to the plugin's create() function. This will in turn return a 
+    QState object which is expected to implement an artificial intelligence which controls the 
+    tank and attempts to destroy other tanks it detects.
+    
+    \snippet examples/statemachine/tankgame/mainwindow.cpp 2
+    
+    Each returned QState object becomes a descendant of a \c region in the "running" state, which is 
+    defined as a parallel state. This means that entering the "running" state will cause each of the 
+    plugged-in QState objects to be entered simultaneously, allowing the tanks to run independently
+    of each other.
+    
+    \snippet examples/statemachine/tankgame/mainwindow.cpp 0
+    
+    The maximum number of tanks on the map is four, and when this number is reached, the 
+    "Add tank" menu item should be disabled. This is implemented by giving the "stopped" state two
+    children which define whether the map is full or not. 
+    
+    To make sure that we go into the correct child state when returning from the "running" state 
+    (if the "Stop game" menu item is selected while the game is running) we also give the "stopped" 
+    state a history state which we make the initial state of "stopped" state.
+    
+    \snippet examples/statemachine/tankgame/mainwindow.cpp 3
+    
+    Since part of the state graph is defined by external plugins, we have no way of controlling
+    whether they contain errors. By default, run-time errors are handled in the state machine by
+    entering a top level state which prints out an error message and never exits. If we were to 
+    use this default behavior, a run-time error in any of the plugins would cause the "running"
+    state to exit, and thus all the other tanks to stop running as well. A better solution would
+    be if the broken plugin was disabled and the rest of the tanks allowed to continue as before.
+    
+    This is done by setting the error state of the plugin's top-most state to a special error state
+    defined specifically for the plugin in question.
+    
+    \snippet examples/statemachine/tankgame/mainwindow.cpp 3
+    
+    If a run-time error occurs in \c pluginState or any of its descendants, the state machine will
+    search the hierarchy of ancestors until it finds a state whose error state is different from
+    \c null. (Note that if we are worried that a plugin could inadvertedly be overriding our 
+    error state, we could search the descendants of \c pluginState and verify that their error 
+    states are set to \c null before accepting the plugin.)
+    
+    The specialized \c errorState sets the "enabled" property of the tank item in question to false, 
+    causing it to be painted with a red cross over it to indicate that it is no longer running. 
+    Since the error state is a child of the same region in the parallel "running" state as 
+    \c pluginState, it will not exit the "running" state, and the other tanks will continue running
+    without disruption.
+
+*/