/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Qt Software Information (qt-info@nokia.com)
**
** This file is part of a Qt Solutions component.
**
** GNU General Public License Usage
** This file may be used under the terms of the GNU
** General Public License versions 2.0 or 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 GNU General Public Licensing requirements will be met:
** http://www.fsf.org/licensing/licenses/info/GPLv2.html and
** http://www.gnu.org/copyleft/gpl.html.  In addition, as a special
** exception, Nokia gives you certain additional rights. These rights
** are described in the Nokia Qt GPL Exception version 1.3, included in
** the file GPL_EXCEPTION.txt in this package.
**
** If you are unsure which license is appropriate for your use, please
** contact the sales department at qt-sales@nokia.com.
**
****************************************************************************/


/*!
  \page statemachine-api.html
  \title The State Machine Framework
  \brief An overview of the State Machine framework for constructing and executing state graphs.

  \tableofcontents

  The State Machine framework provides classes for creating and executing
  state graphs. The concepts and notation are based on those from Harel's
  \l{Statecharts: A visual formalism for complex systems}{Statecharts}, which
  is also the basis of UML state diagrams. The semantics of state machine
  execution are based on \l{State Chart XML: State Machine Notation for
  Control Abstraction}{State Chart XML (SCXML)}.

  Statecharts provide a graphical way of modeling how a system reacts to
  stimuli. This is done by defining the possible \e states that the system can
  be in, and how the system can move from one state to another (\e transitions
  between states). A key characteristic of event-driven systems (such as Qt
  applications) is that behavior often depends not only on the last or current
  event, but also the events that preceded it. With statecharts, this
  information is easy to express.

  The State Machine framework provides an API and execution model that can be
  used to effectively embed the elements and semantics of statecharts in Qt
  applications. The framework integrates tightly with Qt's existing event
  system and meta-object system; for example, transitions between states can
  be triggered by signals, and states can be configured to set properties and
  invoke methods on QObjects.

  \section1 A Simple State Machine

  To demonstrate the core functionality of the State Machine API, let's look
  at a small example: A state machine with three states, \c s1, \c s2 and \c
  s3. The state machine is controlled by a single QPushButton; when the button
  is clicked, the machine transitions to another state. Initially, the state
  machine is in state \c s1. The statechart for this machine is as follows:

    \img statemachine-button.png
    \omit
    \caption This is a caption
    \endomit

  The following snippet shows the code needed to create such a state machine.

  \code
    QtStateMachine machine;
    QtState *s1 = new QtState();
    QtState *s2 = new QtState();
    QtState *s3 = new QtState();

    s1->addTransition(button, SIGNAL(clicked()), s2);
    s2->addTransition(button, SIGNAL(clicked()), s3);
    s3->addTransition(button, SIGNAL(clicked()), s1);

    machine.addState(s1);
    machine.addState(s2);
    machine.addState(s3);
    machine.setInitialState(s1);

    machine.start();
  \endcode

  Once the state machine has been set up, you need to start it by calling
  QtStateMachine::start(). The state machine executes asynchronously, i.e. it
  becomes part of your application's event loop.

  The above state machine is perfectly fine, but it doesn't \e do anything; it
  merely transitions from one state to another. The
  QtState::setPropertyOnEntry() function can be used to have a state set a
  property of a QObject when the state is entered. In the following snippet,
  the value that should be assigned to a QLabel's text property is specified
  for each state:

  \code
    s1->setPropertyOnEntry(label, "text", "In state s1");
    s2->setPropertyOnEntry(label, "text", "In state s2");
    s3->setPropertyOnEntry(label, "text", "In state s3");
  \endcode

  When any of the states is entered, the label's text will be changed
  accordingly.

  The QtState::invokeMethodOnEntry() function can be used to have a state
  invoke a method (a slot) of a QObject when the state is entered. In the
  following snippet, the button's showMaximized() slot will be called when
  state \c s3 is entered:

  \code
    s2->invokeMethodOnEntry(button, "showMaximized");
  \endcode

  \section1 Sharing Transitions By Grouping States

  The state machine defined in the previous section never finishes. In order
  for a state machine to be able to finish, it needs to have a top-level \e
  final state. When the state machine enters a top-level final state, the
  machine will emit the finished() signal and halt.

  Assume we wanted the user to be able to quit the application at any time by
  clicking a Quit button. In order to achieve this, we need to create a final
  state and make it the target of a transition associated with the Quit
  button's clicked() signal. We could add a transition from each of \c s1, \c
  s2 and \c s3; however, this seems redundant, and one would also have to
  remember to add such a transition from every new state that is added in the
  future.

  We can achieve the same behavior (namely that clicking the Quit button quits
  the state machine, regardless of which state the state machine is in) by
  grouping states \c s1, \c s2 and \c s3. This is done by creating a new
  top-level state and making the three original states children of the new
  state. The following diagram shows the new state machine.

    \img statemachine-button-nested.png
    \omit
    \caption This is a caption
    \endomit

  The three original states have been renamed \c s11, \c s12 and \c s13 to
  reflect that they are now children of the new top-level state, \c s1.  Child
  states implicitly inherit the transitions of their parent state. This means
  it is now sufficient to add a single transition from \c s1 to the final
  state \c s2. New states added to \c s1 will also automatically inherit this
  transition.

  All that's needed to group states is to specify the proper parent when the
  state is created. You also need to specify which of the child states is the
  initial one (i.e. which child state the state machine should enter when the
  parent state is the target of a transition).

  \code
    QtState *s1 = new QtState();
    QtState *s11 = new QtState(s1);
    QtState *s12 = new QtState(s1);
    QtState *s13 = new QtState(s1);
    s1->setInitialState(s11);
    machine.addState(s1);
  \endcode

  \code
    QtFinalState *s2 = new QtFinalState();
    s1->addTransition(quitButton, SIGNAL(clicked()), s2);
    machine.addState(s2);

    QObject::connect(&machine, SIGNAL(finished()), QApplication::instance(), SLOT(quit()));
  \endcode

  In this case we want the application to quit when the state machine is
  finished, so the machine's finished() signal is connected to the
  application's quit() slot.

  A child state can override an inherited transition. For example, the
  following code adds a transition that effectively causes the Quit button to
  be ignored when the state machine is in state \c s12.

  \code
    s12>addTransition(quitButton, SIGNAL(clicked()), s12);
  \endcode

  \section1 Using History States to Save and Restore the Current State

  Imagine that we wanted to add an "interrupt" mechanism to the example
  discussed in the previous section; the user should be able to click a button
  to have the state machine perform some non-related task, after which the
  state machine should resume whatever it was doing before (i.e. return to the
  old state, which is one of \c s11, \c s12 and \c s13 in this case).

  Such behavior can easily be modeled using \e{history states}. A history
  state (QtHistoryState object) is a pseudo-state that represents the child
  state that the parent state was in the last time the parent state was
  exited.

  A history state is created as a child of the state for which we wish to
  record the current child state; when the state machine detects the presence
  of such a state at runtime, it automatically records the current (real)
  child state when the parent state is exited. A transition to the history
  state is in fact a transition to the child state that the state machine had
  previously saved; the state machine automatically "forwards" the transition
  to the real child state.

  The following diagram shows the state machine after the interrupt mechanism
  has been added.

    \img statemachine-button-history.png
    \omit
    \caption This is a caption
    \endomit

  The following code shows how it can be implemented; in this example we
  simply display a message box when \c s3 is entered, then immediately return
  to the previous child state of \c s1 via the history state.

  \code
    QtHistoryState *s1h = s1->addHistoryState();

    QtState *s3 = new QtState();
    s3->setPropertyOnEntry(label, "text", "In s3");
    QMessageBox mbox;
    mbox.addButton(QMessageBox::Ok);
    mbox.setText("Interrupted!");
    mbox.setIcon(QMessageBox::Information);
    s3->invokeMethodOnEntry(&mbox, "exec");
    s3->addTransition(s1h);
    machine.addState(s3);

    s1->addTransition(interruptButton, SIGNAL(clicked()), s3);
  \endcode

  \section1 Using Parallel States to Avoid a Combinatorial Explosion of States

  Assume that you wanted to model a set of mutually exclusive properties of a
  car in a single state machine. Let's say the properties we are interested in
  are Clean vs Dirty, and Moving vs Not moving. It would take four mutually
  exclusive states and eight transitions to be able to represent and freely
  move between all possible combinations.

    \img statemachine-nonparallel.png
    \omit
    \caption This is a caption
    \endomit

  If we added a third property (say, Red vs Blue), the total number of states
  would double, to eight; and if we added a fourth property (say, Enclosed vs
  Convertible), the total number of states would double again, to 16.

  Using parallel states, the total number of states and transitions grows
  linearly as we add more properties, instead of exponentially. Furthermore,
  states can be added to or removed from the parallel state without affecting
  any of their sibling states.

    \img statemachine-parallel.png
    \omit
    \caption This is a caption
    \endomit

  The QtParallelStateGroup class is used to create groups of parallel states.

  \code
    QtParallelStateGroup *s1 = new QtParallelStateGroup();
    // s11 and s12 will be entered in parallel
    QtState *s11 = new QtState(s1);
    QtState *s12 = new QtState(s1);
  \endcode

  \section1 Detecting that a Composite State has Finished

  A child state can be final; when a final child state is entered, a
  QtStateFinishedEvent is generated for the parent state. You can use the
  QtStateFinishedTransition class to trigger a transition based on this event.

    \img statemachine-finished.png
    \omit
    \caption This is a caption
    \endomit

  This is useful when you want to hide the internal details of a state;
  i.e. the only thing the outside world should be able to do is enter the
  state, and get a notification when the state has finished (i.e. when a final
  child state has been entered).

  */
