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/****************************************************************************
**
** Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Qt Software Information (qt-info@nokia.com)
**
** This file is part of the $MODULE$ of the Qt Toolkit.
**
** $TROLLTECH_DUAL_LICENSE$
**
****************************************************************************/
/*!
\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
QStateMachine machine;
QState *s1 = new QState();
QState *s2 = new QState();
QState *s3 = new QState();
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
QStateMachine::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 QState::assignProperty()
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->assignProperty(label, "text", "In state s1");
s2->assignProperty(label, "text", "In state s2");
s3->assignProperty(label, "text", "In state s3");
\endcode
When any of the states is entered, the label's text will be changed
accordingly.
The QActionState::entered() signal is emitted when the state is entered. In the
following snippet, the button's showMaximized() slot will be called when
state \c s3 is entered:
\code
QObject::connect(s3, SIGNAL(entered()), button, SLOT(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
QState *s1 = new QState();
QState *s11 = new QState(s1);
QState *s12 = new QState(s1);
QState *s13 = new QState(s1);
s1->setInitialState(s11);
machine.addState(s1);
\endcode
\code
QFinalState *s2 = new QFinalState();
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 (QHistoryState 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
QHistoryState *s1h = s1->addHistoryState();
QState *s3 = new QState();
s3->assignProperty(label, "text", "In s3");
QMessageBox mbox;
mbox.addButton(QMessageBox::Ok);
mbox.setText("Interrupted!");
mbox.setIcon(QMessageBox::Information);
QObject::connect(s3, SIGNAL(entered()), &mbox, SLOT(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
To create a parallel state group, pass QState::ParallelStateGroup to the
QState constructor.
\code
QState *s1 = new QState(QState::ParallelStateGroup);
// s11 and s12 will be entered in parallel
QState *s11 = new QState(s1);
QState *s12 = new QState(s1);
\endcode
\section1 Detecting that a Composite State has Finished
A child state can be final; when a final child state is entered, a
QStateFinishedEvent is generated for the parent state. You can use the
QStateFinishedTransition 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).
*/
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