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/*!

    \group statemachine

    \title State Machine Classes

*/

/*!

  \page statemachine-api.html

  \title The State Machine Framework

  \brief An overview of the State Machine framework for constructing and executing state graphs.

  \ingroup frameworks-technologies

  \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 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.

  Qt's event system is used to drive the state machines.

  The state graph in the State Machine framework is hierarchical. States can be nested inside of

  other states, and the current configuration of the state machine consists of the set of states

  which are currently active. All the states in a valid configuration of the state machine will

  have a common ancestor.

  \section1 Classes in the State Machine Framework

  These classes are provided by qt for creating event-driven state machines.

  \annotatedlist statemachine

  \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.

  First, we create the state machine and states:

  \snippet doc/src/snippets/statemachine/main.cpp 0

  Then, we create the transitions by using the QState::addTransition()

  function:

  \snippet doc/src/snippets/statemachine/main.cpp 1

  Next, we add the states to the machine and set the machine's initial state:

  \snippet doc/src/snippets/statemachine/main.cpp 2

  Finally, we start the state machine:

  \snippet doc/src/snippets/statemachine/main.cpp 3

  The state machine executes asynchronously, i.e. it becomes part of your

  application's event loop.

  \section1 Doing Useful Work on State Entry and Exit

  The above state machine merely transitions from one state to another, it

  doesn't perform any operations. 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:

  \snippet doc/src/snippets/statemachine/main.cpp 4

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

  accordingly.

  The QState::entered() signal is emitted when the state is entered, and the

  QState::exited() signal is emitted when the state is exited. In the

  following snippet, the button's showMaximized() slot will be called when

  state \c s3 is entered, and the button's showMinimized() slot will be called

  when \c s3 is exited:

  \snippet doc/src/snippets/statemachine/main.cpp 5

  Custom states can reimplement QAbstractState::onEntry() and

  QAbstractState::onExit().

  \section1 State Machines That Finish

  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 (QFinalState object). When the state machine enters a top-level

  final state, the machine will emit the QStateMachine::finished() signal and

  halt.

  All you need to do to introduce a final state in the graph is create a

  QFinalState object and use it as the target of one or more transitions.

  \section1 Sharing Transitions By Grouping States

  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).

  \snippet doc/src/snippets/statemachine/main2.cpp 0

  \snippet doc/src/snippets/statemachine/main2.cpp 1

  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.

  \snippet doc/src/snippets/statemachine/main2.cpp 2

  A transition can have any state as its target, i.e. the target state does

  not have to be on the same level in the state hierarchy as the source state.

  \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.

  \snippet doc/src/snippets/statemachine/main2.cpp 3

  \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::ParallelStates to the QState

  constructor.

  \snippet doc/src/snippets/statemachine/main3.cpp 0

  When a parallel state group is entered, all its child states will be

  simultaneously entered. Transitions within the individual child states

  operate normally. However, any of the child states may take a transition which exits the parent

  state. When this happens, the parent state and all of its child states are exited.

  The parallelism in the State Machine framework follows an interleaved semantics. All parallel

  operations will be executed in a single, atomic step of the event processing, so no event can

  interrupt the parallel operations. However, events will still be processed sequentially, since

  the machine itself is single threaded. As an example: Consider the situation where there are two

  transitions that exit the same parallel state group, and their conditions become true

  simultaneously. In this case, the event that is processed last of the two will not have any

  effect, since the first event will already have caused the machine to exit from the parallel

  state.

  \section1 Detecting that a Composite State has Finished

  A child state can be final (a QFinalState object); when a final child state

  is entered, the parent state emits the QState::finished() signal. The

  following diagram shows a composite state \c s1 which does some processing

  before entering a final state:

    \img statemachine-finished.png

    \omit

    \caption This is a caption

    \endomit

  When \c s1 's final state is entered, \c s1 will automatically emit

  finished(). We use a signal transition to cause this event to trigger a

  state change:

  \snippet doc/src/snippets/statemachine/main3.cpp 1

  Using final states in composite states is useful when you want to hide the

  internal details of a composite 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 completed its work. This is a very powerful abstraction and

  encapsulation mechanism when building complex (deeply nested) state

  machines. (In the above example, you could of course create a transition

  directly from \c s1 's \c done state rather than relying on \c s1 's

  finished() signal, but with the consequence that implementation details of

  \c s1 are exposed and depended on).

  For parallel state groups, the QState::finished() signal is emitted when \e

  all the child states have entered final states.

  \section1 Targetless Transitions

  A transition need not have a target state. A transition without a target can

  be triggered the same way as any other transition; the difference is that

  when a targetless transition is triggered, it doesn't cause any state

  changes. This allows you to react to a signal or event when your machine is

  in a certain state, without having to leave that state. Example:

  \code

    QStateMachine machine;

    QState *s1 = new QState(&machine);

    QPushButton button;

    QSignalTransition *trans = new QSignalTransition(&button, SIGNAL(clicked()));

    s1->addTransition(trans);

    QMessageBox msgBox;

    msgBox.setText("The button was clicked; carry on.");

    QObject::connect(trans, SIGNAL(triggered()), &msgBox, SLOT(exec()));

    machine.setInitialState(s1);

  \endcode

  The message box will be displayed each time the button is clicked, but the

  state machine will remain in its current state (s1). If the target state

  were explicitly set to s1, however, s1 would be exited and re-entered each

  time (e.g. the QAbstractState::entered() and QAbstractState::exited()

  signals would be emitted).

  \section1 Events, Transitions and Guards

  A QStateMachine runs its own event loop. For signal transitions

  (QSignalTransition objects), QStateMachine automatically posts a

  QStateMachine::SignalEvent to itself when it intercepts the corresponding

  signal; similarly, for QObject event transitions (QEventTransition objects)

  a QStateMachine::WrappedEvent is posted.

  You can post your own events to the state machine using

  QStateMachine::postEvent().

  When posting a custom event to the state machine, you typically also have

  one or more custom transitions that can be triggered from events of that

  type. To create such a transition, you subclass QAbstractTransition and

  reimplement QAbstractTransition::eventTest(), where you check if an event

  matches your event type (and optionally other criteria, e.g. attributes of

  the event object).

  Here we define our own custom event type, \c StringEvent, for posting

  strings to the state machine:

  \snippet doc/src/snippets/statemachine/main4.cpp 0

  Next, we define a transition that only triggers when the event's string

  matches a particular string (a \e guarded transition):

  \snippet doc/src/snippets/statemachine/main4.cpp 1

  In the eventTest() reimplementation, we first check if the event type is the

  desired one; if so, we cast the event to a StringEvent and perform the

  string comparison.

  The following is a statechart that uses the custom event and transition:

    \img statemachine-customevents.png

    \omit

    \caption This is a caption

    \endomit

  Here's what the implementation of the statechart looks like:

  \snippet doc/src/snippets/statemachine/main4.cpp 2

  Once the machine is started, we can post events to it.

  \snippet doc/src/snippets/statemachine/main4.cpp 3

  An event that is not handled by any relevant transition will be silently

  consumed by the state machine. It can be useful to group states and provide

  a default handling of such events; for example, as illustrated in the

  following statechart:

    \img statemachine-customevents2.png

    \omit

    \caption This is a caption

    \endomit

  For deeply nested statecharts, you can add such "fallback" transitions at

  the level of granularity that's most appropriate.

  \section1 Using Restore Policy To Automatically Restore Properties

  In some state machines it can be useful to focus the attention on assigning properties in states,

  not on restoring them when the state is no longer active. If you know that a property should

  always be restored to its initial value when the machine enters a state that does not explicitly

  give the property a value, you can set the global restore policy to

  QStateMachine::RestoreProperties.

  \code

    QStateMachine machine;

    machine.setGlobalRestorePolicy(QStateMachine::RestoreProperties);

  \endcode

  When this restore policy is set, the machine will automatically restore all properties. If it

  enters a state where a given property is not set, it will first search the hierarchy of ancestors

  to see if the property is defined there. If it is, the property will be restored to the value

  defined by the closest ancestor. If not, it will be restored to its initial value (i.e. the

  value of the property before any property assignments in states were executed.)

  Take the following code:

  \snippet doc/src/snippets/statemachine/main5.cpp 0  

  Lets say the property \c fooBar is 0.0 when the machine starts. When the machine is in state

  \c s1, the property will be 1.0, since the state explicitly assigns this value to it. When the

  machine is in state \c s2, no value is explicitly defined for the property, so it will implicitly

  be restored to 0.0.

  If we are using nested states, the parent defines a value for the property which is inherited by

  all descendants that do not explicitly assign a value to the property.

  \snippet doc/src/snippets/statemachine/main5.cpp 2

  Here \c s1 has two children: \c s2 and \c s3. When \c s2 is entered, the property \c fooBar

  will have the value 2.0, since this is explicitly defined for the state. When the machine is in

  state \c s3, no value is defined for the state, but \c s1 defines the property to be 1.0, so this

  is the value that will be assigned to \c fooBar.

  \section1 Animating Property Assignments

  The State Machine API connects with the Animation API in Qt to allow automatically animating

  properties as they are assigned in states.

  Say we have the following code:

  \snippet doc/src/snippets/statemachine/main5.cpp 3

  Here we define two states of a user interface. In \c s1 the \c button is small, and in \c s2

  it is bigger. If we click the button to transition from \c s1 to \c s2, the geometry of the button

  will be set immediately when a given state has been entered. If we want the transition to be

  smooth, however, all we need to do is make a QPropertyAnimation and add this to the transition

  object.

  \snippet doc/src/snippets/statemachine/main5.cpp 4

  Adding an animation for the property in question means that the property assignment will no

  longer take immediate effect when the state has been entered. Instead, the animation will start

  playing when the state has been entered and smoothly animate the property assignment. Since we

  do not set the start value or end value of the animation, these will be set implicitly. The

  start value of the animation will be the property's current value when the animation starts, and

  the end value will be set based on the property assignments defined for the state.

  If the global restore policy of the state machine is set to QStateMachine::RestoreProperties,

  it is possible to also add animations for the property restorations.

  \section1 Detecting That All Properties Have Been Set In A State

  When animations are used to assign properties, a state no longer defines the exact values that a

  property will have when the machine is in the given state. While the animation is running, the

  property can potentially have any value, depending on the animation.

  In some cases, it can be useful to be able to detect when the property has actually been assigned

  the value defined by a state.

  Say we have the following code:

  \snippet doc/src/snippets/statemachine/main5.cpp 5

  When \c button is clicked, the machine will transition into state \c s2, which will set the

  geometry of the button, and then pop up a message box to alert the user that the geometry has

  been changed.

  In the normal case, where animations are not used, this will operate as expected. However, if

  an animation for the \c geometry of \c button is set on the transition between \c s1 and \c s2,

  the animation will be started when \c s2 is entered, but the \c geometry property will not

  actually reach its defined value before the animation is finished running. In this case, the

  message box will pop up before the geometry of the button has actually been set.

  To ensure that the message box does not pop up until the geometry actually reaches its final

  value, we can use the state's propertiesAssigned() signal. The propertiesAssigned() signal will be

  emitted when the property is assigned its final value, whether this is done immediately or

  after the animation has finished playing.

  \snippet doc/src/snippets/statemachine/main5.cpp 6

  In this example, when \c button is clicked, the machine will enter \c s2. It will remain in state

  \c s2 until the \c geometry property has been set to \c QRect(0, 0, 50, 50). Then it will

  transition into \c s3. When \c s3 is entered, the message box will pop up. If the transition into

  \c s2 has an animation for the \c geometry property, then the machine will stay in \c s2 until the

  animation has finished playing. If there is no such animation, it will simply set the property and

  immediately enter state \c s3.

  Either way, when the machine is in state \c s3, you are guaranteed that the property \c geometry

  has been assigned the defined value.

  If the global restore policy is set to QStateMachine::RestoreProperties, the state will not emit

  the propertiesAssigned() signal until these have been executed as well.

  \section1 What Happens If A State Is Exited Before The Animation Has Finished

  If a state has property assignments, and the transition into the state has animations for the

  properties, the state can potentially be exited before the properties have been assigned to the

  values defines by the state. This is true in particular when there are transitions out from the

  state that do not depend on the propertiesAssigned signal, as described in the previous section.

  The State Machine API guarantees that a property assigned by the state machine either:

  \list

  \o Has a value explicitly assigned to the property.

  \o Is currently being animated into a value explicitly assigned to the property.

  \endlist