QStateMachine Class

The QStateMachine class provides a hierarchical finite state machine. More...

Header: #include <QStateMachine>
qmake: QT += core
Since: Qt 4.6
Inherits: QState

This class was introduced in Qt 4.6.

Note: All functions in this class are reentrant.

Note: These functions are also thread-safe:

Public Types

class SignalEvent
class WrappedEvent
enum Error { NoError, NoInitialStateError, NoDefaultStateInHistoryStateError, NoCommonAncestorForTransitionError, StateMachineChildModeSetToParallelError }
enum EventPriority { NormalPriority, HighPriority }


Public Functions

QStateMachine(QObject *parent = nullptr)
virtual ~QStateMachine()
void addDefaultAnimation(QAbstractAnimation *animation)
void addState(QAbstractState *state)
bool cancelDelayedEvent(int id)
void clearError()
QSet<QAbstractState *> configuration() const
QList<QAbstractAnimation *> defaultAnimations() const
QStateMachine::Error error() const
QString errorString() const
QState::RestorePolicy globalRestorePolicy() const
bool isAnimated() const
bool isRunning() const
int postDelayedEvent(QEvent *event, int delay)
void postEvent(QEvent *event, QStateMachine::EventPriority priority = NormalPriority)
void removeDefaultAnimation(QAbstractAnimation *animation)
void removeState(QAbstractState *state)
void setAnimated(bool enabled)
void setGlobalRestorePolicy(QState::RestorePolicy restorePolicy)

Reimplemented Public Functions

virtual bool eventFilter(QObject *watched, QEvent *event) override

Public Slots

void setRunning(bool running)
void start()
void stop()


void runningChanged(bool running)
void started()
void stopped()

Reimplemented Protected Functions

virtual bool event(QEvent *e) override
virtual void onEntry(QEvent *event) override
virtual void onExit(QEvent *event) override

Detailed Description

QStateMachine is based on the concepts and notation of Statecharts. QStateMachine is part of The State Machine Framework.

A state machine manages a set of states (classes that inherit from QAbstractState) and transitions (descendants of QAbstractTransition) between those states; these states and transitions define a state graph. Once a state graph has been built, the state machine can execute it. QStateMachine's execution algorithm is based on the State Chart XML (SCXML) algorithm. The framework's overview gives several state graphs and the code to build them.

Use the addState() function to add a top-level state to the state machine. States are removed with the removeState() function. Removing states while the machine is running is discouraged.

Before the machine can be started, the initial state must be set. The initial state is the state that the machine enters when started. You can then start() the state machine. The started() signal is emitted when the initial state is entered.

The machine is event driven and keeps its own event loop. Events are posted to the machine through postEvent(). Note that this means that it executes asynchronously, and that it will not progress without a running event loop. You will normally not have to post events to the machine directly as Qt's transitions, e.g., QEventTransition and its subclasses, handle this. But for custom transitions triggered by events, postEvent() is useful.

The state machine processes events and takes transitions until a top-level final state is entered; the state machine then emits the finished() signal. You can also stop() the state machine explicitly. The stopped() signal is emitted in this case.

The following snippet shows a state machine that will finish when a button is clicked:

QPushButton button;

QStateMachine machine;
QState *s1 = new QState();
s1->assignProperty(&button, "text", "Click me");

QFinalState *s2 = new QFinalState();
s1->addTransition(&button, SIGNAL(clicked()), s2);


This code example uses QState, which inherits QAbstractState. The QState class provides a state that you can use to set properties and invoke methods on QObjects when the state is entered or exited. It also contains convenience functions for adding transitions, e.g., QSignalTransitions as in this example. See the QState class description for further details.

If an error is encountered, the machine will look for an error state, and if one is available, it will enter this state. The types of errors possible are described by the Error enum. After the error state is entered, the type of the error can be retrieved with error(). The execution of the state graph will not stop when the error state is entered. If no error state applies to the erroneous state, the machine will stop executing and an error message will be printed to the console.

Note: Important: setting the ChildMode of a state machine to parallel (ParallelStates) results in an invalid state machine. It can only be set to (or kept as) ExclusiveStates.

See also QAbstractState, QAbstractTransition, QState, and The State Machine Framework.

Member Type Documentation

enum QStateMachine::Error

This enum type defines errors that can occur in the state machine at run time. When the state machine encounters an unrecoverable error at run time, it will set the error code returned by error(), the error message returned by errorString(), and enter an error state based on the context of the error.

QStateMachine::NoError0No error has occurred.
QStateMachine::NoInitialStateError1The machine has entered a QState with children which does not have an initial state set. The context of this error is the state which is missing an initial state.
QStateMachine::NoDefaultStateInHistoryStateError2The machine has entered a QHistoryState which does not have a default state set. The context of this error is the QHistoryState which is missing a default state.
QStateMachine::NoCommonAncestorForTransitionError3The machine has selected a transition whose source and targets are not part of the same tree of states, and thus are not part of the same state machine. Commonly, this could mean that one of the states has not been given any parent or added to any machine. The context of this error is the source state of the transition.
QStateMachine::StateMachineChildModeSetToParallelError4The machine's childMode property was set to QState::ParallelStates. This is illegal. Only states may be declared as parallel, not the state machine itself. This enum value was added in Qt 5.14.

See also setErrorState().

enum QStateMachine::EventPriority

This enum type specifies the priority of an event posted to the state machine using postEvent().

Events of high priority are processed before events of normal priority.

QStateMachine::NormalPriority0The event has normal priority.
QStateMachine::HighPriority1The event has high priority.

Property Documentation

animated : bool

This property holds whether animations are enabled

The default value of this property is true.

Access functions:

bool isAnimated() const
void setAnimated(bool enabled)

See also QAbstractTransition::addAnimation().

errorString : const QString

This property holds the error string of this state machine

Access functions:

QString errorString() const

globalRestorePolicy : QState::RestorePolicy

This property holds the restore policy for states of this state machine.

The default value of this property is QState::DontRestoreProperties.

Access functions:

QState::RestorePolicy globalRestorePolicy() const
void setGlobalRestorePolicy(QState::RestorePolicy restorePolicy)

running : bool

This property holds the running state of this state machine

This property was introduced in Qt 5.4.

Access functions:

bool isRunning() const
void setRunning(bool running)

Notifier signal:

void runningChanged(bool running)

See also start(), stop(), started(), stopped(), and runningChanged().

Member Function Documentation

QStateMachine::QStateMachine(QObject *parent = nullptr)

Constructs a new state machine with the given parent.

[signal] void QStateMachine::runningChanged(bool running)

This signal is emitted when the running property is changed with running as argument.

This function was introduced in Qt 5.4.

Note: Notifier signal for property running.

See also QStateMachine::running.

[slot] void QStateMachine::start()

Starts this state machine. The machine will reset its configuration and transition to the initial state. When a final top-level state (QFinalState) is entered, the machine will emit the finished() signal.

Note: A state machine will not run without a running event loop, such as the main application event loop started with QCoreApplication::exec() or QApplication::exec().

See also started(), finished(), stop(), initialState(), and setRunning().

[signal] void QStateMachine::started()

This signal is emitted when the state machine has entered its initial state (QStateMachine::initialState).

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also QStateMachine::finished() and QStateMachine::start().

[slot] void QStateMachine::stop()

Stops this state machine. The state machine will stop processing events and then emit the stopped() signal.

See also stopped(), start(), and setRunning().

[signal] void QStateMachine::stopped()

This signal is emitted when the state machine has stopped.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also QStateMachine::stop() and QStateMachine::finished().

[virtual] QStateMachine::~QStateMachine()

Destroys this state machine.

void QStateMachine::addDefaultAnimation(QAbstractAnimation *animation)

Adds a default animation to be considered for any transition.

void QStateMachine::addState(QAbstractState *state)

Adds the given state to this state machine. The state becomes a top-level state and the state machine takes ownership of the state.

If the state is already in a different machine, it will first be removed from its old machine, and then added to this machine.

See also removeState() and setInitialState().

bool QStateMachine::cancelDelayedEvent(int id)

Cancels the delayed event identified by the given id. The id should be a value returned by a call to postDelayedEvent(). Returns true if the event was successfully cancelled, otherwise returns false.

Note: This function is thread-safe.

See also postDelayedEvent().

void QStateMachine::clearError()

Clears the error string and error code of the state machine.

QSet<QAbstractState *> QStateMachine::configuration() const

Returns the maximal consistent set of states (including parallel and final states) that this state machine is currently in. If a state s is in the configuration, it is always the case that the parent of s is also in c. Note, however, that the machine itself is not an explicit member of the configuration.

QList<QAbstractAnimation *> QStateMachine::defaultAnimations() const

Returns the list of default animations that will be considered for any transition.

QStateMachine::Error QStateMachine::error() const

Returns the error code of the last error that occurred in the state machine.

QString QStateMachine::errorString() const

Returns the error string of the last error that occurred in the state machine.

Note: Getter function for property errorString.

[override virtual protected] bool QStateMachine::event(QEvent *e)

Reimplements: QState::event(QEvent *e).

[override virtual] bool QStateMachine::eventFilter(QObject *watched, QEvent *event)

Reimplements: QObject::eventFilter(QObject *watched, QEvent *event).

QState::RestorePolicy QStateMachine::globalRestorePolicy() const

Returns the restore policy of the state machine.

Note: Getter function for property globalRestorePolicy.

See also setGlobalRestorePolicy().

bool QStateMachine::isAnimated() const

Returns whether animations are enabled for this state machine.

Note: Getter function for property animated.

[override virtual protected] void QStateMachine::onEntry(QEvent *event)

Reimplements: QState::onEntry(QEvent *event).

This function will call start() to start the state machine.

[override virtual protected] void QStateMachine::onExit(QEvent *event)

Reimplements: QState::onExit(QEvent *event).

This function will call stop() to stop the state machine and subsequently emit the stopped() signal.

int QStateMachine::postDelayedEvent(QEvent *event, int delay)

Posts the given event for processing by this state machine, with the given delay in milliseconds. Returns an identifier associated with the delayed event, or -1 if the event could not be posted.

This function returns immediately. When the delay has expired, the event will be added to the state machine's event queue for processing. The state machine takes ownership of the event and deletes it once it has been processed.

You can only post events when the state machine is running.

Note: This function is thread-safe.

See also cancelDelayedEvent() and postEvent().

void QStateMachine::postEvent(QEvent *event, QStateMachine::EventPriority priority = NormalPriority)

Posts the given event of the given priority for processing by this state machine.

This function returns immediately. The event is added to the state machine's event queue. Events are processed in the order posted. The state machine takes ownership of the event and deletes it once it has been processed.

You can only post events when the state machine is running or when it is starting up.

Note: This function is thread-safe.

See also postDelayedEvent().

void QStateMachine::removeDefaultAnimation(QAbstractAnimation *animation)

Removes animation from the list of default animations.

void QStateMachine::removeState(QAbstractState *state)

Removes the given state from this state machine. The state machine releases ownership of the state.

See also addState().

void QStateMachine::setAnimated(bool enabled)

Sets whether animations are enabled for this state machine.

Note: Setter function for property animated.

See also isAnimated().

void QStateMachine::setGlobalRestorePolicy(QState::RestorePolicy restorePolicy)

Sets the restore policy of the state machine to restorePolicy. The default restore policy is QState::DontRestoreProperties.

Note: Setter function for property globalRestorePolicy.

See also globalRestorePolicy().

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