QStateMachine

The QStateMachine class provides a hierarchical finite state machine. More

Inheritance diagram of PySide2.QtCore.QStateMachine

New in version 4.6.

Synopsis

Functions

Virtual functions

Slots

Signals

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:

button = QPushButton()

machine = QStateMachine()
s1 = QState()
s1.assignProperty(button, "text", "Click me")

s2 = QFinalState()
s1.addTransition(button, SIGNAL('clicked()'), s2)

machine.addState(s1)
machine.addState(s2)
machine.setInitialState(s1)
machine.start()

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 QObject s when the state is entered or exited. It also contains convenience functions for adding transitions, e.g., QSignalTransition s 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.

class QStateMachine([parent=None])

QStateMachine(childMode[, parent=None])

param parent

QObject

param childMode

ChildMode

Constructs a new state machine with the given parent .

Constructs a new state machine with the given childMode and parent .

PySide2.QtCore.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.

Constant

Description

QStateMachine.NormalPriority

The event has normal priority.

QStateMachine.HighPriority

The event has high priority.

PySide2.QtCore.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.

Constant

Description

QStateMachine.NoError

No error has occurred.

QStateMachine.NoInitialStateError

The 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.NoDefaultStateInHistoryStateError

The 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.NoCommonAncestorForTransitionError

The 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.

See also

setErrorState()

PySide2.QtCore.QStateMachine.addDefaultAnimation(animation)
Parameters

animationQAbstractAnimation

Adds a default animation to be considered for any transition.

PySide2.QtCore.QStateMachine.addState(state)
Parameters

stateQAbstractState

Adds the given state to this state machine. The state becomes a top-level 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.

PySide2.QtCore.QStateMachine.beginMicrostep(event)
Parameters

eventQEvent

This function is called when the state machine is about to do a microstep.

The default implementation does nothing.

PySide2.QtCore.QStateMachine.beginSelectTransitions(event)
Parameters

eventQEvent

This function is called when the state machine is about to select transitions based on the given event .

The default implementation does nothing.

PySide2.QtCore.QStateMachine.cancelDelayedEvent(id)
Parameters

idint

Return type

bool

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 .

PySide2.QtCore.QStateMachine.clearError()

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

PySide2.QtCore.QStateMachine.configuration()
Return type

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.

PySide2.QtCore.QStateMachine.endMicrostep(event)
Parameters

eventQEvent

This function is called when the state machine has finished doing a microstep.

The default implementation does nothing.

PySide2.QtCore.QStateMachine.endSelectTransitions(event)
Parameters

eventQEvent

This function is called when the state machine has finished selecting transitions based on the given event .

The default implementation does nothing.

PySide2.QtCore.QStateMachine.error()
Return type

Error

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

PySide2.QtCore.QStateMachine.errorString()
Return type

unicode

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

PySide2.QtCore.QStateMachine.globalRestorePolicy()
Return type

RestorePolicy

Returns the restore policy of the state machine.

PySide2.QtCore.QStateMachine.isAnimated()
Return type

bool

Returns whether animations are enabled for this state machine.

PySide2.QtCore.QStateMachine.isRunning()
Return type

bool

PySide2.QtCore.QStateMachine.postDelayedEvent(event, delay)
Parameters
  • eventQEvent

  • delayint

Return type

int

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.

PySide2.QtCore.QStateMachine.postEvent(event[, priority=NormalPriority])
Parameters

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.

PySide2.QtCore.QStateMachine.removeDefaultAnimation(animation)
Parameters

animationQAbstractAnimation

Removes animation from the list of default animations.

PySide2.QtCore.QStateMachine.removeState(state)
Parameters

stateQAbstractState

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

See also

addState()

PySide2.QtCore.QStateMachine.runningChanged(running)
Parameters

runningbool

PySide2.QtCore.QStateMachine.setAnimated(enabled)
Parameters

enabledbool

Sets whether animations are enabled for this state machine.

See also

isAnimated()

PySide2.QtCore.QStateMachine.setGlobalRestorePolicy(restorePolicy)
Parameters

restorePolicyRestorePolicy

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

PySide2.QtCore.QStateMachine.setRunning(running)
Parameters

runningbool

See also

isRunning()

PySide2.QtCore.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 exec() or exec() .

See also

started() finished() stop() initialState() setRunning()

PySide2.QtCore.QStateMachine.stop()

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

See also

stopped() start() setRunning()