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# QStateMachine Class Reference
[QtCore module]

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

Inherits QState.

### Types

- enum Error { NoError, NoInitialStateError, NoDefaultStateInHistoryStateError, NoCommonAncestorForTransitionError }
- enum EventPriority { NormalPriority, HighPriority }
- enum RestorePolicy { DontRestoreProperties, RestoreProperties }
- class SignalEvent
- class WrappedEvent

### Methods

- __init__ (self, QObject parent = None)
- addDefaultAnimation (self, QAbstractAnimation animation)
- addState (self, QAbstractState state)
- bool cancelDelayedEvent (self, int id)
- clearError (self)
- unknown-type configuration (self)
- unknown-type defaultAnimations (self)
- Error error (self)
- QString errorString (self)
- bool event (self, QEvent e)
- bool eventFilter (self, QObject watched, QEvent event)
- RestorePolicy globalRestorePolicy (self)
- bool isAnimated (self)
- bool isRunning (self)
- onEntry (self, QEvent event)
- onExit (self, QEvent event)
- int postDelayedEvent (self, QEvent event, int delay)
- postEvent (self, QEvent event, EventPriority priority = QStateMachine.NormalPriority)
- removeDefaultAnimation (self, QAbstractAnimation animation)
- removeState (self, QAbstractState state)
- setAnimated (self, bool enabled)
- setGlobalRestorePolicy (self, RestorePolicy restorePolicy)
- start (self)
- stop (self)

### Qt Signals

- void started ()
- void stopped ()

## Detailed Description

The QStateMachine class provides a hierarchical finite state machine.

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);

     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 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.

## Type Documentation

### 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 | Value | Description
--- | --- | ---
`QStateMachine.NoError` | `0` | No error has occurred.
`QStateMachine.NoInitialStateError` | `1` | 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` | `2` | 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` | `3` | 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().

### 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 | Value | Description
--- | --- | ---
`QStateMachine.NormalPriority` | `0` | The event has normal priority.
`QStateMachine.HighPriority` | `1` | The event has high priority.

### QStateMachine.RestorePolicy

This enum specifies the restore policy type. The restore policy takes effect when the machine enters a state which sets one or more properties. If the restore policy is set to RestoreProperties, the state machine will save the original value of the property before the new value is set.

Later, when the machine either enters a state which does not set a value for the given property, the property will automatically be restored to its initial value.

Only one initial value will be saved for any given property. If a value for a property has already been saved by the state machine, it will not be overwritten until the property has been successfully restored.

Constant | Value | Description
--- | --- | ---
`QStateMachine.DontRestoreProperties` | `0` | The state machine should not save the initial values of properties and restore them later.
`QStateMachine.RestoreProperties` | `1` | The state machine should save the initial values of properties and restore them later.

See also QStateMachine.globalRestorePolicy and QState.assignProperty().

## Method Documentation

### QStateMachine.__init__ (self, QObject parent = None)

The parent argument, if not None, causes self to be owned by Qt instead of PyQt.

Constructs a new state machine with the given parent.

### QStateMachine.addDefaultAnimation (self, QAbstractAnimation animation)

Adds a default animation to be considered for any transition.

### QStateMachine.addState (self, QAbstractState state)

The state argument has it's ownership transferred to Qt.

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.

See also removeState() and setInitialState().

### bool QStateMachine.cancelDelayedEvent (self, 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().

### QStateMachine.clearError (self)

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

### unknown-type QStateMachine.configuration (self)

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.

### unknown-type QStateMachine.defaultAnimations (self)

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

### Error QStateMachine.error (self)

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

### QString QStateMachine.errorString (self)

### bool QStateMachine.event (self, QEvent e)

Reimplemented from QObject.event().

### bool QStateMachine.eventFilter (self, QObject watched, QEvent event)

Reimplemented from QObject.eventFilter().

### RestorePolicy QStateMachine.globalRestorePolicy (self)

### bool QStateMachine.isAnimated (self)

### bool QStateMachine.isRunning (self)

Returns whether this state machine is running.

start(), stop()

### QStateMachine.onEntry (self, QEvent event)

Reimplemented from QAbstractState.onEntry().

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

### QStateMachine.onExit (self, QEvent event)

Reimplemented from QAbstractState.onExit().

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

### int QStateMachine.postDelayedEvent (self, QEvent event, int delay)

The event argument has it's ownership transferred to Qt.

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().

### QStateMachine.postEvent (self, QEvent event, EventPriority priority = QStateMachine.NormalPriority)

The event argument has it's ownership transferred to Qt.

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.

Note: This function is thread-safe.

See also postDelayedEvent().

### QStateMachine.removeDefaultAnimation (self, QAbstractAnimation animation)

Removes animation from the list of default animations.

### QStateMachine.removeState (self, QAbstractState state)

The state argument

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

See also addState().

### QStateMachine.setAnimated (self, bool enabled)

### QStateMachine.setGlobalRestorePolicy (self, RestorePolicy restorePolicy)

### QStateMachine.start (self)

This method is also a Qt slot with the C++ signature `void 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(), and initialState().

### QStateMachine.stop (self)

This method is also a Qt slot with the C++ signature `void stop()`.

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

See also stopped() and start().

## Qt Signal Documentation

### void started ()

This is the default overload of this signal.

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

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

### void stopped ()

This is the default overload of this signal.

This signal is emitted when the state machine has stopped.

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