- Qt 5.15
- Qt Core
- C++ Classes
- QHistoryState
- Qt 5.15.15 Reference Documentation

# QHistoryState Class

The QHistoryState class provides a means of returning to a previously active substate. More...

Header: | #include <QHistoryState>
qmake: | QT += core
Since: | Qt 4.6
Inherits: | QAbstractState

This class was introduced in Qt 4.6.

- List of all members, including inherited members

## Public Types

enum | HistoryType { ShallowHistory, DeepHistory }

## Properties

- defaultState : QAbstractState*
- defaultTransition : QAbstractTransition*
- historyType : HistoryType

## Public Functions

| QHistoryState(HistoryType type, QState *parent = nullptr)
| QHistoryState(QState *parent = nullptr)
virtual | ~QHistoryState()
QAbstractState * | defaultState() const
QAbstractTransition * | defaultTransition() const
HistoryType | historyType() const
void | setDefaultState(QAbstractState *state)
void | setDefaultTransition(QAbstractTransition *transition)
void | setHistoryType(HistoryType type)

## Signals

void | defaultStateChanged(QPrivateSignal)
void | defaultTransitionChanged(QPrivateSignal)
void | historyTypeChanged(QPrivateSignal)

## Reimplemented Protected Functions

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

## Detailed Description

A history state is a pseudo-state that represents the child state that the parent state was in the last time the parent state was exited. A transition with a history state as its target is in fact a transition to one or more other child states of the parent state. QHistoryState is part of The State Machine Framework.

Use the setDefaultState() function to set the state that should be entered if the parent state has never been entered. Example:

     QStateMachine machine;

     QState *s1 = new QState();
     QState *s11 = new QState(s1);
     QState *s12 = new QState(s1);

     QHistoryState *s1h = new QHistoryState(s1);
     s1h->setDefaultState(s11);

     machine.addState(s1);

     QState *s2 = new QState();
     machine.addState(s2);

     QPushButton *button = new QPushButton();
     // Clicking the button will cause the state machine to enter the child state
     // that s1 was in the last time s1 was exited, or the history state's default
     // state if s1 has never been entered.
     s1->addTransition(button, SIGNAL(clicked()), s1h);

If more than one default state has to be entered, or if the transition to the default state(s) has to be acted upon, the defaultTransition should be set instead. Note that the eventTest() method of that transition will never be called: the selection and execution of the transition is done automatically when entering the history state.

By default a history state is shallow, meaning that it won't remember nested states. This can be configured through the historyType property.

## Member Type Documentation

### enum QHistoryState::HistoryType

This enum specifies the type of history that a QHistoryState records.

Constant | Value | Description
--- | --- | ---
`QHistoryState::ShallowHistory` | `0` | Only the immediate child states of the parent state are recorded. In this case a transition with the history state as its target will end up in the immediate child state that the parent was in the last time it was exited. This is the default.
`QHistoryState::DeepHistory` | `1` | Nested states are recorded. In this case a transition with the history state as its target will end up in the most deeply nested descendant state the parent was in the last time it was exited.

## Property Documentation

### defaultState : QAbstractState*

This property holds the default state of this history state

Access functions:

QAbstractState * | defaultState() const
void | setDefaultState(QAbstractState *state)

Notifier signal:

void | defaultStateChanged(QPrivateSignal)

### defaultTransition : QAbstractTransition*

This property holds the default transition of this history state

Access functions:

QAbstractTransition * | defaultTransition() const
void | setDefaultTransition(QAbstractTransition *transition)

Notifier signal:

void | defaultTransitionChanged(QPrivateSignal)

### historyType : HistoryType

This property holds the type of history that this history state records

The default value of this property is QHistoryState::ShallowHistory.

Access functions:

HistoryType | historyType() const
void | setHistoryType(HistoryType type)

Notifier signal:

void | historyTypeChanged(QPrivateSignal)

## Member Function Documentation

### QHistoryState::QHistoryState(HistoryType type, QState *parent = nullptr)

Constructs a new history state of the given type, with the given parent state.

### QHistoryState::QHistoryState(QState *parent = nullptr)

Constructs a new shallow history state with the given parent state.

### `[virtual]` QHistoryState::~QHistoryState()

Destroys this history state.

### QAbstractState *QHistoryState::defaultState() const

Returns this history state's default state. The default state indicates the state to transition to if the parent state has never been entered before.

Note: Getter function for property defaultState.

See also setDefaultState().

### QAbstractTransition *QHistoryState::defaultTransition() const

Returns this history state's default transition. The default transition is taken when the history state has never been entered before. The target states of the default transition therefore make up the default state.

Note: Getter function for property defaultTransition.

This function was introduced in Qt 5.6.

See also setDefaultTransition().

### `[override virtual protected]` bool QHistoryState::event(QEvent *e)

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

### HistoryType QHistoryState::historyType() const

Returns the type of history that this history state records.

Note: Getter function for property historyType.

See also setHistoryType().

### `[override virtual protected]` void QHistoryState::onEntry(QEvent *event)

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

### `[override virtual protected]` void QHistoryState::onExit(QEvent *event)

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

### void QHistoryState::setDefaultState(QAbstractState *state)

Sets this history state's default state to be the given state. state must be a sibling of this history state.

Note that this function does not set state as the initial state of its parent.

Note: Setter function for property defaultState.

See also defaultState().

### void QHistoryState::setDefaultTransition(QAbstractTransition *transition)

Sets this history state's default transition to be the given transition. This will set the source state of the transition to the history state.

Note that the eventTest method of the transition will never be called.

Note: Setter function for property defaultTransition.

This function was introduced in Qt 5.6.

See also defaultTransition().

### void QHistoryState::setHistoryType(HistoryType type)

Sets the type of history that this history state records.

Note: Setter function for property historyType.

See also historyType().