- Qt 6.10
- Qt SCXML
- C++ Classes
- QScxmlCppDataModel
- Qt 6.10.2 Reference Documentation

# QScxmlCppDataModel Class

The QScxmlCppDataModel class is a C++ data model for a Qt SCXML state machine. More...

Header: | `#include <QScxmlCppDataModel>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Scxml)` `target_link_libraries(mytarget PRIVATE Qt6::Scxml)`
qmake: | `QT += scxml`
Inherits: | QScxmlDataModel

- List of all members, including inherited members

## Public Functions

| QScxmlCppDataModel(QObject *parent = nullptr)
bool | inState(const QString &stateName) const
const QScxmlEvent & | scxmlEvent() const

## Reimplemented Public Functions

virtual void | evaluateAssignment(QScxmlExecutableContent::EvaluatorId id, bool *ok) override
virtual void | evaluateForeach(QScxmlExecutableContent::EvaluatorId id, bool *ok, QScxmlDataModel::ForeachLoopBody *body) override
virtual void | evaluateInitialization(QScxmlExecutableContent::EvaluatorId id, bool *ok) override
virtual bool | hasScxmlProperty(const QString &name) const override
virtual QVariant | scxmlProperty(const QString &name) const override
virtual void | setScxmlEvent(const QScxmlEvent &event) override
virtual bool | setScxmlProperty(const QString &name, const QVariant &value, const QString &context) override
virtual bool | setup(const QVariantMap &initialDataValues) override

## Detailed Description

The C++ data model for SCXML lets you write C++ code for expr attributes and `<script>` elements. The data part of the data model is backed by a subclass of QScxmlCppDataModel, for which the Qt SCXML compiler (`qscxmlc`) will generate the dispatch methods. It cannot be used when loading an SCXML file at runtime.

Usage is through the datamodel attribute of the `<scxml>` element:

     <scxml datamodel="cplusplus:TheDataModel:thedatamodel.h"  ....>

The format of the datamodel attribute is: `cplusplus:<class-name>:<classdef-header>`. So, for the example above, there should be a file thedatamodel.h containing a subclass of QScxmlCppDataModel, containing at least the following:

     #include <QScxmlCppDataModel>

     class TheDataModel: public QScxmlCppDataModel
     {
         Q_OBJECT
         Q_SCXML_DATAMODEL
         ...
     };

The Q_SCXML_DATAMODEL has to appear in the private section of the class definition, for example right after the opening bracket, or after a Q_OBJECT macro. This macro expands to the declaration of some virtual methods whose implementation is generated by the Qt SCXML compiler.

The Qt SCXML compiler will generate the various `evaluateTo` methods, and convert expressions and scripts into lambdas inside those methods. For example:

     <scxml datamodel="cplusplus:TheDataModel:thedatamodel.h" xmlns="http://www.w3.org/2005/07/scxml" version="1.0" name="MediaPlayerStateMachine">
         <state id="stopped">
             <transition event="tap" cond="isValidMedia()" target="playing"/>
         </state>

         <state id="playing">
             <onentry>
                 <script>
                     media = eventData().value(QStringLiteral(&quot;media&quot;)).toString();
                 </script>
                 <send event="playbackStarted">
                     <param name="media" expr="media"/>
                 </send>
             </onentry>
         </state>
     </scxml>

This will result in:

     bool TheDataModel::evaluateToBool(QScxmlExecutableContent::EvaluatorId id, bool *ok) {
         // ....
             return [this]()->bool{ return isValidMedia(); }();
         // ....
     }

     QVariant TheDataModel::evaluateToVariant(QScxmlExecutableContent::EvaluatorId id, bool *ok) {
         // ....
             return [this]()->QVariant{ return media; }();
         // ....
     }

     void TheDataModel::evaluateToVoid(QScxmlExecutableContent::EvaluatorId id, bool *ok) {
         // ....
             [this]()->void{ media = eventData().value(QStringLiteral("media")).toString(); }();
         // ....
     }

So, you are not limited to call functions. In a `<script>` element you can put zero or more C++ statements, and in cond or expr attributes you can use any C++ expression that can be converted to the respective bool or QVariant. And, as the `this` pointer is also captured, you can call or access the data model (the media attribute in the example above). For the full example, see SCXML Media Player.

See also QScxmlStateMachine and QScxmlDataModel.

## Member Function Documentation

### `[explicit]` QScxmlCppDataModel::QScxmlCppDataModel(QObject *parent = nullptr)

Creates a new C++ data model with the parent object parent.

### `[override virtual]` void QScxmlCppDataModel::evaluateAssignment(QScxmlExecutableContent::EvaluatorId id, bool *ok)

Reimplements: QScxmlDataModel::evaluateAssignment(QScxmlExecutableContent::EvaluatorId id, bool *ok).

This method does not perform any action, ignores id, and sets ok to `false`. Override it in your specific data model in order to implement `<assign>`.

### `[override virtual]` void QScxmlCppDataModel::evaluateForeach(QScxmlExecutableContent::EvaluatorId id, bool *ok, QScxmlDataModel::ForeachLoopBody *body)

Reimplements: QScxmlDataModel::evaluateForeach(QScxmlExecutableContent::EvaluatorId id, bool *ok, QScxmlDataModel::ForeachLoopBody *body).

This method does not perform any action, ignores id and body, and sets ok to `false`. Override it in your specific data model in order to implement `<foreach>`.

### `[override virtual]` void QScxmlCppDataModel::evaluateInitialization(QScxmlExecutableContent::EvaluatorId id, bool *ok)

Reimplements: QScxmlDataModel::evaluateInitialization(QScxmlExecutableContent::EvaluatorId id, bool *ok).

This method does not perform any action, ignores id, and sets ok to `false`. Override it in your specific data model in order to implement `<data>`.

### `[override virtual]` bool QScxmlCppDataModel::hasScxmlProperty(const QString &name) const

Reimplements: QScxmlDataModel::hasScxmlProperty(const QString &name) const.

This method always returns `false` and ignores name. Override it to implement the lookup of data model properties via the `location` attribute of various elements.

### bool QScxmlCppDataModel::inState(const QString &stateName) const

Returns `true` if the state machine is in the state specified by stateName, `false` otherwise.

### const QScxmlEvent &QScxmlCppDataModel::scxmlEvent() const

Holds the current event that is being processed by the state machine.

See also SCXML Specification - 5.10 System Variables for the description of the `_event` variable.

Returns the event currently being processed.

See also setScxmlEvent().

### `[override virtual]` QVariant QScxmlCppDataModel::scxmlProperty(const QString &name) const

Reimplements: QScxmlDataModel::scxmlProperty(const QString &name) const.

This method always returns an empty QVariant and ignores name. Override it to implement the lookup of data model properties via the `location` attribute of various elements.

See also setScxmlProperty().

### `[override virtual]` void QScxmlCppDataModel::setScxmlEvent(const QScxmlEvent &event)

Reimplements: QScxmlDataModel::setScxmlEvent(const QScxmlEvent &event).

Sets the event that will be processed next.

See also QScxmlCppDataModel::scxmlEvent.

### `[override virtual]` bool QScxmlCppDataModel::setScxmlProperty(const QString &name, const QVariant &value, const QString &context)

Reimplements: QScxmlDataModel::setScxmlProperty(const QString &name, const QVariant &value, const QString &context).

This method always returns `false` and ignores name, value, and context. Override it to implement the lookup of data model properties via the `location` attribute of various elements.

See also scxmlProperty().

### `[override virtual invokable]` bool QScxmlCppDataModel::setup(const QVariantMap &initialDataValues)

Reimplements: QScxmlDataModel::setup(const QVariantMap &initialDataValues).

Called during state machine initialization to set up a state machine using the initial values for data model variables specified by their keys, initialDataValues. These are the values specified by `<param>` tags in an `<invoke>` element.

Returns `true` on success.

Note: This function can be invoked via the meta-object system and from QML. See Q_INVOKABLE.

See also QScxmlStateMachine::init.