QScxmlCppDataModel Class

Detailed Description

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

The C++ data model for SCXML that 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.h"

  class TheDataModel: public QScxmlCppDataModel
  {
      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.

Note: You can of course inherit from both QScxmlCppDataModel and QObject.

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 type="qt:signal" 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 Qt SCXML: Media Player QML Example (C++ Data Model).