Qt 5.13 | Qt SCXML | Qt SCXML Media Player QML Example (C++ Data Model)

Qt 5.13.2 Reference Documentation

# Qt SCXML Media Player QML Example (C++ Data Model)

Sends data to and receives it from a C++ data model.

Media Player QML Example (C++ Data Model) demonstrates how to access data from a C++ data model. The data model enables writing 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`) generates the dispatch methods.

The UI is created using Qt Quick.

#### Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

#### Using the C++ Data Model

We specify the data model as a value of the datamodel attribute of the `<scxml>` element in the SCXML file:

      <scxml
          xmlns="http://www.w3.org/2005/07/scxml"
          version="1.0"
          name="MediaPlayerStateMachine"
          initial="stopped"
          datamodel="cplusplus:TheDataModel:thedatamodel.h"

The format of the datamodel attribute is: `cplusplus:<class-name>:<classdef-header>`. Therefore, we need a file called thedatamodel.h that contains a subclass of QScxmlCppDataModel:

      #include "qscxmlcppdatamodel.h"

      class TheDataModel: public QScxmlCppDataModel
      {
          Q_OBJECT
          Q_SCXML_DATAMODEL

QScxmlCppDataModel derives from QObject, so we add the `Q_OBJECT` macro in the private section of the definition, right after the opening bracket. We then place the `Q_SCXML_DATAMODEL` macro after `Q_OBJECT`. The macro expands to the declaration of virtual methods, the implementation of which is generated by the Qt SCXML compiler.

In the SCXML file, we specify C++ statements in the `<script>` element and use the expr attribute to access the data model:

          <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>

              <onexit>
                  <send event="playbackStopped">
                      <param name="media" expr="media"/>
                  </send>
              </onexit>

              <transition event="tap" cond="!isValidMedia() || media == eventData().value(QStringLiteral(&quot;media&quot;))" target="stopped"/>
              <transition event="tap" cond="isValidMedia() &amp;&amp; media != eventData().value(QStringLiteral(&quot;media&quot;))" target="playing"/>
          </state>

The Qt SCXML compiler generates the various `evaluateTo` methods and converts the expressions and scripts into lambdas inside those methods in mediaplayer-cppdatamodel.cpp:

      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(); }();
      ....
      }

Example project @ code.qt.io