SCXML Media Player

Media Player 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

You can run the example from:

• Qt Creator

  Open the Welcome mode and select the example from Examples. For more information, see Qt Creator: Tutorial: Build and run.

• Qt Extension for Visual Studio Code

  Run the Qt: Open Qt examples command from the Command Palette, and select the example from the list. For more information, see Qt Extension for Visual Studio Code: Tutorial: Build and run.

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"
 #include <QtQml/qqml.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.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