- Qt 5.15
- Qt SCXML
- Qt SCXML Examples
- Qt SCXML Media Player QML Example (C++ Data Model)
- Qt 5.15.18 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