SCXML Media Player

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

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The Qt Company Ltd.

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Publié le 14 février 2024 - Mis à jour le 14 février 2024

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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▲

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:

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&lt;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:

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#include "qscxmlcppdatamodel.h"
#include &lt;QtQml/qqml.h&gt;

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:

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    &lt;state id="stopped"&gt;
        &lt;transition event="tap" cond="isValidMedia()" target="playing"/&gt;
    &lt;/state&gt;

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

        &lt;onexit&gt;
            &lt;send event="playbackStopped"&gt;
                &lt;param name="media" expr="media"/&gt;
            &lt;/send&gt;
        &lt;/onexit&gt;

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

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

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

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

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

Example project▲

Example project @ code.qt.io

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