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

Image non disponible

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:

 
Sélectionnez
&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:

 
Sélectionnez
#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:

 
Sélectionnez
    &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;transiti