Qt SCXML Media Player Example (Static)

A widget-based application that sends data to and receives it from a compiled ECMAScript data model.

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The Qt Company Ltd.

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Publié le 26 janvier 2024 - Mis à jour le 26 janvier 2024

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Qt SCXML Media Player Example (Static)▲

Media Player Example (Static) demonstrates how to access data from an ECMAScript data model that is compiled into a C++ class.

The UI is created using Qt Widgets.

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 ECMAScript Data Model▲

We specify the data model as a value of the datamodel attribute of the <scxml> element in mediaplayer-common/mediaplayer.scxml:

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&lt;scxml
    xmlns="http://www.w3.org/2005/07/scxml"
    version="1.0"
    name="MediaPlayerStateMachine"
    initial="stopped"
    datamodel="ecmascript"
&gt;
    &lt;datamodel&gt;
        &lt;data id="media"/&gt;
    &lt;/datamodel&gt;

Compiling the State Machine▲

We link against the Qt SCXML module by adding the following lines to the example's build files.

.pro when using qmake:▲

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QT += widgets scxml

We then specify the state machine to compile:

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STATECHARTS = ../mediaplayer-common/mediaplayer.scxml

CMakeLists.txt when using cmake:▲

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find_package(Qt6 REQUIRED COMPONENTS Core Gui Scxml Widgets)

target_link_libraries(mediaplayer-widgets-static PUBLIC
    Qt6::Core
    Qt6::Gui
    Qt6::Scxml
    Qt6::Widgets
)

We then specify the state machine to compile:

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qt6_add_statecharts(mediaplayer-widgets-static
    ../mediaplayer-common/mediaplayer.scxml
)

The statechart directives STATECHARTS or qt6_add_statecharts invoke the Qt SCXML Compiler, qscxmlc, which is run automatically to generate a header and a source file, which are then added appropriately for compilation.

Instantiating the State Machine▲

We instantiate the generated MediaPlayerStateMachine class in mediaplayer-widgets-static.cpp:

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#include "mediaplayer.h"
#include "../mediaplayer-common/mainwindow.h"

#include &lt;QApplication&gt;

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    MediaPlayerStateMachine machine;
    MainWindow mainWindow(&amp;machine);

Connecting to States▲

The media player state machine will send out events when users tap a control and when playback starts or stops, as specified in the SCXML file:

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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;assign location="media" expr="_event.data.media"/&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 === _event.data.media" target="stopped"/&gt;
        &lt;transition event="tap" cond="isValidMedia() &amp;amp;&amp;amp; media !== _event.data.media" target="playing"/&gt;
    &lt;/state&gt;

To be notified when a state machine sends out an event, we connect to the corresponding signals:

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    stateMachine-&gt;connectToEvent("playbackStarted", this, &amp;MainWindow::started);
    stateMachine-&gt;connectToEvent("playbackStopped", this, &amp;MainWindow::stopped);

Example project▲

Example project @ code.qt.io

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