Qt SCXML Media Player Example (Static)

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

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:

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

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:

QT += widgets scxml

We then specify the state machine to compile:

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

CMakeLists.txt when using cmake:

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:

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:

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

#include <QApplication>

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

    MediaPlayerStateMachine machine;
    MainWindow mainWindow(&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:

    <state id="stopped">
        <transition event="tap" cond="isValidMedia()" target="playing"/>
    </state>

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

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

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

Example project @ code.qt.io

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