Qt 5.13 | Qt SCXML | Qt SCXML Media Player Example (Static)

Qt 5.13.2 Reference Documentation

# 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 line to the .pro file:

      QT += widgets scxml

We then specify the state machine to compile:

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

The Qt SCXML Compiler, `qscxmlc`, is run automatically to generate statemachine.h and statemachine.cpp, and to add them to the `HEADERS` and `SOURCES` variables 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