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SCXML Tutorial

This project is an attempt to illustrate the current SCXML standard

New! Video version of the tutorial

Hello world

<scxml name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<final id="Final">
		<onentry>
			<log expr="Hello, world!" label="INFO"/>
		</onentry>
	</final>
</scxml>

Questions & Answers

Table of contents

• Core Constructs
  • <scxml>
  • <state>
  • <parallel>
  • <initial>
  • <final>
  • <transition>
  • <onentry>
  • <onexit>
  • <history>
• Executable Content
  • <raise>
  • <if>
  • <else>
  • <elseif>
  • <foreach>
  • <log>
• Data Model and Data Manipulation
  • <datamodel>
  • <data>
  • <assign>
  • <donedata>
  • <content>
  • <param>
  • <script>
• External Communications
  • <send>
  • <cancel>
  • <invoke>
  • <finalize>
  • Basic HTTP Event I/O Processor

SCXML Frameworks W3C Standard Specification Compliance

Framework	Datamodel	W3C Mandatory	W3C Optional	Compliance
uSCXML 2.0	ecmascript	159 of 159	33 of 33	Full
Qt SCXML 5.15	ecmascript	141 of 159	17 of 33	Partial
SCION CLI 1.0.37	ecmascript	156 of 159	18 of 33	Partial
PyBlendSCXML 1.0.0	python	159 of 159	22 of 33	Partial

Examples

1. Microwave owen

2. Microwave owen (using parallel)

3. Calculator

4. Salus RT500 (Digital Room Thermostat) Simulator

5. Morse Code Trainer

6. StopWatch

7. Infotainment Radio Bolero Simulator

Qt SCXML Examples

Articles

1. Inheritance (Visual) in SCXML (State Machines)

W3C Examples

W3C IRP tests

Donations

When we started the project in 2016 there were no SCXML illustrations, no simple explanation of SCXML details, no good examples. We felt the power of SCXML but understood that without simple explanations, illustrative examples and convinient modelling tools it’s worth nothing.

If you are interested in the development of SCXML tutorials and examples we would be highly appreciated for any help.

Paypal

SCXML Overview

Basic State Machine Notation

The most basic state machine concepts are <state>, <transition> and event. Each state contains a set of transitions that define how it reacts to events. Events can be generated by the state machine itself or by external entities. In a traditional state machine, the machine is always in a single state. This state is called the active state. When an event occurs, the state machine checks the transitions that are defined in the active state. If it finds one that matches the event, it moves from the active state to the state specified by the transition (called the “target” of the transition.) Thus the target state becomes the new active state.

<scxml initial="Start" name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<final id="Final">
		<onentry>
			<log expr="Finished!" label="INFO"/>
		</onentry>
	</final>
	<state id="Start">
		<transition event="Event" target="Final"/>
	</state>
</scxml>

Atomic state

Does not contain any child states

<scxml name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<state id="Level 1"/>
</scxml>

Compound states

May contain nested <state> elements and the nesting may proceed to any depth

<scxml name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<state id="Level 1">
		<state id="Level 2">
			<state id="Level 3"/>
		</state>
	</state>
</scxml>

Parallel states

The <parallel> element represents a state whose children are executed in parallel.

<scxml name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<parallel id="Airplane_Engines">
		<state id="Engine_1" initial="Engine_1_Off">
			<state id="Engine_1_Off">
				<transition event="Start.1" target="Engine_1_On"/>
			</state>
			<state id="Engine_1_On">
				<transition event="Shutdown.1" target="Engine_1_Off"/>
			</state>
		</state>
		<state id="Engine_2" initial="Engine_2_Off">
			<state id="Engine_2_Off">
				<transition event="Start.2" target="Engine_2_On"/>
			</state>
			<state id="Engine_2_On">
				<transition event="Shutdown.2" target="Engine_2_Off"/>
			</state>
		</state>
	</parallel>
</scxml>

Initial state

Represents the default initial state for a complex <state> element

<scxml name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<state id="Work">
		<initial>
			<transition target="Ready"/>
		</initial>
		<state id="Ready"/>
	</state>
</scxml>

Final state

Represents a final state of an <scxml> or compound <state> element.

<scxml name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<state id="Work">
		<transition event="done.state.Work" target="WorkFinished"/>
		<state id="CompletingTask">
			<transition target="Completed"/>
		</state>
		<final id="Completed"/>
	</state>
	<final id="WorkFinished"/>
</scxml>

History state

The <history> pseudo-state allows a state machine to remember its state configuration. A <transition> taking the <history> state as its target will return the state machine to this recorded configuration.

<scxml name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<state id="Work">
		<transition event="Pause" target="Expecting"/>
		<state id="Off">
			<transition event="Switch" target="On"/>
		</state>
		<state id="On">
			<transition event="Switch" target="Off"/>
		</state>
		<initial>
			<transition target="HistoryPoint"/>
		</initial>
		<history id="HistoryPoint">
			<transition target="Off"/>
		</history>
	</state>
	<state id="Expecting">
		<transition event="Resume" target="HistoryPoint"/>
	</state>
</scxml>

Transitions

Transitions between states are triggered by events and conditionalized via guard conditions. They may contain executable content, which is executed when the transition is taken.

<scxml datamodel="lua" name="Scxml" version="1.0" xmlns="http://www.w3.org/2005/07/scxml">
	<state id="Work" initial="Off">
		<transition event="Update" type="internal">
			<log expr="'Updated'" label="OnUpdate"/>
		</transition>
		<transition event="ReInit" target="Work" type="internal"/>
		<transition event="Quit" target="End"/>
		<transition event="error.*" target="Fail"/>
		<state id="Off">
			<transition cond="_event.data==1" event="Switch" target="On"/>
		</state>
		<state id="On">
			<transition cond="_event.data==0" event="Switch" target="Off"/>
		</state>
	</state>
	<final id="End"/>
	<final id="Fail"/>
</scxml>

Invoke

SCXML provides an element <invoke> which can create external services. For example: it can create instances of external state machines

Traffic light example

Source code

```xml ```

Time generator example

Source code

```xml ```

StopWatch example

Source code

```xml ```

Qt Bug Tracking Workflow example

Source code

```xml ```

Microwave owen example

Source code

```xml ```

Dining Philosophers Problem example

Source code - main.scxml

```xml ```

Source code - invoked.scxml

```xml ```

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