SCXML-tutorial

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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>

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>

Microwave owen example

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