A tutorial on SCXML History Element - Remembering State Information

An SCXML with a complex state (A state with substates/parallel elements) can be made to preserve its state information on exit. When a complex state with a history is entered again, it enters the state it was last in rather than the initial state which it would have otherwise entered.

Use Case

To define an error-handler. In many cases, it might be desirable to have the error-handling logic at one place which can be re-used in all the error conditions. Normally, the flow would be, action1-state [act] do action1 on error do error-handling back to action1-state else action2-state [act] do action1 on error do error-handling back to action2-state else action3-state [act] do action1 on error do error-handling back to action3-state else

Example based on the use case above (without history)

<scxml
  xmlns="http://www.w3.org/2005/07/SCXML"
  version="1.0" 
  xmlns:somespace="http://stack.mera.com/CCXML"
  initialstate="action1-state">

  <state id="action1-state">
    <transition event="act" target="action2-state"/>
  </state>
  <state id="action2-state">
    <onentry>
      <somespace:some-action/>
    </onentry>
    <transition event="act" target="action3-state"/>
    <transition event="application.error" target="action2-error-handler"/>
  </state>
  <state id="action3-state">
    <onentry>
      <somespace:some-action/>
    </onentry>
    <transition event="act" target="action4-state"/>
    <transition event="application.error" target="action3-error-handler"/>
  </state>
  <state id="action4-state" final="true"/>
  <state id="action2-error-handler">
    <onentry>
      <somespace:handle-error/>
    </onentry>
    <transition target="action2-state"/>
  </state>
  <state id="action3-error-handler">
    <onentry>
      <somespace:handle-error/>
    </onentry>
    <transition target="action3-state"/>
  </state>
  <state id="action4-state" final="true"/>
</scxml>

SCXML Using history element

<?xml version="1.0"?>
<scxml xmlns="http://www.w3.org/2005/07/SCXML"
       xmlns:my="http://my.custom-actions.domain/CUSTOM"
       version="1.0"
       initialstate="universe">
    <state id="universe">
        <initial>
                <transition target="history-actions"/>
        </initial>
        <transition event="application.error" target="error-handler"/>
        <history type="deep" id="history-actions">
                <transition target="action-1"/>
        </history>
        <state id="action-1">
                <transition event="action.done" target="action-2"/>
        </state>
        <state id="action-2">
                <transition event="action.done" target="action-3"/>
        </state>
        <state id="action-3" final="true"/>
    </state>
    <state id="error-handler">
        <transition event="error.handled" target="history-actions"/>
    </state>
</scxml>

Explanation

TODO

Source Code

TODO


Contributed By: Fasihullah Askiri

SCXML/Tutorials/History (last edited 2009-09-20 23:48:22 by localhost)