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| <h1 align="center">The Boost Statechart Library</h1> |
| |
| <h2 align="center">Definitions</h2> |
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| |
| <h2>Introduction</h2> |
| |
| <p>The Boost.Statechart documentation uses a lot of terminology specific to |
| state machines. Most of it is equal to the one used in the UML |
| specifications. This document contains only definitions for terminology not |
| used by the <a href="http://www.omg.org/cgi-bin/doc?formal/03-03-01">UML |
| standard</a>. A short tour around UML terminology can be found <a href= |
| "http://www.sts.tu-harburg.de/teaching/ws-99.00/OOA+D/StateDiagrams.pdf">here</a>.</p> |
| |
| <h2>Definitions</h2> |
| |
| <dl class="page-index"> |
| <dt><a href="#Context">Context</a></dt> |
| |
| <dt><a href="#InnermostCommonContext">Innermost common context</a></dt> |
| |
| <dt><a href="#InnermostState">Innermost state</a></dt> |
| |
| <dt><a href="#InStateReaction">In-state reaction</a></dt> |
| |
| <dt><a href="#OutermostState">Outermost state</a></dt> |
| |
| <dt><a href="#PolymorphicEvents">Polymorphic events</a></dt> |
| |
| <dt><a href="#Reaction">Reaction</a></dt> |
| |
| <dt><a href="#UnstableState">Unstable state</a></dt> |
| |
| <dt><a href="#UnstableStateMachine">Unstable state machine</a></dt> |
| </dl> |
| |
| <h3><a name="Context" id="Context">Context</a></h3> |
| |
| <p>The contexts of a state define its location in the state hierarchy. A |
| state's <b>direct</b> context is defined by what is passed as the |
| <code>Context</code> template parameter of the <code><a href= |
| "reference.html#ClassTemplatesimple_state">simple_state</a></code> and |
| <code><a href="reference.html#ClassTemplatestate">state</a></code> class |
| templates. This can either be the state machine (which makes the state an |
| <a href="#OutermostState">outermost state</a>) or its direct outer state. A |
| state's <b>indirect</b> contexts follow from the direct context of its |
| direct context and the direct context of the direct context of its direct |
| context and so on. Examples:</p> |
| |
| <p><img alt="OutermostUnstableState" src="OutermostUnstableState.gif" |
| border="0" width="467" height="572"></p> |
| |
| <ul> |
| <li>A's <b>direct</b> context is the state machine (not visible in this |
| picture). A does not have any indirect contexts</li> |
| |
| <li>B's <b>direct</b> context is A. B's <b>indirect</b> context is the |
| state machine (not visible in this picture)</li> |
| |
| <li>C's <b>direct</b> context is B. C's <b>indirect</b> contexts are B, A |
| and the state machine (not visible in this picture)</li> |
| |
| <li>D's <b>direct</b> context is A. D's <b>indirect</b> context is the |
| state machine (not visible in this picture)</li> |
| </ul> |
| |
| <h3><a name="InnermostCommonContext" id="InnermostCommonContext">Innermost |
| common context</a></h3> |
| |
| <p>The innermost common context of two states is the first direct or |
| indirect context that both states have in common. Also known as Least |
| Common Ancestor (UML).</p> |
| |
| <h3><a name="InnermostState" id="InnermostState">Innermost state</a></h3> |
| |
| <p>An innermost state is a state that does not itself have inner states. |
| Also known as leaf state or simple state (UML). Note that <code><a href= |
| "reference.html#ClassTemplatesimple_state">boost::statechart::simple_state<></a></code> |
| is <b>not</b> a model of the UML simple state.</p> |
| |
| <h3><a name="InStateReaction" id="InStateReaction">In-state |
| reaction</a></h3> |
| |
| <p>An in-state reaction is a <a href="#Reaction">reaction</a> that neither |
| exits nor enters any states. Also known as inner transition or internal |
| transition (UML).</p> |
| |
| <h3><a name="OutermostState" id="OutermostState">Outermost state</a></h3> |
| |
| <p>An outermost state is a state that does not itself have outer states. |
| Note that an outermost state is different from the UML top state. A state |
| machine can have an arbitrary number of the former but only exactly one of |
| the latter. Boost.Statechart only supports outermost states.</p> |
| |
| <h3><a name="PolymorphicEvents" id="PolymorphicEvents">Polymorphic |
| events</a></h3> |
| |
| <p>An FSM library supports polymorphic events if events can inherit from |
| each other without restrictions <b>and</b> if it allows the definition of |
| reactions for leafs and nodes of the resulting event inheritance |
| tree.</p> |
| |
| <p>Example (using a hypothetical FSM library, as Boost.Statechart does not |
| support polymorphic events):</p> |
| <pre> |
| struct EvButtonPressed : Event // node |
| { |
| /* common button pressed properties */ |
| }; |
| |
| struct EvPlayButtonPressed : EvButtonPressed {}; // leaf |
| struct EvStopButtonPressed : EvButtonPressed {}; // leaf |
| struct EvForwardButtonPressed : EvButtonPressed {}; // leaf |
| </pre> |
| |
| <p>If a state machine needs to react whenever <b>any</b> button (including |
| the ones that may be added in the future) is pressed, a reaction for |
| <code>EvButtonPressed</code> can be defined.</p> |
| |
| <h3><a name="Reaction" id="Reaction">Reaction</a></h3> |
| |
| <p>A reaction consists of all the side effects caused by the processing of |
| one event. Reactions can be categorized as follows:</p> |
| |
| <ol> |
| <li>In-state reaction</li> |
| |
| <li>Event deferral</li> |
| |
| <li>Transition</li> |
| |
| <li>Termination, also known as transition to the final state (UML)</li> |
| </ol> |
| |
| <p>Note that it is possible to mix a reaction of type 1 with one of the |
| other types (the in-state reaction is always executed first) but it is not |
| possible to mix a reaction of type 2-4 with anything else but type 1.</p> |
| |
| <p>A reaction is always associated with exactly one state type and exactly |
| one event type.</p> |
| |
| <h3><a name="UnstableState" id="UnstableState">Unstable state</a></h3> |
| |
| <p>A state is unstable from the moment when it has been entered until after |
| its last <b>direct</b> inner state has been entered. A state is also |
| unstable from the moment just before its first <b>direct</b> inner state is |
| exited until right before the state itself is exited.</p> |
| |
| <h3><a name="UnstableStateMachine" id="UnstableStateMachine">Unstable state |
| machine</a></h3> |
| |
| <p>A state machine is unstable if at least one of its currently active |
| states is unstable. This is the case during the following three |
| operations:</p> |
| |
| <ul> |
| <li>Initiation: From the moment after the first state has been entered |
| until after the last state of the initial state configuration has been |
| entered</li> |
| |
| <li>Transition: From the moment just before the first state of the |
| current state configuration is exited until after the last state of the |
| destination state configuration has been entered</li> |
| |
| <li>Termination: From the moment just before the first state is exited |
| until right before the last terminated state is exited. A successfully |
| executed termination (no exception was thrown) never leaves any states |
| unstable. For example, consider the active state A with two orthogonal |
| regions in which the inner states B and C are each active. Terminating |
| either B or C does not make A unstable. Neither does terminating both, as |
| that inevitably also terminates A</li> |
| </ul> |
| |
| <p>Under normal circumstances a state machine has Run-To-Completion |
| semantics, that is, it is always stable before the machine returns to the |
| client or before the next event is dequeued. So, a state machine is usually |
| only unstable when it is busy processing an event and becomes stable again |
| right before it has finished processing the event. However, this can not be |
| guaranteed when entry, exit or transition actions fail. Such a failure is |
| reported by an event, which must be processed while the state machine is |
| unstable. However, exception event processing rules ensure that a state |
| machine is never unstable when it returns to the client (see <code><a href= |
| "reference.html#process_event">state_machine<>::process_event()</a></code> |
| for details).</p> |
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| <p>Revised |
| <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->29 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38526" --></p> |
| |
| <p><i>Copyright © 2003-<!--webbot bot="Timestamp" s-type="EDITED" s-format="%Y" startspan -->2006<!--webbot bot="Timestamp" endspan i-checksum="770" --> |
| <a href="contact.html">Andreas Huber Dönni</a></i></p> |
| |
| <p><i>Distributed under the Boost Software License, Version 1.0. (See |
| accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or |
| copy at <a href= |
| "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> |
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