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   <title>Chapter&nbsp;6.&nbsp;Internals</title><link rel="stylesheet" href="boostbook.css" type="text/css"><meta name="generator" content="DocBook XSL-NS Stylesheets V1.75.2"><link rel="home" href="index.html" title="Meta State Machine (MSM) V2.12"><link rel="up" href="pt01.html" title="Part&nbsp;I.&nbsp;User' guide"><link rel="prev" href="ch05.html" title="Chapter&nbsp;5.&nbsp;Questions &amp; Answers"><link rel="next" href="ch06s02.html" title="Frontend / Backend interface"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter&nbsp;6.&nbsp;Internals</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch05.html">Prev</a>&nbsp;</td><th width="60%" align="center">Part&nbsp;I.&nbsp;User' guide</th><td width="20%" align="right">&nbsp;<a accesskey="n" href="ch06s02.html">Next</a></td></tr></table><hr></div><div class="chapter" title="Chapter&nbsp;6.&nbsp;Internals"><div class="titlepage"><div><div><h2 class="title"><a name="d0e2651"></a>Chapter&nbsp;6.&nbsp;Internals</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="ch06.html#d0e2656">Backend: Run To Completion</a></span></dt><dt><span class="sect1"><a href="ch06s02.html">Frontend / Backend
                    interface</a></span></dt><dt><span class="sect1"><a href="ch06s03.html"> Generated state ids </a></span></dt><dt><span class="sect1"><a href="ch06s04.html">Metaprogramming tools</a></span></dt></dl></div><p>This chapter describes the internal machinery of the back-end, which can be useful
                for UML experts but can be safely ignored for most users. For implementers, the
                interface between front- and back- end is also described in detail.</p><div class="sect1" title="Backend: Run To Completion"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="d0e2656"></a><span class="command"><strong><a name="run-to-completion"></a></strong></span>Backend: Run To Completion</h2></div></div></div><p>The back-end implements the following run-to completion algorithm:</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>Check if one region of the concrete state machine is in a
                                terminate or interrupt state. If yes, event processing is disabled
                                while the condition lasts (forever for a terminate pseudo-state,
                                while active for an interrupt pseudo-state).</p></li><li class="listitem"><p>If the message queue feature is enabled and if the state machine
                                is already processing an event, push the currently processed event
                                into the queue and end processing. Otherwise, remember that the
                                state machine is now processing an event and continue.</p></li><li class="listitem"><p>If the state machine detected that no deferred event is used, skip
                                this step. Otherwise, mark the first deferred event from the
                                deferred queue as active.</p></li><li class="listitem"><p>Now start the core of event dispatching. If exception handling is
                                activated, this will happen inside a try/catch block and the
                                front-end <code class="code">exception_caught</code> is called if an exception
                                occurs. </p></li><li class="listitem"><p>The event is now dispatched in turn to every region, in the order
                                defined by the initial state front-end definition. This will, for
                                every region, call the corresponding front-end transition definition
                                (the "row" or "Row" of the transition table).</p></li><li class="listitem"><p>Without transition conflict, if for a given region a transition is
                                possible, the guard condition is checked. If it returns
                                    <code class="code">true</code>, the transition processing continues and the
                                current state's exit action is called, followed by the transition
                                action behavior and the new active state's entry behavior.</p></li><li class="listitem"><p>With transition conflicts (several possible transitions,
                                disambiguated by mutually exclusive guard conditions), the guard
                                conditions are tried in reverse order of their transition definition
                                in the transition table. The first one returning <code class="code">true</code>
                                selects its transition. Note that this is not defined by the UML
                                standard, which simply specifies that if the guard conditions are
                                not mutually exclusive, the state machine is ill-formed and the
                                behaviour undefined. Relying on this implementation-specific
                                behaviour will make it harder for the developer to support another
                                state machine framework.</p></li><li class="listitem"><p>If at least one region processes the event, this event is seen as
                                having been accepted. If not, the library calls
                                    <code class="code">no_transition</code> on the state machine for every
                                contained region.</p></li><li class="listitem"><p>If the currently active state is a submachine, the behaviour is
                                slightly different. The UML standard specifies that internal
                                transitions have to be tried first, so the event is first dispatched
                                to the submachine. Only if the submachine does not accept the event
                                are other (non internal) transitions tried.</p></li><li class="listitem"><p>This back-end supports simple states' and submachines' internal
                                transitions. These are provided in the state's
                                    <code class="code">internal_transition_table</code> type. Transitions defined
                                in this table are added at the end of the main state machine's
                                transition table, but with a lesser priority than the submachine's
                                transitions (defined in <code class="code">transition_table</code>). This means,
                                for simple states, that these transitions have higher priority than
                                non-internal transitions, conform to the UML standard which gives
                                higher priority to deeper-level transitions. For submachines, this
                                is a non-standard addition which can help make event processing
                                faster by giving a chance to bypass subregion processing. With
                                standard UML, one would need to add a subregion only to process
                                these internal transitions, which would be slower.</p></li><li class="listitem"><p>After the dispatching itself, the deferred event marked in step 3
                                (if any) now gets a chance of processing.</p></li><li class="listitem"><p>Then, events queued in the message queue also get a dispatching
                                chance</p></li><li class="listitem"><p>Finally, completion / anonymous transitions, if to be found in the
                                transition table, also get their dispatching chance.</p></li></ul></div><p>This algorithm illustrates how the back-end configures itself at compile-time
                    as much as possible. Every feature not found in a given state machine definition
                    is deactivated and has therefore no runtime cost. Completion events, deferred
                    events, terminate states, dispatching to several regions, internal transitions
                    are all deactivated if not used. User configuration is only for exception
                    handling and message queue necessary.</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch05.html">Prev</a>&nbsp;</td><td width="20%" align="center"><a accesskey="u" href="pt01.html">Up</a></td><td width="40%" align="right">&nbsp;<a accesskey="n" href="ch06s02.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter&nbsp;5.&nbsp;Questions &amp; Answers&nbsp;</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top">&nbsp;Frontend / Backend
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