1<html><head> 2 <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 3 <title>Frontend / Backend interface</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)"><link rel="up" href="ch06.html" title="Chapter 6. Internals"><link rel="prev" href="ch06.html" title="Chapter 6. Internals"><link rel="next" href="ch06s03.html" title="Generated state ids"></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">Frontend / Backend 4 interface</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch06.html">Prev</a> </td><th width="60%" align="center">Chapter 6. Internals</th><td width="20%" align="right"> <a accesskey="n" href="ch06s03.html">Next</a></td></tr></table><hr></div><div class="sect1" title="Frontend / Backend interface"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="d0e3079"></a><span class="command"><strong><a name="internals-front-back-interface"></a></strong></span>Frontend / Backend 5 interface</h2></div></div></div><p>The design of MSM tries to make front-ends and back-ends (later) to be as 6 interchangeable as possible. Of course, no back-end will ever implement every 7 feature defined by any possible front-end and inversely, but the goal is to make 8 it as easy as possible to extend the current state of the library.</p><p>To achieve this, MSM divides the functionality between both sides: the 9 front-end is a sort of user interface and is descriptive, the back-end 10 implements the state machine engine.</p><p>MSM being based on a transition table, a concrete state machine (or a given 11 front-end) must provide a transition_table. This transition table must be made 12 of rows. And each row must tell what kind of transition it is and implement the 13 calls to the actions and guards. A state machine must also define its regions 14 (marked by initial states) And that is about the only constraints for 15 front-ends. How the rows are described is implementer's choice. </p><p>Every row must provide:</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>A <code class="code">Source</code> typedef indicating, well, the type of the source 16 state.</p></li><li class="listitem"><p>A <code class="code">Target</code> typedef indicating, well, the type of the target 17 state.</p></li><li class="listitem"><p>A <code class="code">Evt</code> typedef indicating the type of the event triggering 18 the transition.</p></li><li class="listitem"><p>A <code class="code">row_type_tag</code> typedef indicating the type of the 19 transition.</p></li><li class="listitem"><p>Rows having a type requiring transition actions must provide a static 20 function <code class="code">action_call</code> with the following signature: <code class="code"> 21 template <class Fsm,class SourceState,class TargetState,class 22 AllStates> </code></p><p><code class="code">static void action_call (Fsm& fsm, Event const& evt, 23 SourceState&, TargetState&, AllStates&) </code></p><p>The function gets as parameters the (back-end) state machine, the 24 event, source and target states and a container (in the current 25 back-end, a fusion::set) of all the states defined in the state machine. 26 For example, as the back-end has the front-end as basic class, 27 <code class="code">action_call</code> is simply defined as 28 <code class="code">(fsm.*action)(evt)</code>.</p></li><li class="listitem"><p>Rows having a type requiring a guard must provide a static function 29 <code class="code">guard_call</code> with the following signature:<code class="code"> </code></p><p><code class="code">template <class Fsm,class SourceState,class TargetState,class 30 AllStates></code></p><p><code class="code">static bool guard_call (Fsm&, Event const&, 31 SourceState&, TargetState&, AllStates&)</code></p></li><li class="listitem"><p>The possible transition (row) types are:</p><div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem"><p>a_row_tag: a transition with actions and no guard</p></li><li class="listitem"><p>g_row_type: a transition with a guard and no 32 actions</p></li><li class="listitem"><p>_row_tag: a transition without actions or guard</p></li><li class="listitem"><p>row_tag: a transition with guard and actions</p></li><li class="listitem"><p>a_irow_tag: an internal transition (defined inside the 33 <code class="code">transition_table</code>) with actions</p></li><li class="listitem"><p>g_irow_tag: an internal transition (defined inside the 34 <code class="code">transition_table</code>) with guard</p></li><li class="listitem"><p>irow_tag: an internal transition (defined inside the 35 <code class="code">transition_table</code>) with actions and 36 guards</p></li><li class="listitem"><p>_irow_tag: an internal transition (defined inside the 37 <code class="code">transition_table</code>) without action or guard. 38 Due to higher priority for internal transitions, this is 39 equivalent to a "ignore event"</p></li><li class="listitem"><p>sm_a_i_row_tag: an internal transition (defined inside the 40 <code class="code">internal_transition_table</code>) with 41 actions</p></li><li class="listitem"><p>sm_g_i_row_tag: an internal transition (defined inside the 42 <code class="code">internal_transition_table</code>) with 43 guard</p></li><li class="listitem"><p>sm_i_row_tag: an internal transition (defined inside the 44 <code class="code">internal_transition_table</code>) with actions and 45 guards</p></li><li class="listitem"><p>sm__i_row_tag: an internal transition (defined inside the 46 <code class="code">internal_transition_table</code>) without action 47 or guard. Due to higher priority for internal transitions, 48 this is quivalent to a "ignore event"</p></li></ul></div></li></ul></div><p>Furthermore, a front-end must provide the definition of states and state 49 machines. State machine definitions must provide (the implementer is free to 50 provide it or let it be done by every concrete state machine. Different MSM 51 front-ends took one or the other approach):</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p><code class="code">initial_state</code>: This typedef can be a single state or 52 a mpl container and provides the initial states defining one or 53 several orthogonal regions.</p></li><li class="listitem"><p><code class="code">transition_table</code>: This typedef is a MPL sequence of 54 transition rows.</p></li><li class="listitem"><p><code class="code">configuration</code>: this typedef is a MPL sequence of 55 known types triggering special behavior in the back-end, for example 56 if a concrete fsm requires a message queue or exception 57 catching.</p></li></ul></div><p>States and state machines must both provide a (possibly empty) definition of:</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p><code class="code">flag_list</code>: the flags being active when this state or 58 state machine become the current state of the fsm.</p></li><li class="listitem"><p><code class="code">deferred_events</code>: events being automatically deferred 59 when the state is the current state of the fsm.</p></li><li class="listitem"><p><code class="code">internal_transition_table</code>: the internal transitions 60 of this state.</p></li><li class="listitem"><p><code class="code">on_entry</code> and <code class="code">on_exit</code> methods.</p></li></ul></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch06.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch06.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch06s03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 6. 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