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13.6 事件排序：wait_order()</div>
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<span style="color: #0000FF">wait_order</span>结构会挂起调用进程直到所有指定的事件以指定的顺序（从左到右）被触发，或者任何为触发的事件被乱序触发，并因此会导致操作失败。 </p>
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<span style="color: #0000FF">wait_order</span>结构的语法如下： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00521"><pre class="Element12">wait_statement ::= <span style="color: #008000">// 引用自附录A.6.5</span>
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| <strong><span style="color: #FF0000">wait_order</span></strong> <strong><span style="color: #FF0000">(</span></strong>hierarchical_identifier[<strong><span style="color: #FF0000">,</span></strong> hierarchical_identifier]<strong><span style="color: #FF0000">)</span></strong> action_block
action_block ::=
statement_or_null
| [statement] <strong><span style="color: #FF0000">else</span></strong> statement</pre></div></div>
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<strong>语法13-2 — wait_order事件排序语法（摘录自附录A）</strong></p><p class="Element10">
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为了使<span style="color: #0000FF">wait_order</span>成功，在序列的任何点上，后续的事件—在这一点上它必须是未触发的，或者序列已经失败—必须以规定的顺序被触发。以前的事件并没有被限制成只发生一次。也就是说，一旦一个事件以规定的顺序发生，那么它可以被再次触发而不会引起结构失败。 </p>
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只有列表中的第一个事件能够等待稳固的触发特性。 </p>
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结构失败是采取的动作依赖于是否指定了可选的<span style="color: #0000FF">else</span>语句（失败语句）。如果指定了失败语句，那么一旦结构失败就会执行指定的语句。如果没有指定失败语句，那么失败会产生一个运行时错误。 </p>
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例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00522"><pre class="Element12"><span style="color: #0000FF">wait_order</span><span style="color: #800080">(</span>a<span style="color: #800080">,</span> b<span style="color: #800080">,</span> c<span style="color: #800080">);</span></pre></div></div>
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上面的例子会挂起当前的进程直到事件<span class="Element146">a</span>、<span class="Element146">b</span>和<span class="Element146">c</span>以<span class="Element146">a</span> –> <span class="Element146">b</span> –> <span class="Element146">c</span>的顺序触发。如果事件被乱序触发，那么会产生一个运行时错误。 </p>
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例子： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00523"><pre class="Element12"><span style="color: #0000FF">wait_order</span><span style="color: #800080">(</span>a<span style="color: #800080">,</span> b<span style="color: #800080">,</span> c<span style="color: #800080">)</span> <span style="color: #0000FF">else $display</span><span style="color: #800080">(</span><span style="color: #575D94">"Error: events out of order"</span><span style="color: #800080">);</span></pre></div></div>
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在这个例子中，失败语句指出：一旦结构失败应该显示一条用户消息，但不会产生一个错误。 </p>
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例子： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00524"><pre class="Element12"><span style="color: #0000FF">bit</span> success<span style="color: #800080">;</span>
<span style="color: #0000FF">wait_order</span><span style="color: #800080">(</span>a<span style="color: #800080">,</span> b<span style="color: #800080">,</span> c<span style="color: #800080">)</span>
success <span style="color: #800080">= </span><span style="color: #FF0000">1</span><span style="color: #800080">;</span>
<span style="color: #0000FF">else</span>
success <span style="color: #800080">= </span><span style="color: #FF0000">0</span><span style="color: #800080">;</span></pre></div></div>
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在这个例子中，完成状态被存储在变量<span class="Element146">success</span>中，并且不会产生一个错误。</p></div>
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