php-doc-en/reference/event/event.persistence.xml

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<chapter  xml:id="event.persistence" xmlns:phpdoc="http://php.net/ns/phpdoc" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>About event persistence</title>
 <para>
  By default, whenever a pending event becomes active (because its file
  descriptor is ready to read or write, or because its timeout expires), it
  becomes non-pending right before its callback is executed. Thus, to make the
  event pending again one may call
  <methodname>Event::add</methodname>
  on it again from inside the callback function.
 </para>
 <para>
  If the
  <constant>Event::PERSIST</constant>
  flag is set on an event, however, the event is
  <emphasis>persistent</emphasis>
  . This means that event remains pending even when its callback is activated.
  <methodname>Event::del</methodname>
  method can be called to make it non-pending.
 </para>
 <para>
  The timeout on a persistent event resets whenever the event's callback runs.
  Thus, if one has an event with flags
  <constant>Event::READ</constant>
  <literal>|</literal>
  <constant>Event::PERSIST</constant>
  and a timeout of five seconds, the event will become active:
 </para>
 <orderedlist>
  <listitem>
   <para>
    Whenever the socket or file descriptor is ready for reading.
   </para>
  </listitem>
  <listitem>
   <para>
    Whenever five seconds have passed since the event last became active.
   </para>
  </listitem>
 </orderedlist>
 <para>
  See also
  <link
 xlink:href="http://www.wangafu.net/~nickm/libevent-book/Ref4_event.html#_about_event_persistence">Fast
 portable non-blocking network programming with Libevent, About Event
 Persistence</link>
 </para>
</chapter>
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