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The event system allows custom handling in addition to or instead of the standard processing for any part of DokuWiki which signals its activity via the event system. The custom handlers, or hooks, can be included in any plugin or template script (or even in DokuWiki itself). Action Plugins are specifically designed to take advantage of the events system. They are guaranteed to be loaded at the start of processing and to have an opportunity to register for events before any events have taken place. Other parts of DokuWiki may not be executed immediately or at all for any given page and execution pathway.

It is also possible for custom DokuWiki content to create and signal events of their own.

The event system consists of three parts

 * the [[#event object]].
 * the main event handler or controller.  This is a DokuWiki global, ''$EVENT_HANDLER''. Scripts that wish to receive an event need to register their interest with this object.  When an event is signalled it will cycle through all the registered hooks (or event handlers) passing them the event in turn.
 * individual event handlers or hooks.  These are functions that wish to receive a particular event.  The [[devel:Action Plugins|Action Plugin]] is a vehicle specifically for these functions, however they can also be part of templates, plugins of other types or the main DokuWiki scripts.

Detailed information describing existing events and when they occur is provided in the events list.

**class name Doku_Event**

An event object consists of:

 * **Public** properties
   * ''name'', (readonly) hooks must use this to register to process a particular event
   * ''data'', (read/write) data pertaining to the event, hooks have an opportunity to inspect and modify this
   * ''result'', (read/write) available after the default action has taken place to hooks that have registered for the ''after'' advise.
   * ''canPreventDefault'', (readonly) informs a hook whether or not the default action can be prevented
 * **Private** properties
   * ''_default'' (boolean, initial value ''true''), whether or not the default action associated with the event should be carried out.  Interact with this property via the ''preventDefault()'' method.
   * ''_continue'' (boolean, initial value ''true''), whether or not to continue sending the event to registered hooks that have yet to receive it.  Interact with this property via the ''stopPropagation()'' method.

 * **Public** methods
   * ''[[xref>trigger()]]'' - automated signalling of events.  This method accepts two optional parameters, the default action (callback), and whether or not it may be prevented (bool) and returns the results of the event.  It looks after the whole event process, signalling the "_BEFORE" advise, triggering the default action and signalling the "_AFTER" advise.
   * ''[[xref>stopPropagation()]]'' - stop any further processing of the event by event handlers this function does not prevent the default action taking place
   * ''[[xref>preventDefault()]]'' - prevent the default action taking place
 * ''advise_*()'' methods - for use when the signalling script wishes to handle the complete event signalling process (perhaps when functionalising a default action is not appropriate).
   * ''[[xref>advise_before()]]'' - accepts one parameter, a boolean indicating whether the default action can be prevented, issues the "_BEFORE" signal.
   * ''[[xref>advise_after()]]'' - issues the "_AFTER" signal.

To register a hook to receive an event, call the register_hook() method of the $EVENT_HANDLER. Action plugins can do this using the $controller parameter from within their own register() method. Other parts of DokuWiki should ensure they are either in global scope or declare $EVENT_HANDLER as a global. e.g.

global $EVENT_HANDLER;
$EVENT_HANDLER->register_hook( ... )

For up-to-date details of the register_hook() function and its parameters refer to its declaration in inc/events.php.

Use register_hook($event, $advise, $obj, $method, $param=null, $seq=0) with the arguments:

 * ''$event'' //string//,  name used by the event
 * ''$advise'' //string//, ''BEFORE'' or ''AFTER'', the advise the hook wished to receive
 * ''$obj'' //object//, object in whose scope method is to be executed. If ''null'' the method is assumed to be a globally available function
 * ''$method'' //function//, event handler function. More info at the [[Event handlers]] page.
 * ''$param'' //mixed// (optional), the data to be passed to the event handler. Default null.
 * ''$seq'' //int// (optional), sequence number used to control the order in which hooks are executed. Hooks are executed in ascending $seq order.  If two or more hooks have the same $seq value, their order (relative to each other) is undefined. Hooks can use ''-PHP_INT_MAX'' or ''[[http://www.php.net/manual/en/reserved.constants.php#constant.php-int-max|PHP_INT_MAX]]'', in an attempt to be first or last. Be aware that these values provide no guarantee of being first/last as more than one plugin can use them.

An event can be signalled in three ways.

 - The simplest is to use the function wrapper ''[[xref>trigger_event()]]''. This function takes all the parameters necessary to create an event object and trigger it. \\ Use ''trigger_event($name, &$data, $action=null, $canPreventDefault=true)'' with the arguments:     
    * ''$name'' //string//, name for the event
    * ''$data'' //mixed//, event data
    * ''$action'' //callback// (optional), default action given as php callback function. Default null.
    * ''$canPreventDefault'' //boolean// (optional), can hooks prevent the default action. Default true.
    * return //mixed//, the event result value after all event processing is complete. By default this is the return value of the default action. However it can be set or modified by event handlers hooks as it is stored in ''result'' attribute of the ''[[xref>Doku_Event]]'' object, where the Doku_Event is available in handlers. 

             , ) 

 - using the [[xref>trigger()]] method.  This isn't recommended as it is better to use the ''trigger_event()'' function wrapper described above. 

 - managing the whole event signalling process with [[xref>advise_before()|advise_before($enablePreventDefault = true)]] and [[xref>advise_after()|advise_after()]] on the [[xref>Doku_Event]] object.  Use this method when there is a default action but it not possible to package it as a PHP callback function. 

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